1///===- SimpleLoopUnswitch.cpp - Hoist loop-invariant control flow ---------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8
9#include "llvm/Transforms/Scalar/SimpleLoopUnswitch.h"
10#include "llvm/ADT/DenseMap.h"
11#include "llvm/ADT/STLExtras.h"
12#include "llvm/ADT/Sequence.h"
13#include "llvm/ADT/SetVector.h"
14#include "llvm/ADT/SmallPtrSet.h"
15#include "llvm/ADT/SmallVector.h"
16#include "llvm/ADT/Statistic.h"
17#include "llvm/ADT/Twine.h"
18#include "llvm/Analysis/AssumptionCache.h"
19#include "llvm/Analysis/BlockFrequencyInfo.h"
20#include "llvm/Analysis/CFG.h"
21#include "llvm/Analysis/CodeMetrics.h"
22#include "llvm/Analysis/DomTreeUpdater.h"
23#include "llvm/Analysis/GuardUtils.h"
24#include "llvm/Analysis/LoopAnalysisManager.h"
25#include "llvm/Analysis/LoopInfo.h"
26#include "llvm/Analysis/LoopIterator.h"
27#include "llvm/Analysis/MemorySSA.h"
28#include "llvm/Analysis/MemorySSAUpdater.h"
29#include "llvm/Analysis/MustExecute.h"
30#include "llvm/Analysis/ScalarEvolution.h"
31#include "llvm/Analysis/TargetTransformInfo.h"
32#include "llvm/Analysis/ValueTracking.h"
33#include "llvm/IR/BasicBlock.h"
34#include "llvm/IR/Constant.h"
35#include "llvm/IR/Constants.h"
36#include "llvm/IR/Dominators.h"
37#include "llvm/IR/Function.h"
38#include "llvm/IR/IRBuilder.h"
39#include "llvm/IR/InstrTypes.h"
40#include "llvm/IR/Instruction.h"
41#include "llvm/IR/Instructions.h"
42#include "llvm/IR/IntrinsicInst.h"
43#include "llvm/IR/MDBuilder.h"
44#include "llvm/IR/Module.h"
45#include "llvm/IR/PatternMatch.h"
46#include "llvm/IR/ProfDataUtils.h"
47#include "llvm/IR/Use.h"
48#include "llvm/IR/Value.h"
49#include "llvm/Support/Casting.h"
50#include "llvm/Support/CommandLine.h"
51#include "llvm/Support/Debug.h"
52#include "llvm/Support/ErrorHandling.h"
53#include "llvm/Support/GenericDomTree.h"
54#include "llvm/Support/InstructionCost.h"
55#include "llvm/Support/raw_ostream.h"
56#include "llvm/Transforms/Scalar/LoopPassManager.h"
57#include "llvm/Transforms/Utils/BasicBlockUtils.h"
58#include "llvm/Transforms/Utils/Cloning.h"
59#include "llvm/Transforms/Utils/Local.h"
60#include "llvm/Transforms/Utils/LoopUtils.h"
61#include "llvm/Transforms/Utils/ValueMapper.h"
62#include <algorithm>
63#include <cassert>
64#include <iterator>
65#include <numeric>
66#include <optional>
67#include <utility>
68
69#define DEBUG_TYPE "simple-loop-unswitch"
70
71using namespace llvm;
72using namespace llvm::PatternMatch;
73
74STATISTIC(NumBranches, "Number of branches unswitched");
75STATISTIC(NumSwitches, "Number of switches unswitched");
76STATISTIC(NumSelects, "Number of selects turned into branches for unswitching");
77STATISTIC(NumGuards, "Number of guards turned into branches for unswitching");
78STATISTIC(NumTrivial, "Number of unswitches that are trivial");
79STATISTIC(
80 NumCostMultiplierSkipped,
81 "Number of unswitch candidates that had their cost multiplier skipped");
82STATISTIC(NumInvariantConditionsInjected,
83 "Number of invariant conditions injected and unswitched");
84
85namespace llvm {
86static cl::opt<bool> EnableNonTrivialUnswitch(
87 "enable-nontrivial-unswitch", cl::init(Val: false), cl::Hidden,
88 cl::desc("Forcibly enables non-trivial loop unswitching rather than "
89 "following the configuration passed into the pass."));
90
91static cl::opt<int>
92 UnswitchThreshold("unswitch-threshold", cl::init(Val: 50), cl::Hidden,
93 cl::desc("The cost threshold for unswitching a loop."));
94
95static cl::opt<bool> EnableUnswitchCostMultiplier(
96 "enable-unswitch-cost-multiplier", cl::init(Val: true), cl::Hidden,
97 cl::desc("Enable unswitch cost multiplier that prohibits exponential "
98 "explosion in nontrivial unswitch."));
99static cl::opt<int> UnswitchSiblingsToplevelDiv(
100 "unswitch-siblings-toplevel-div", cl::init(Val: 2), cl::Hidden,
101 cl::desc("Toplevel siblings divisor for cost multiplier."));
102static cl::opt<int> UnswitchParentBlocksDiv(
103 "unswitch-parent-blocks-div", cl::init(Val: 8), cl::Hidden,
104 cl::desc("Outer loop size divisor for cost multiplier."));
105static cl::opt<int> UnswitchNumInitialUnscaledCandidates(
106 "unswitch-num-initial-unscaled-candidates", cl::init(Val: 8), cl::Hidden,
107 cl::desc("Number of unswitch candidates that are ignored when calculating "
108 "cost multiplier."));
109static cl::opt<bool> UnswitchGuards(
110 "simple-loop-unswitch-guards", cl::init(Val: true), cl::Hidden,
111 cl::desc("If enabled, simple loop unswitching will also consider "
112 "llvm.experimental.guard intrinsics as unswitch candidates."));
113static cl::opt<bool> DropNonTrivialImplicitNullChecks(
114 "simple-loop-unswitch-drop-non-trivial-implicit-null-checks",
115 cl::init(Val: false), cl::Hidden,
116 cl::desc("If enabled, drop make.implicit metadata in unswitched implicit "
117 "null checks to save time analyzing if we can keep it."));
118static cl::opt<unsigned>
119 MSSAThreshold("simple-loop-unswitch-memoryssa-threshold",
120 cl::desc("Max number of memory uses to explore during "
121 "partial unswitching analysis"),
122 cl::init(Val: 100), cl::Hidden);
123static cl::opt<bool> FreezeLoopUnswitchCond(
124 "freeze-loop-unswitch-cond", cl::init(Val: true), cl::Hidden,
125 cl::desc("If enabled, the freeze instruction will be added to condition "
126 "of loop unswitch to prevent miscompilation."));
127
128static cl::opt<bool> InjectInvariantConditions(
129 "simple-loop-unswitch-inject-invariant-conditions", cl::Hidden,
130 cl::desc("Whether we should inject new invariants and unswitch them to "
131 "eliminate some existing (non-invariant) conditions."),
132 cl::init(Val: true));
133
134static cl::opt<unsigned> InjectInvariantConditionHotnesThreshold(
135 "simple-loop-unswitch-inject-invariant-condition-hotness-threshold",
136 cl::Hidden,
137 cl::desc("Only try to inject loop invariant conditions and "
138 "unswitch on them to eliminate branches that are "
139 "not-taken 1/<this option> times or less."),
140 cl::init(Val: 16));
141
142static cl::opt<bool> EstimateProfile("simple-loop-unswitch-estimate-profile",
143 cl::Hidden, cl::init(Val: true));
144extern cl::opt<bool> ProfcheckDisableMetadataFixes;
145} // namespace llvm
146
147AnalysisKey ShouldRunExtraSimpleLoopUnswitch::Key;
148namespace {
149struct CompareDesc {
150 BranchInst *Term;
151 Value *Invariant;
152 BasicBlock *InLoopSucc;
153
154 CompareDesc(BranchInst *Term, Value *Invariant, BasicBlock *InLoopSucc)
155 : Term(Term), Invariant(Invariant), InLoopSucc(InLoopSucc) {}
156};
157
158struct InjectedInvariant {
159 ICmpInst::Predicate Pred;
160 Value *LHS;
161 Value *RHS;
162 BasicBlock *InLoopSucc;
163
164 InjectedInvariant(ICmpInst::Predicate Pred, Value *LHS, Value *RHS,
165 BasicBlock *InLoopSucc)
166 : Pred(Pred), LHS(LHS), RHS(RHS), InLoopSucc(InLoopSucc) {}
167};
168
169struct NonTrivialUnswitchCandidate {
170 Instruction *TI = nullptr;
171 TinyPtrVector<Value *> Invariants;
172 std::optional<InstructionCost> Cost;
173 std::optional<InjectedInvariant> PendingInjection;
174 NonTrivialUnswitchCandidate(
175 Instruction *TI, ArrayRef<Value *> Invariants,
176 std::optional<InstructionCost> Cost = std::nullopt,
177 std::optional<InjectedInvariant> PendingInjection = std::nullopt)
178 : TI(TI), Invariants(Invariants), Cost(Cost),
179 PendingInjection(PendingInjection) {};
180
181 bool hasPendingInjection() const { return PendingInjection.has_value(); }
182};
183} // end anonymous namespace.
184
185// Helper to skip (select x, true, false), which matches both a logical AND and
186// OR and can confuse code that tries to determine if \p Cond is either a
187// logical AND or OR but not both.
188static Value *skipTrivialSelect(Value *Cond) {
189 Value *CondNext;
190 while (match(V: Cond, P: m_Select(C: m_Value(V&: CondNext), L: m_One(), R: m_Zero())))
191 Cond = CondNext;
192 return Cond;
193}
194
195/// Collect all of the loop invariant input values transitively used by the
196/// homogeneous instruction graph from a given root.
197///
198/// This essentially walks from a root recursively through loop variant operands
199/// which have perform the same logical operation (AND or OR) and finds all
200/// inputs which are loop invariant. For some operations these can be
201/// re-associated and unswitched out of the loop entirely.
202static TinyPtrVector<Value *>
203collectHomogenousInstGraphLoopInvariants(const Loop &L, Instruction &Root,
204 const LoopInfo &LI) {
205 assert(!L.isLoopInvariant(&Root) &&
206 "Only need to walk the graph if root itself is not invariant.");
207 TinyPtrVector<Value *> Invariants;
208
209 bool IsRootAnd = match(V: &Root, P: m_LogicalAnd());
210 bool IsRootOr = match(V: &Root, P: m_LogicalOr());
211
212 // Build a worklist and recurse through operators collecting invariants.
213 SmallVector<Instruction *, 4> Worklist;
214 SmallPtrSet<Instruction *, 8> Visited;
215 Worklist.push_back(Elt: &Root);
216 Visited.insert(Ptr: &Root);
217 do {
218 Instruction &I = *Worklist.pop_back_val();
219 for (Value *OpV : I.operand_values()) {
220 // Skip constants as unswitching isn't interesting for them.
221 if (isa<Constant>(Val: OpV))
222 continue;
223
224 // Add it to our result if loop invariant.
225 if (L.isLoopInvariant(V: OpV)) {
226 Invariants.push_back(NewVal: OpV);
227 continue;
228 }
229
230 // If not an instruction with the same opcode, nothing we can do.
231 Instruction *OpI = dyn_cast<Instruction>(Val: skipTrivialSelect(Cond: OpV));
232
233 if (OpI && ((IsRootAnd && match(V: OpI, P: m_LogicalAnd())) ||
234 (IsRootOr && match(V: OpI, P: m_LogicalOr())))) {
235 // Visit this operand.
236 if (Visited.insert(Ptr: OpI).second)
237 Worklist.push_back(Elt: OpI);
238 }
239 }
240 } while (!Worklist.empty());
241
242 return Invariants;
243}
244
245static void replaceLoopInvariantUses(const Loop &L, Value *Invariant,
246 Constant &Replacement) {
247 assert(!isa<Constant>(Invariant) && "Why are we unswitching on a constant?");
248
249 // Replace uses of LIC in the loop with the given constant.
250 // We use make_early_inc_range as set invalidates the iterator.
251 for (Use &U : llvm::make_early_inc_range(Range: Invariant->uses())) {
252 Instruction *UserI = dyn_cast<Instruction>(Val: U.getUser());
253
254 // Replace this use within the loop body.
255 if (UserI && L.contains(Inst: UserI))
256 U.set(&Replacement);
257 }
258}
259
260/// Check that all the LCSSA PHI nodes in the loop exit block have trivial
261/// incoming values along this edge.
262static bool areLoopExitPHIsLoopInvariant(const Loop &L,
263 const BasicBlock &ExitingBB,
264 const BasicBlock &ExitBB) {
265 for (const Instruction &I : ExitBB) {
266 auto *PN = dyn_cast<PHINode>(Val: &I);
267 if (!PN)
268 // No more PHIs to check.
269 return true;
270
271 // If the incoming value for this edge isn't loop invariant the unswitch
272 // won't be trivial.
273 if (!L.isLoopInvariant(V: PN->getIncomingValueForBlock(BB: &ExitingBB)))
274 return false;
275 }
276 llvm_unreachable("Basic blocks should never be empty!");
277}
278
279/// Copy a set of loop invariant values \p Invariants and insert them at the
280/// end of \p BB and conditionally branch on the copied condition. We only
281/// branch on a single value.
282/// We attempt to estimate the profile of the resulting conditional branch from
283/// \p ComputeProfFrom, which is the original conditional branch we're
284/// unswitching.
285/// When \p Direction is true, the \p Invariants form a disjunction, and the
286/// branch conditioned on it exits the loop on the "true" case. When \p
287/// Direction is false, the \p Invariants form a conjunction and the branch
288/// exits on the "false" case.
289static void buildPartialUnswitchConditionalBranch(
290 BasicBlock &BB, ArrayRef<Value *> Invariants, bool Direction,
291 BasicBlock &UnswitchedSucc, BasicBlock &NormalSucc, bool InsertFreeze,
292 const Instruction *I, AssumptionCache *AC, const DominatorTree &DT,
293 const BranchInst &ComputeProfFrom) {
294
295 SmallVector<uint32_t> BranchWeights;
296 bool HasBranchWeights = EstimateProfile && !ProfcheckDisableMetadataFixes &&
297 extractBranchWeights(I: ComputeProfFrom, Weights&: BranchWeights);
298 // If Direction is true, that means we had a disjunction and that the "true"
299 // case exits. The probability of the disjunction of the subset of terms is at
300 // most as high as the original one. So, if the probability is higher than the
301 // one we'd assign in absence of a profile (i.e. 0.5), we will use 0.5,
302 // but if it's lower, we will use the original probability.
303 // Conversely, if Direction is false, that means we had a conjunction, and the
304 // probability of exiting is captured in the second branch weight. That
305 // probability is a disjunction (of the negation of the original terms). The
306 // same reasoning applies as above.
307 // Issue #165649: should we expect BFI to conserve, and use that to calculate
308 // the branch weights?
309 if (HasBranchWeights &&
310 static_cast<double>(BranchWeights[Direction ? 0 : 1]) /
311 static_cast<double>(sum_of(Range&: BranchWeights)) >
312 0.5)
313 HasBranchWeights = false;
314
315 IRBuilder<> IRB(&BB);
316 IRB.SetCurrentDebugLocation(DebugLoc::getCompilerGenerated());
317
318 SmallVector<Value *> FrozenInvariants;
319 for (Value *Inv : Invariants) {
320 if (InsertFreeze && !isGuaranteedNotToBeUndefOrPoison(V: Inv, AC, CtxI: I, DT: &DT))
321 Inv = IRB.CreateFreeze(V: Inv, Name: Inv->getName() + ".fr");
322 FrozenInvariants.push_back(Elt: Inv);
323 }
324
325 Value *Cond = Direction ? IRB.CreateOr(Ops: FrozenInvariants)
326 : IRB.CreateAnd(Ops: FrozenInvariants);
327 auto *BR = IRB.CreateCondBr(
328 Cond, True: Direction ? &UnswitchedSucc : &NormalSucc,
329 False: Direction ? &NormalSucc : &UnswitchedSucc,
330 BranchWeights: HasBranchWeights ? ComputeProfFrom.getMetadata(KindID: LLVMContext::MD_prof)
331 : nullptr);
332 if (!HasBranchWeights)
333 setExplicitlyUnknownBranchWeightsIfProfiled(I&: *BR, DEBUG_TYPE);
334}
335
336/// Copy a set of loop invariant values, and conditionally branch on them.
337static void buildPartialInvariantUnswitchConditionalBranch(
338 BasicBlock &BB, ArrayRef<Value *> ToDuplicate, bool Direction,
339 BasicBlock &UnswitchedSucc, BasicBlock &NormalSucc, Loop &L,
340 MemorySSAUpdater *MSSAU, const BranchInst &OriginalBranch) {
341 ValueToValueMapTy VMap;
342 for (auto *Val : reverse(C&: ToDuplicate)) {
343 Instruction *Inst = cast<Instruction>(Val);
344 Instruction *NewInst = Inst->clone();
345
346 if (const DebugLoc &DL = Inst->getDebugLoc())
347 mapAtomInstance(DL, VMap);
348
349 NewInst->insertInto(ParentBB: &BB, It: BB.end());
350 RemapInstruction(I: NewInst, VM&: VMap,
351 Flags: RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
352 VMap[Val] = NewInst;
353
354 if (!MSSAU)
355 continue;
356
357 MemorySSA *MSSA = MSSAU->getMemorySSA();
358 if (auto *MemUse =
359 dyn_cast_or_null<MemoryUse>(Val: MSSA->getMemoryAccess(I: Inst))) {
360 auto *DefiningAccess = MemUse->getDefiningAccess();
361 // Get the first defining access before the loop.
362 while (L.contains(BB: DefiningAccess->getBlock())) {
363 // If the defining access is a MemoryPhi, get the incoming
364 // value for the pre-header as defining access.
365 if (auto *MemPhi = dyn_cast<MemoryPhi>(Val: DefiningAccess))
366 DefiningAccess =
367 MemPhi->getIncomingValueForBlock(BB: L.getLoopPreheader());
368 else
369 DefiningAccess = cast<MemoryDef>(Val: DefiningAccess)->getDefiningAccess();
370 }
371 MSSAU->createMemoryAccessInBB(I: NewInst, Definition: DefiningAccess,
372 BB: NewInst->getParent(),
373 Point: MemorySSA::BeforeTerminator);
374 }
375 }
376
377 IRBuilder<> IRB(&BB);
378 IRB.SetCurrentDebugLocation(DebugLoc::getCompilerGenerated());
379 Value *Cond = VMap[ToDuplicate[0]];
380 // The expectation is that ToDuplicate[0] is the condition used by the
381 // OriginalBranch, case in which we can clone the profile metadata from there.
382 auto *ProfData =
383 !ProfcheckDisableMetadataFixes &&
384 ToDuplicate[0] == skipTrivialSelect(Cond: OriginalBranch.getCondition())
385 ? OriginalBranch.getMetadata(KindID: LLVMContext::MD_prof)
386 : nullptr;
387 auto *BR =
388 IRB.CreateCondBr(Cond, True: Direction ? &UnswitchedSucc : &NormalSucc,
389 False: Direction ? &NormalSucc : &UnswitchedSucc, BranchWeights: ProfData);
390 if (!ProfData)
391 setExplicitlyUnknownBranchWeightsIfProfiled(I&: *BR, DEBUG_TYPE);
392}
393
394/// Rewrite the PHI nodes in an unswitched loop exit basic block.
395///
396/// Requires that the loop exit and unswitched basic block are the same, and
397/// that the exiting block was a unique predecessor of that block. Rewrites the
398/// PHI nodes in that block such that what were LCSSA PHI nodes become trivial
399/// PHI nodes from the old preheader that now contains the unswitched
400/// terminator.
401static void rewritePHINodesForUnswitchedExitBlock(BasicBlock &UnswitchedBB,
402 BasicBlock &OldExitingBB,
403 BasicBlock &OldPH) {
404 for (PHINode &PN : UnswitchedBB.phis()) {
405 // When the loop exit is directly unswitched we just need to update the
406 // incoming basic block. We loop to handle weird cases with repeated
407 // incoming blocks, but expect to typically only have one operand here.
408 for (auto i : seq<int>(Begin: 0, End: PN.getNumOperands())) {
409 assert(PN.getIncomingBlock(i) == &OldExitingBB &&
410 "Found incoming block different from unique predecessor!");
411 PN.setIncomingBlock(i, BB: &OldPH);
412 }
413 }
414}
415
416/// Rewrite the PHI nodes in the loop exit basic block and the split off
417/// unswitched block.
418///
419/// Because the exit block remains an exit from the loop, this rewrites the
420/// LCSSA PHI nodes in it to remove the unswitched edge and introduces PHI
421/// nodes into the unswitched basic block to select between the value in the
422/// old preheader and the loop exit.
423static void rewritePHINodesForExitAndUnswitchedBlocks(BasicBlock &ExitBB,
424 BasicBlock &UnswitchedBB,
425 BasicBlock &OldExitingBB,
426 BasicBlock &OldPH,
427 bool FullUnswitch) {
428 assert(&ExitBB != &UnswitchedBB &&
429 "Must have different loop exit and unswitched blocks!");
430 BasicBlock::iterator InsertPt = UnswitchedBB.begin();
431 for (PHINode &PN : ExitBB.phis()) {
432 auto *NewPN = PHINode::Create(Ty: PN.getType(), /*NumReservedValues*/ 2,
433 NameStr: PN.getName() + ".split");
434 NewPN->insertBefore(InsertPos: InsertPt);
435
436 // Walk backwards over the old PHI node's inputs to minimize the cost of
437 // removing each one. We have to do this weird loop manually so that we
438 // create the same number of new incoming edges in the new PHI as we expect
439 // each case-based edge to be included in the unswitched switch in some
440 // cases.
441 // FIXME: This is really, really gross. It would be much cleaner if LLVM
442 // allowed us to create a single entry for a predecessor block without
443 // having separate entries for each "edge" even though these edges are
444 // required to produce identical results.
445 for (int i = PN.getNumIncomingValues() - 1; i >= 0; --i) {
446 if (PN.getIncomingBlock(i) != &OldExitingBB)
447 continue;
448
449 Value *Incoming = PN.getIncomingValue(i);
450 if (FullUnswitch)
451 // No more edge from the old exiting block to the exit block.
452 PN.removeIncomingValue(Idx: i);
453
454 NewPN->addIncoming(V: Incoming, BB: &OldPH);
455 }
456
457 // Now replace the old PHI with the new one and wire the old one in as an
458 // input to the new one.
459 PN.replaceAllUsesWith(V: NewPN);
460 NewPN->addIncoming(V: &PN, BB: &ExitBB);
461 }
462}
463
464/// Hoist the current loop up to the innermost loop containing a remaining exit.
465///
466/// Because we've removed an exit from the loop, we may have changed the set of
467/// loops reachable and need to move the current loop up the loop nest or even
468/// to an entirely separate nest.
469static void hoistLoopToNewParent(Loop &L, BasicBlock &Preheader,
470 DominatorTree &DT, LoopInfo &LI,
471 MemorySSAUpdater *MSSAU, ScalarEvolution *SE) {
472 // If the loop is already at the top level, we can't hoist it anywhere.
473 Loop *OldParentL = L.getParentLoop();
474 if (!OldParentL)
475 return;
476
477 SmallVector<BasicBlock *, 4> Exits;
478 L.getExitBlocks(ExitBlocks&: Exits);
479 Loop *NewParentL = nullptr;
480 for (auto *ExitBB : Exits)
481 if (Loop *ExitL = LI.getLoopFor(BB: ExitBB))
482 if (!NewParentL || NewParentL->contains(L: ExitL))
483 NewParentL = ExitL;
484
485 if (NewParentL == OldParentL)
486 return;
487
488 // The new parent loop (if different) should always contain the old one.
489 if (NewParentL)
490 assert(NewParentL->contains(OldParentL) &&
491 "Can only hoist this loop up the nest!");
492
493 // The preheader will need to move with the body of this loop. However,
494 // because it isn't in this loop we also need to update the primary loop map.
495 assert(OldParentL == LI.getLoopFor(&Preheader) &&
496 "Parent loop of this loop should contain this loop's preheader!");
497 LI.changeLoopFor(BB: &Preheader, L: NewParentL);
498
499 // Remove this loop from its old parent.
500 OldParentL->removeChildLoop(Child: &L);
501
502 // Add the loop either to the new parent or as a top-level loop.
503 if (NewParentL)
504 NewParentL->addChildLoop(NewChild: &L);
505 else
506 LI.addTopLevelLoop(New: &L);
507
508 // Remove this loops blocks from the old parent and every other loop up the
509 // nest until reaching the new parent. Also update all of these
510 // no-longer-containing loops to reflect the nesting change.
511 for (Loop *OldContainingL = OldParentL; OldContainingL != NewParentL;
512 OldContainingL = OldContainingL->getParentLoop()) {
513 llvm::erase_if(C&: OldContainingL->getBlocksVector(),
514 P: [&](const BasicBlock *BB) {
515 return BB == &Preheader || L.contains(BB);
516 });
517
518 OldContainingL->getBlocksSet().erase(Ptr: &Preheader);
519 for (BasicBlock *BB : L.blocks())
520 OldContainingL->getBlocksSet().erase(Ptr: BB);
521
522 // Because we just hoisted a loop out of this one, we have essentially
523 // created new exit paths from it. That means we need to form LCSSA PHI
524 // nodes for values used in the no-longer-nested loop.
525 formLCSSA(L&: *OldContainingL, DT, LI: &LI, SE);
526
527 // We shouldn't need to form dedicated exits because the exit introduced
528 // here is the (just split by unswitching) preheader. However, after trivial
529 // unswitching it is possible to get new non-dedicated exits out of parent
530 // loop so let's conservatively form dedicated exit blocks and figure out
531 // if we can optimize later.
532 formDedicatedExitBlocks(L: OldContainingL, DT: &DT, LI: &LI, MSSAU,
533 /*PreserveLCSSA*/ true);
534 }
535}
536
537// Return the top-most loop containing ExitBB and having ExitBB as exiting block
538// or the loop containing ExitBB, if there is no parent loop containing ExitBB
539// as exiting block.
540static Loop *getTopMostExitingLoop(const BasicBlock *ExitBB,
541 const LoopInfo &LI) {
542 Loop *TopMost = LI.getLoopFor(BB: ExitBB);
543 Loop *Current = TopMost;
544 while (Current) {
545 if (Current->isLoopExiting(BB: ExitBB))
546 TopMost = Current;
547 Current = Current->getParentLoop();
548 }
549 return TopMost;
550}
551
552/// Unswitch a trivial branch if the condition is loop invariant.
553///
554/// This routine should only be called when loop code leading to the branch has
555/// been validated as trivial (no side effects). This routine checks if the
556/// condition is invariant and one of the successors is a loop exit. This
557/// allows us to unswitch without duplicating the loop, making it trivial.
558///
559/// If this routine fails to unswitch the branch it returns false.
560///
561/// If the branch can be unswitched, this routine splits the preheader and
562/// hoists the branch above that split. Preserves loop simplified form
563/// (splitting the exit block as necessary). It simplifies the branch within
564/// the loop to an unconditional branch but doesn't remove it entirely. Further
565/// cleanup can be done with some simplifycfg like pass.
566///
567/// If `SE` is not null, it will be updated based on the potential loop SCEVs
568/// invalidated by this.
569static bool unswitchTrivialBranch(Loop &L, BranchInst &BI, DominatorTree &DT,
570 LoopInfo &LI, ScalarEvolution *SE,
571 MemorySSAUpdater *MSSAU) {
572 assert(BI.isConditional() && "Can only unswitch a conditional branch!");
573 LLVM_DEBUG(dbgs() << " Trying to unswitch branch: " << BI << "\n");
574
575 // The loop invariant values that we want to unswitch.
576 TinyPtrVector<Value *> Invariants;
577
578 // When true, we're fully unswitching the branch rather than just unswitching
579 // some input conditions to the branch.
580 bool FullUnswitch = false;
581
582 Value *Cond = skipTrivialSelect(Cond: BI.getCondition());
583 if (L.isLoopInvariant(V: Cond)) {
584 Invariants.push_back(NewVal: Cond);
585 FullUnswitch = true;
586 } else {
587 if (auto *CondInst = dyn_cast<Instruction>(Val: Cond))
588 Invariants = collectHomogenousInstGraphLoopInvariants(L, Root&: *CondInst, LI);
589 if (Invariants.empty()) {
590 LLVM_DEBUG(dbgs() << " Couldn't find invariant inputs!\n");
591 return false;
592 }
593 }
594
595 // Check that one of the branch's successors exits, and which one.
596 bool ExitDirection = true;
597 int LoopExitSuccIdx = 0;
598 auto *LoopExitBB = BI.getSuccessor(i: 0);
599 if (L.contains(BB: LoopExitBB)) {
600 ExitDirection = false;
601 LoopExitSuccIdx = 1;
602 LoopExitBB = BI.getSuccessor(i: 1);
603 if (L.contains(BB: LoopExitBB)) {
604 LLVM_DEBUG(dbgs() << " Branch doesn't exit the loop!\n");
605 return false;
606 }
607 }
608 auto *ContinueBB = BI.getSuccessor(i: 1 - LoopExitSuccIdx);
609 auto *ParentBB = BI.getParent();
610 if (!areLoopExitPHIsLoopInvariant(L, ExitingBB: *ParentBB, ExitBB: *LoopExitBB)) {
611 LLVM_DEBUG(dbgs() << " Loop exit PHI's aren't loop-invariant!\n");
612 return false;
613 }
614
615 // When unswitching only part of the branch's condition, we need the exit
616 // block to be reached directly from the partially unswitched input. This can
617 // be done when the exit block is along the true edge and the branch condition
618 // is a graph of `or` operations, or the exit block is along the false edge
619 // and the condition is a graph of `and` operations.
620 if (!FullUnswitch) {
621 if (ExitDirection ? !match(V: Cond, P: m_LogicalOr())
622 : !match(V: Cond, P: m_LogicalAnd())) {
623 LLVM_DEBUG(dbgs() << " Branch condition is in improper form for "
624 "non-full unswitch!\n");
625 return false;
626 }
627 }
628
629 LLVM_DEBUG({
630 dbgs() << " unswitching trivial invariant conditions for: " << BI
631 << "\n";
632 for (Value *Invariant : Invariants) {
633 dbgs() << " " << *Invariant << " == true";
634 if (Invariant != Invariants.back())
635 dbgs() << " ||";
636 dbgs() << "\n";
637 }
638 });
639
640 // If we have scalar evolutions, we need to invalidate them including this
641 // loop, the loop containing the exit block and the topmost parent loop
642 // exiting via LoopExitBB.
643 if (SE) {
644 if (const Loop *ExitL = getTopMostExitingLoop(ExitBB: LoopExitBB, LI))
645 SE->forgetLoop(L: ExitL);
646 else
647 // Forget the entire nest as this exits the entire nest.
648 SE->forgetTopmostLoop(L: &L);
649 SE->forgetBlockAndLoopDispositions();
650 }
651
652 if (MSSAU && VerifyMemorySSA)
653 MSSAU->getMemorySSA()->verifyMemorySSA();
654
655 // Split the preheader, so that we know that there is a safe place to insert
656 // the conditional branch. We will change the preheader to have a conditional
657 // branch on LoopCond.
658 BasicBlock *OldPH = L.getLoopPreheader();
659 BasicBlock *NewPH = SplitEdge(From: OldPH, To: L.getHeader(), DT: &DT, LI: &LI, MSSAU);
660
661 // Now that we have a place to insert the conditional branch, create a place
662 // to branch to: this is the exit block out of the loop that we are
663 // unswitching. We need to split this if there are other loop predecessors.
664 // Because the loop is in simplified form, *any* other predecessor is enough.
665 BasicBlock *UnswitchedBB;
666 if (FullUnswitch && LoopExitBB->getUniquePredecessor()) {
667 assert(LoopExitBB->getUniquePredecessor() == BI.getParent() &&
668 "A branch's parent isn't a predecessor!");
669 UnswitchedBB = LoopExitBB;
670 } else {
671 UnswitchedBB =
672 SplitBlock(Old: LoopExitBB, SplitPt: LoopExitBB->begin(), DT: &DT, LI: &LI, MSSAU, BBName: "");
673 }
674
675 if (MSSAU && VerifyMemorySSA)
676 MSSAU->getMemorySSA()->verifyMemorySSA();
677
678 // Actually move the invariant uses into the unswitched position. If possible,
679 // we do this by moving the instructions, but when doing partial unswitching
680 // we do it by building a new merge of the values in the unswitched position.
681 OldPH->getTerminator()->eraseFromParent();
682 if (FullUnswitch) {
683 // If fully unswitching, we can use the existing branch instruction.
684 // Splice it into the old PH to gate reaching the new preheader and re-point
685 // its successors.
686 BI.moveBefore(BB&: *OldPH, I: OldPH->end());
687 BI.setCondition(Cond);
688 if (MSSAU) {
689 // Temporarily clone the terminator, to make MSSA update cheaper by
690 // separating "insert edge" updates from "remove edge" ones.
691 BI.clone()->insertInto(ParentBB, It: ParentBB->end());
692 } else {
693 // Create a new unconditional branch that will continue the loop as a new
694 // terminator.
695 Instruction *NewBI = BranchInst::Create(IfTrue: ContinueBB, InsertBefore: ParentBB);
696 NewBI->setDebugLoc(BI.getDebugLoc());
697 }
698 BI.setSuccessor(idx: LoopExitSuccIdx, NewSucc: UnswitchedBB);
699 BI.setSuccessor(idx: 1 - LoopExitSuccIdx, NewSucc: NewPH);
700 } else {
701 // Only unswitching a subset of inputs to the condition, so we will need to
702 // build a new branch that merges the invariant inputs.
703 if (ExitDirection)
704 assert(match(skipTrivialSelect(BI.getCondition()), m_LogicalOr()) &&
705 "Must have an `or` of `i1`s or `select i1 X, true, Y`s for the "
706 "condition!");
707 else
708 assert(match(skipTrivialSelect(BI.getCondition()), m_LogicalAnd()) &&
709 "Must have an `and` of `i1`s or `select i1 X, Y, false`s for the"
710 " condition!");
711 buildPartialUnswitchConditionalBranch(
712 BB&: *OldPH, Invariants, Direction: ExitDirection, UnswitchedSucc&: *UnswitchedBB, NormalSucc&: *NewPH,
713 InsertFreeze: FreezeLoopUnswitchCond, I: OldPH->getTerminator(), AC: nullptr, DT, ComputeProfFrom: BI);
714 }
715
716 // Update the dominator tree with the added edge.
717 DT.insertEdge(From: OldPH, To: UnswitchedBB);
718
719 // After the dominator tree was updated with the added edge, update MemorySSA
720 // if available.
721 if (MSSAU) {
722 SmallVector<CFGUpdate, 1> Updates;
723 Updates.push_back(Elt: {cfg::UpdateKind::Insert, OldPH, UnswitchedBB});
724 MSSAU->applyInsertUpdates(Updates, DT);
725 }
726
727 // Finish updating dominator tree and memory ssa for full unswitch.
728 if (FullUnswitch) {
729 if (MSSAU) {
730 Instruction *Term = ParentBB->getTerminator();
731 // Remove the cloned branch instruction and create unconditional branch
732 // now.
733 Instruction *NewBI = BranchInst::Create(IfTrue: ContinueBB, InsertBefore: ParentBB);
734 NewBI->setDebugLoc(Term->getDebugLoc());
735 Term->eraseFromParent();
736 MSSAU->removeEdge(From: ParentBB, To: LoopExitBB);
737 }
738 DT.deleteEdge(From: ParentBB, To: LoopExitBB);
739 }
740
741 if (MSSAU && VerifyMemorySSA)
742 MSSAU->getMemorySSA()->verifyMemorySSA();
743
744 // Rewrite the relevant PHI nodes.
745 if (UnswitchedBB == LoopExitBB)
746 rewritePHINodesForUnswitchedExitBlock(UnswitchedBB&: *UnswitchedBB, OldExitingBB&: *ParentBB, OldPH&: *OldPH);
747 else
748 rewritePHINodesForExitAndUnswitchedBlocks(ExitBB&: *LoopExitBB, UnswitchedBB&: *UnswitchedBB,
749 OldExitingBB&: *ParentBB, OldPH&: *OldPH, FullUnswitch);
750
751 // The constant we can replace all of our invariants with inside the loop
752 // body. If any of the invariants have a value other than this the loop won't
753 // be entered.
754 ConstantInt *Replacement = ExitDirection
755 ? ConstantInt::getFalse(Context&: BI.getContext())
756 : ConstantInt::getTrue(Context&: BI.getContext());
757
758 // Since this is an i1 condition we can also trivially replace uses of it
759 // within the loop with a constant.
760 for (Value *Invariant : Invariants)
761 replaceLoopInvariantUses(L, Invariant, Replacement&: *Replacement);
762
763 // If this was full unswitching, we may have changed the nesting relationship
764 // for this loop so hoist it to its correct parent if needed.
765 if (FullUnswitch)
766 hoistLoopToNewParent(L, Preheader&: *NewPH, DT, LI, MSSAU, SE);
767
768 if (MSSAU && VerifyMemorySSA)
769 MSSAU->getMemorySSA()->verifyMemorySSA();
770
771 LLVM_DEBUG(dbgs() << " done: unswitching trivial branch...\n");
772 ++NumTrivial;
773 ++NumBranches;
774 return true;
775}
776
777/// Unswitch a trivial switch if the condition is loop invariant.
778///
779/// This routine should only be called when loop code leading to the switch has
780/// been validated as trivial (no side effects). This routine checks if the
781/// condition is invariant and that at least one of the successors is a loop
782/// exit. This allows us to unswitch without duplicating the loop, making it
783/// trivial.
784///
785/// If this routine fails to unswitch the switch it returns false.
786///
787/// If the switch can be unswitched, this routine splits the preheader and
788/// copies the switch above that split. If the default case is one of the
789/// exiting cases, it copies the non-exiting cases and points them at the new
790/// preheader. If the default case is not exiting, it copies the exiting cases
791/// and points the default at the preheader. It preserves loop simplified form
792/// (splitting the exit blocks as necessary). It simplifies the switch within
793/// the loop by removing now-dead cases. If the default case is one of those
794/// unswitched, it replaces its destination with a new basic block containing
795/// only unreachable. Such basic blocks, while technically loop exits, are not
796/// considered for unswitching so this is a stable transform and the same
797/// switch will not be revisited. If after unswitching there is only a single
798/// in-loop successor, the switch is further simplified to an unconditional
799/// branch. Still more cleanup can be done with some simplifycfg like pass.
800///
801/// If `SE` is not null, it will be updated based on the potential loop SCEVs
802/// invalidated by this.
803static bool unswitchTrivialSwitch(Loop &L, SwitchInst &SI, DominatorTree &DT,
804 LoopInfo &LI, ScalarEvolution *SE,
805 MemorySSAUpdater *MSSAU) {
806 LLVM_DEBUG(dbgs() << " Trying to unswitch switch: " << SI << "\n");
807 Value *LoopCond = SI.getCondition();
808
809 // If this isn't switching on an invariant condition, we can't unswitch it.
810 if (!L.isLoopInvariant(V: LoopCond))
811 return false;
812
813 auto *ParentBB = SI.getParent();
814
815 // The same check must be used both for the default and the exit cases. We
816 // should never leave edges from the switch instruction to a basic block that
817 // we are unswitching, hence the condition used to determine the default case
818 // needs to also be used to populate ExitCaseIndices, which is then used to
819 // remove cases from the switch.
820 auto IsTriviallyUnswitchableExitBlock = [&](BasicBlock &BBToCheck) {
821 // BBToCheck is not an exit block if it is inside loop L.
822 if (L.contains(BB: &BBToCheck))
823 return false;
824 // BBToCheck is not trivial to unswitch if its phis aren't loop invariant.
825 if (!areLoopExitPHIsLoopInvariant(L, ExitingBB: *ParentBB, ExitBB: BBToCheck))
826 return false;
827 // We do not unswitch a block that only has an unreachable statement, as
828 // it's possible this is a previously unswitched block. Only unswitch if
829 // either the terminator is not unreachable, or, if it is, it's not the only
830 // instruction in the block.
831 auto *TI = BBToCheck.getTerminator();
832 bool isUnreachable = isa<UnreachableInst>(Val: TI);
833 return !isUnreachable || &*BBToCheck.getFirstNonPHIOrDbg() != TI;
834 };
835
836 SmallVector<int, 4> ExitCaseIndices;
837 for (auto Case : SI.cases())
838 if (IsTriviallyUnswitchableExitBlock(*Case.getCaseSuccessor()))
839 ExitCaseIndices.push_back(Elt: Case.getCaseIndex());
840 BasicBlock *DefaultExitBB = nullptr;
841 SwitchInstProfUpdateWrapper::CaseWeightOpt DefaultCaseWeight =
842 SwitchInstProfUpdateWrapper::getSuccessorWeight(SI, idx: 0);
843 if (IsTriviallyUnswitchableExitBlock(*SI.getDefaultDest())) {
844 DefaultExitBB = SI.getDefaultDest();
845 } else if (ExitCaseIndices.empty())
846 return false;
847
848 LLVM_DEBUG(dbgs() << " unswitching trivial switch...\n");
849
850 if (MSSAU && VerifyMemorySSA)
851 MSSAU->getMemorySSA()->verifyMemorySSA();
852
853 // We may need to invalidate SCEVs for the outermost loop reached by any of
854 // the exits.
855 Loop *OuterL = &L;
856
857 if (DefaultExitBB) {
858 // Check the loop containing this exit.
859 Loop *ExitL = getTopMostExitingLoop(ExitBB: DefaultExitBB, LI);
860 if (!ExitL || ExitL->contains(L: OuterL))
861 OuterL = ExitL;
862 }
863 for (unsigned Index : ExitCaseIndices) {
864 auto CaseI = SI.case_begin() + Index;
865 // Compute the outer loop from this exit.
866 Loop *ExitL = getTopMostExitingLoop(ExitBB: CaseI->getCaseSuccessor(), LI);
867 if (!ExitL || ExitL->contains(L: OuterL))
868 OuterL = ExitL;
869 }
870
871 if (SE) {
872 if (OuterL)
873 SE->forgetLoop(L: OuterL);
874 else
875 SE->forgetTopmostLoop(L: &L);
876 }
877
878 if (DefaultExitBB) {
879 // Clear out the default destination temporarily to allow accurate
880 // predecessor lists to be examined below.
881 SI.setDefaultDest(nullptr);
882 }
883
884 // Store the exit cases into a separate data structure and remove them from
885 // the switch.
886 SmallVector<std::tuple<ConstantInt *, BasicBlock *,
887 SwitchInstProfUpdateWrapper::CaseWeightOpt>,
888 4> ExitCases;
889 ExitCases.reserve(N: ExitCaseIndices.size());
890 SwitchInstProfUpdateWrapper SIW(SI);
891 // We walk the case indices backwards so that we remove the last case first
892 // and don't disrupt the earlier indices.
893 for (unsigned Index : reverse(C&: ExitCaseIndices)) {
894 auto CaseI = SI.case_begin() + Index;
895 // Save the value of this case.
896 auto W = SIW.getSuccessorWeight(idx: CaseI->getSuccessorIndex());
897 ExitCases.emplace_back(Args: CaseI->getCaseValue(), Args: CaseI->getCaseSuccessor(), Args&: W);
898 // Delete the unswitched cases.
899 SIW.removeCase(I: CaseI);
900 }
901
902 // Check if after this all of the remaining cases point at the same
903 // successor.
904 BasicBlock *CommonSuccBB = nullptr;
905 if (SI.getNumCases() > 0 &&
906 all_of(Range: drop_begin(RangeOrContainer: SI.cases()), P: [&SI](const SwitchInst::CaseHandle &Case) {
907 return Case.getCaseSuccessor() == SI.case_begin()->getCaseSuccessor();
908 }))
909 CommonSuccBB = SI.case_begin()->getCaseSuccessor();
910 if (!DefaultExitBB) {
911 // If we're not unswitching the default, we need it to match any cases to
912 // have a common successor or if we have no cases it is the common
913 // successor.
914 if (SI.getNumCases() == 0)
915 CommonSuccBB = SI.getDefaultDest();
916 else if (SI.getDefaultDest() != CommonSuccBB)
917 CommonSuccBB = nullptr;
918 }
919
920 // Split the preheader, so that we know that there is a safe place to insert
921 // the switch.
922 BasicBlock *OldPH = L.getLoopPreheader();
923 BasicBlock *NewPH = SplitEdge(From: OldPH, To: L.getHeader(), DT: &DT, LI: &LI, MSSAU);
924 OldPH->getTerminator()->eraseFromParent();
925
926 // Now add the unswitched switch. This new switch instruction inherits the
927 // debug location of the old switch, because it semantically replace the old
928 // one.
929 auto *NewSI = SwitchInst::Create(Value: LoopCond, Default: NewPH, NumCases: ExitCases.size(), InsertBefore: OldPH);
930 NewSI->setDebugLoc(SIW->getDebugLoc());
931 SwitchInstProfUpdateWrapper NewSIW(*NewSI);
932
933 // Rewrite the IR for the unswitched basic blocks. This requires two steps.
934 // First, we split any exit blocks with remaining in-loop predecessors. Then
935 // we update the PHIs in one of two ways depending on if there was a split.
936 // We walk in reverse so that we split in the same order as the cases
937 // appeared. This is purely for convenience of reading the resulting IR, but
938 // it doesn't cost anything really.
939 SmallPtrSet<BasicBlock *, 2> UnswitchedExitBBs;
940 SmallDenseMap<BasicBlock *, BasicBlock *, 2> SplitExitBBMap;
941 // Handle the default exit if necessary.
942 // FIXME: It'd be great if we could merge this with the loop below but LLVM's
943 // ranges aren't quite powerful enough yet.
944 if (DefaultExitBB) {
945 if (pred_empty(BB: DefaultExitBB)) {
946 UnswitchedExitBBs.insert(Ptr: DefaultExitBB);
947 rewritePHINodesForUnswitchedExitBlock(UnswitchedBB&: *DefaultExitBB, OldExitingBB&: *ParentBB, OldPH&: *OldPH);
948 } else {
949 auto *SplitBB =
950 SplitBlock(Old: DefaultExitBB, SplitPt: DefaultExitBB->begin(), DT: &DT, LI: &LI, MSSAU);
951 rewritePHINodesForExitAndUnswitchedBlocks(ExitBB&: *DefaultExitBB, UnswitchedBB&: *SplitBB,
952 OldExitingBB&: *ParentBB, OldPH&: *OldPH,
953 /*FullUnswitch*/ true);
954 DefaultExitBB = SplitExitBBMap[DefaultExitBB] = SplitBB;
955 }
956 }
957 // Note that we must use a reference in the for loop so that we update the
958 // container.
959 for (auto &ExitCase : reverse(C&: ExitCases)) {
960 // Grab a reference to the exit block in the pair so that we can update it.
961 BasicBlock *ExitBB = std::get<1>(t&: ExitCase);
962
963 // If this case is the last edge into the exit block, we can simply reuse it
964 // as it will no longer be a loop exit. No mapping necessary.
965 if (pred_empty(BB: ExitBB)) {
966 // Only rewrite once.
967 if (UnswitchedExitBBs.insert(Ptr: ExitBB).second)
968 rewritePHINodesForUnswitchedExitBlock(UnswitchedBB&: *ExitBB, OldExitingBB&: *ParentBB, OldPH&: *OldPH);
969 continue;
970 }
971
972 // Otherwise we need to split the exit block so that we retain an exit
973 // block from the loop and a target for the unswitched condition.
974 BasicBlock *&SplitExitBB = SplitExitBBMap[ExitBB];
975 if (!SplitExitBB) {
976 // If this is the first time we see this, do the split and remember it.
977 SplitExitBB = SplitBlock(Old: ExitBB, SplitPt: ExitBB->begin(), DT: &DT, LI: &LI, MSSAU);
978 rewritePHINodesForExitAndUnswitchedBlocks(ExitBB&: *ExitBB, UnswitchedBB&: *SplitExitBB,
979 OldExitingBB&: *ParentBB, OldPH&: *OldPH,
980 /*FullUnswitch*/ true);
981 }
982 // Update the case pair to point to the split block.
983 std::get<1>(t&: ExitCase) = SplitExitBB;
984 }
985
986 // Now add the unswitched cases. We do this in reverse order as we built them
987 // in reverse order.
988 for (auto &ExitCase : reverse(C&: ExitCases)) {
989 ConstantInt *CaseVal = std::get<0>(t&: ExitCase);
990 BasicBlock *UnswitchedBB = std::get<1>(t&: ExitCase);
991
992 NewSIW.addCase(OnVal: CaseVal, Dest: UnswitchedBB, W: std::get<2>(t&: ExitCase));
993 }
994
995 // If the default was unswitched, re-point it and add explicit cases for
996 // entering the loop.
997 if (DefaultExitBB) {
998 NewSIW->setDefaultDest(DefaultExitBB);
999 NewSIW.setSuccessorWeight(idx: 0, W: DefaultCaseWeight);
1000
1001 // We removed all the exit cases, so we just copy the cases to the
1002 // unswitched switch.
1003 for (const auto &Case : SI.cases())
1004 NewSIW.addCase(OnVal: Case.getCaseValue(), Dest: NewPH,
1005 W: SIW.getSuccessorWeight(idx: Case.getSuccessorIndex()));
1006 } else if (DefaultCaseWeight) {
1007 // We have to set branch weight of the default case.
1008 uint64_t SW = *DefaultCaseWeight;
1009 for (const auto &Case : SI.cases()) {
1010 auto W = SIW.getSuccessorWeight(idx: Case.getSuccessorIndex());
1011 assert(W &&
1012 "case weight must be defined as default case weight is defined");
1013 SW += *W;
1014 }
1015 NewSIW.setSuccessorWeight(idx: 0, W: SW);
1016 }
1017
1018 // If we ended up with a common successor for every path through the switch
1019 // after unswitching, rewrite it to an unconditional branch to make it easy
1020 // to recognize. Otherwise we potentially have to recognize the default case
1021 // pointing at unreachable and other complexity.
1022 if (CommonSuccBB) {
1023 BasicBlock *BB = SI.getParent();
1024 // We may have had multiple edges to this common successor block, so remove
1025 // them as predecessors. We skip the first one, either the default or the
1026 // actual first case.
1027 bool SkippedFirst = DefaultExitBB == nullptr;
1028 for (auto Case : SI.cases()) {
1029 assert(Case.getCaseSuccessor() == CommonSuccBB &&
1030 "Non-common successor!");
1031 (void)Case;
1032 if (!SkippedFirst) {
1033 SkippedFirst = true;
1034 continue;
1035 }
1036 CommonSuccBB->removePredecessor(Pred: BB,
1037 /*KeepOneInputPHIs*/ true);
1038 }
1039 // Now nuke the switch and replace it with a direct branch.
1040 Instruction *NewBI = BranchInst::Create(IfTrue: CommonSuccBB, InsertBefore: BB);
1041 NewBI->setDebugLoc(SIW->getDebugLoc());
1042 SIW.eraseFromParent();
1043 } else if (DefaultExitBB) {
1044 assert(SI.getNumCases() > 0 &&
1045 "If we had no cases we'd have a common successor!");
1046 // Move the last case to the default successor. This is valid as if the
1047 // default got unswitched it cannot be reached. This has the advantage of
1048 // being simple and keeping the number of edges from this switch to
1049 // successors the same, and avoiding any PHI update complexity.
1050 auto LastCaseI = std::prev(x: SI.case_end());
1051
1052 SI.setDefaultDest(LastCaseI->getCaseSuccessor());
1053 SIW.setSuccessorWeight(
1054 idx: 0, W: SIW.getSuccessorWeight(idx: LastCaseI->getSuccessorIndex()));
1055 SIW.removeCase(I: LastCaseI);
1056 }
1057
1058 // Walk the unswitched exit blocks and the unswitched split blocks and update
1059 // the dominator tree based on the CFG edits. While we are walking unordered
1060 // containers here, the API for applyUpdates takes an unordered list of
1061 // updates and requires them to not contain duplicates.
1062 SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
1063 for (auto *UnswitchedExitBB : UnswitchedExitBBs) {
1064 DTUpdates.push_back(Elt: {DT.Delete, ParentBB, UnswitchedExitBB});
1065 DTUpdates.push_back(Elt: {DT.Insert, OldPH, UnswitchedExitBB});
1066 }
1067 for (auto SplitUnswitchedPair : SplitExitBBMap) {
1068 DTUpdates.push_back(Elt: {DT.Delete, ParentBB, SplitUnswitchedPair.first});
1069 DTUpdates.push_back(Elt: {DT.Insert, OldPH, SplitUnswitchedPair.second});
1070 }
1071
1072 if (MSSAU) {
1073 MSSAU->applyUpdates(Updates: DTUpdates, DT, /*UpdateDT=*/UpdateDTFirst: true);
1074 if (VerifyMemorySSA)
1075 MSSAU->getMemorySSA()->verifyMemorySSA();
1076 } else {
1077 DT.applyUpdates(Updates: DTUpdates);
1078 }
1079
1080 assert(DT.verify(DominatorTree::VerificationLevel::Fast));
1081
1082 // We may have changed the nesting relationship for this loop so hoist it to
1083 // its correct parent if needed.
1084 hoistLoopToNewParent(L, Preheader&: *NewPH, DT, LI, MSSAU, SE);
1085
1086 if (MSSAU && VerifyMemorySSA)
1087 MSSAU->getMemorySSA()->verifyMemorySSA();
1088
1089 ++NumTrivial;
1090 ++NumSwitches;
1091 LLVM_DEBUG(dbgs() << " done: unswitching trivial switch...\n");
1092 return true;
1093}
1094
1095/// This routine scans the loop to find a branch or switch which occurs before
1096/// any side effects occur. These can potentially be unswitched without
1097/// duplicating the loop. If a branch or switch is successfully unswitched the
1098/// scanning continues to see if subsequent branches or switches have become
1099/// trivial. Once all trivial candidates have been unswitched, this routine
1100/// returns.
1101///
1102/// The return value indicates whether anything was unswitched (and therefore
1103/// changed).
1104///
1105/// If `SE` is not null, it will be updated based on the potential loop SCEVs
1106/// invalidated by this.
1107static bool unswitchAllTrivialConditions(Loop &L, DominatorTree &DT,
1108 LoopInfo &LI, ScalarEvolution *SE,
1109 MemorySSAUpdater *MSSAU) {
1110 bool Changed = false;
1111
1112 // If loop header has only one reachable successor we should keep looking for
1113 // trivial condition candidates in the successor as well. An alternative is
1114 // to constant fold conditions and merge successors into loop header (then we
1115 // only need to check header's terminator). The reason for not doing this in
1116 // LoopUnswitch pass is that it could potentially break LoopPassManager's
1117 // invariants. Folding dead branches could either eliminate the current loop
1118 // or make other loops unreachable. LCSSA form might also not be preserved
1119 // after deleting branches. The following code keeps traversing loop header's
1120 // successors until it finds the trivial condition candidate (condition that
1121 // is not a constant). Since unswitching generates branches with constant
1122 // conditions, this scenario could be very common in practice.
1123 BasicBlock *CurrentBB = L.getHeader();
1124 SmallPtrSet<BasicBlock *, 8> Visited;
1125 Visited.insert(Ptr: CurrentBB);
1126 do {
1127 // Check if there are any side-effecting instructions (e.g. stores, calls,
1128 // volatile loads) in the part of the loop that the code *would* execute
1129 // without unswitching.
1130 if (MSSAU) // Possible early exit with MSSA
1131 if (auto *Defs = MSSAU->getMemorySSA()->getBlockDefs(BB: CurrentBB))
1132 if (!isa<MemoryPhi>(Val: *Defs->begin()) || (++Defs->begin() != Defs->end()))
1133 return Changed;
1134 if (llvm::any_of(Range&: *CurrentBB,
1135 P: [](Instruction &I) { return I.mayHaveSideEffects(); }))
1136 return Changed;
1137
1138 Instruction *CurrentTerm = CurrentBB->getTerminator();
1139
1140 if (auto *SI = dyn_cast<SwitchInst>(Val: CurrentTerm)) {
1141 // Don't bother trying to unswitch past a switch with a constant
1142 // condition. This should be removed prior to running this pass by
1143 // simplifycfg.
1144 if (isa<Constant>(Val: SI->getCondition()))
1145 return Changed;
1146
1147 if (!unswitchTrivialSwitch(L, SI&: *SI, DT, LI, SE, MSSAU))
1148 // Couldn't unswitch this one so we're done.
1149 return Changed;
1150
1151 // Mark that we managed to unswitch something.
1152 Changed = true;
1153
1154 // If unswitching turned the terminator into an unconditional branch then
1155 // we can continue. The unswitching logic specifically works to fold any
1156 // cases it can into an unconditional branch to make it easier to
1157 // recognize here.
1158 auto *BI = dyn_cast<BranchInst>(Val: CurrentBB->getTerminator());
1159 if (!BI || BI->isConditional())
1160 return Changed;
1161
1162 CurrentBB = BI->getSuccessor(i: 0);
1163 continue;
1164 }
1165
1166 auto *BI = dyn_cast<BranchInst>(Val: CurrentTerm);
1167 if (!BI)
1168 // We do not understand other terminator instructions.
1169 return Changed;
1170
1171 // Don't bother trying to unswitch past an unconditional branch or a branch
1172 // with a constant value. These should be removed by simplifycfg prior to
1173 // running this pass.
1174 if (!BI->isConditional() ||
1175 isa<Constant>(Val: skipTrivialSelect(Cond: BI->getCondition())))
1176 return Changed;
1177
1178 // Found a trivial condition candidate: non-foldable conditional branch. If
1179 // we fail to unswitch this, we can't do anything else that is trivial.
1180 if (!unswitchTrivialBranch(L, BI&: *BI, DT, LI, SE, MSSAU))
1181 return Changed;
1182
1183 // Mark that we managed to unswitch something.
1184 Changed = true;
1185
1186 // If we only unswitched some of the conditions feeding the branch, we won't
1187 // have collapsed it to a single successor.
1188 BI = cast<BranchInst>(Val: CurrentBB->getTerminator());
1189 if (BI->isConditional())
1190 return Changed;
1191
1192 // Follow the newly unconditional branch into its successor.
1193 CurrentBB = BI->getSuccessor(i: 0);
1194
1195 // When continuing, if we exit the loop or reach a previous visited block,
1196 // then we can not reach any trivial condition candidates (unfoldable
1197 // branch instructions or switch instructions) and no unswitch can happen.
1198 } while (L.contains(BB: CurrentBB) && Visited.insert(Ptr: CurrentBB).second);
1199
1200 return Changed;
1201}
1202
1203/// Build the cloned blocks for an unswitched copy of the given loop.
1204///
1205/// The cloned blocks are inserted before the loop preheader (`LoopPH`) and
1206/// after the split block (`SplitBB`) that will be used to select between the
1207/// cloned and original loop.
1208///
1209/// This routine handles cloning all of the necessary loop blocks and exit
1210/// blocks including rewriting their instructions and the relevant PHI nodes.
1211/// Any loop blocks or exit blocks which are dominated by a different successor
1212/// than the one for this clone of the loop blocks can be trivially skipped. We
1213/// use the `DominatingSucc` map to determine whether a block satisfies that
1214/// property with a simple map lookup.
1215///
1216/// It also correctly creates the unconditional branch in the cloned
1217/// unswitched parent block to only point at the unswitched successor.
1218///
1219/// This does not handle most of the necessary updates to `LoopInfo`. Only exit
1220/// block splitting is correctly reflected in `LoopInfo`, essentially all of
1221/// the cloned blocks (and their loops) are left without full `LoopInfo`
1222/// updates. This also doesn't fully update `DominatorTree`. It adds the cloned
1223/// blocks to them but doesn't create the cloned `DominatorTree` structure and
1224/// instead the caller must recompute an accurate DT. It *does* correctly
1225/// update the `AssumptionCache` provided in `AC`.
1226static BasicBlock *buildClonedLoopBlocks(
1227 Loop &L, BasicBlock *LoopPH, BasicBlock *SplitBB,
1228 ArrayRef<BasicBlock *> ExitBlocks, BasicBlock *ParentBB,
1229 BasicBlock *UnswitchedSuccBB, BasicBlock *ContinueSuccBB,
1230 const SmallDenseMap<BasicBlock *, BasicBlock *, 16> &DominatingSucc,
1231 ValueToValueMapTy &VMap,
1232 SmallVectorImpl<DominatorTree::UpdateType> &DTUpdates, AssumptionCache &AC,
1233 DominatorTree &DT, LoopInfo &LI, MemorySSAUpdater *MSSAU,
1234 ScalarEvolution *SE) {
1235 SmallVector<BasicBlock *, 4> NewBlocks;
1236 NewBlocks.reserve(N: L.getNumBlocks() + ExitBlocks.size());
1237
1238 // We will need to clone a bunch of blocks, wrap up the clone operation in
1239 // a helper.
1240 auto CloneBlock = [&](BasicBlock *OldBB) {
1241 // Clone the basic block and insert it before the new preheader.
1242 BasicBlock *NewBB = CloneBasicBlock(BB: OldBB, VMap, NameSuffix: ".us", F: OldBB->getParent());
1243 NewBB->moveBefore(MovePos: LoopPH);
1244
1245 // Record this block and the mapping.
1246 NewBlocks.push_back(Elt: NewBB);
1247 VMap[OldBB] = NewBB;
1248
1249 return NewBB;
1250 };
1251
1252 // We skip cloning blocks when they have a dominating succ that is not the
1253 // succ we are cloning for.
1254 auto SkipBlock = [&](BasicBlock *BB) {
1255 auto It = DominatingSucc.find(Val: BB);
1256 return It != DominatingSucc.end() && It->second != UnswitchedSuccBB;
1257 };
1258
1259 // First, clone the preheader.
1260 auto *ClonedPH = CloneBlock(LoopPH);
1261
1262 // Then clone all the loop blocks, skipping the ones that aren't necessary.
1263 for (auto *LoopBB : L.blocks())
1264 if (!SkipBlock(LoopBB))
1265 CloneBlock(LoopBB);
1266
1267 // Split all the loop exit edges so that when we clone the exit blocks, if
1268 // any of the exit blocks are *also* a preheader for some other loop, we
1269 // don't create multiple predecessors entering the loop header.
1270 for (auto *ExitBB : ExitBlocks) {
1271 if (SkipBlock(ExitBB))
1272 continue;
1273
1274 // When we are going to clone an exit, we don't need to clone all the
1275 // instructions in the exit block and we want to ensure we have an easy
1276 // place to merge the CFG, so split the exit first. This is always safe to
1277 // do because there cannot be any non-loop predecessors of a loop exit in
1278 // loop simplified form.
1279 auto *MergeBB = SplitBlock(Old: ExitBB, SplitPt: ExitBB->begin(), DT: &DT, LI: &LI, MSSAU);
1280
1281 // Rearrange the names to make it easier to write test cases by having the
1282 // exit block carry the suffix rather than the merge block carrying the
1283 // suffix.
1284 MergeBB->takeName(V: ExitBB);
1285 ExitBB->setName(Twine(MergeBB->getName()) + ".split");
1286
1287 // Now clone the original exit block.
1288 auto *ClonedExitBB = CloneBlock(ExitBB);
1289 assert(ClonedExitBB->getTerminator()->getNumSuccessors() == 1 &&
1290 "Exit block should have been split to have one successor!");
1291 assert(ClonedExitBB->getTerminator()->getSuccessor(0) == MergeBB &&
1292 "Cloned exit block has the wrong successor!");
1293
1294 // Remap any cloned instructions and create a merge phi node for them.
1295 for (auto ZippedInsts : llvm::zip_first(
1296 t: llvm::make_range(x: ExitBB->begin(), y: std::prev(x: ExitBB->end())),
1297 u: llvm::make_range(x: ClonedExitBB->begin(),
1298 y: std::prev(x: ClonedExitBB->end())))) {
1299 Instruction &I = std::get<0>(t&: ZippedInsts);
1300 Instruction &ClonedI = std::get<1>(t&: ZippedInsts);
1301
1302 // The only instructions in the exit block should be PHI nodes and
1303 // potentially a landing pad.
1304 assert(
1305 (isa<PHINode>(I) || isa<LandingPadInst>(I) || isa<CatchPadInst>(I)) &&
1306 "Bad instruction in exit block!");
1307 // We should have a value map between the instruction and its clone.
1308 assert(VMap.lookup(&I) == &ClonedI && "Mismatch in the value map!");
1309
1310 // Forget SCEVs based on exit phis in case SCEV looked through the phi.
1311 if (SE)
1312 if (auto *PN = dyn_cast<PHINode>(Val: &I))
1313 SE->forgetLcssaPhiWithNewPredecessor(L: &L, V: PN);
1314
1315 BasicBlock::iterator InsertPt = MergeBB->getFirstInsertionPt();
1316
1317 auto *MergePN =
1318 PHINode::Create(Ty: I.getType(), /*NumReservedValues*/ 2, NameStr: ".us-phi");
1319 MergePN->insertBefore(InsertPos: InsertPt);
1320 MergePN->setDebugLoc(InsertPt->getDebugLoc());
1321 I.replaceAllUsesWith(V: MergePN);
1322 MergePN->addIncoming(V: &I, BB: ExitBB);
1323 MergePN->addIncoming(V: &ClonedI, BB: ClonedExitBB);
1324 }
1325 }
1326
1327 // Rewrite the instructions in the cloned blocks to refer to the instructions
1328 // in the cloned blocks. We have to do this as a second pass so that we have
1329 // everything available. Also, we have inserted new instructions which may
1330 // include assume intrinsics, so we update the assumption cache while
1331 // processing this.
1332 Module *M = ClonedPH->getParent()->getParent();
1333 for (auto *ClonedBB : NewBlocks)
1334 for (Instruction &I : *ClonedBB) {
1335 RemapDbgRecordRange(M, Range: I.getDbgRecordRange(), VM&: VMap,
1336 Flags: RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
1337 RemapInstruction(I: &I, VM&: VMap,
1338 Flags: RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
1339 if (auto *II = dyn_cast<AssumeInst>(Val: &I))
1340 AC.registerAssumption(CI: II);
1341 }
1342
1343 // Update any PHI nodes in the cloned successors of the skipped blocks to not
1344 // have spurious incoming values.
1345 for (auto *LoopBB : L.blocks())
1346 if (SkipBlock(LoopBB))
1347 for (auto *SuccBB : successors(BB: LoopBB))
1348 if (auto *ClonedSuccBB = cast_or_null<BasicBlock>(Val: VMap.lookup(Val: SuccBB)))
1349 for (PHINode &PN : ClonedSuccBB->phis())
1350 PN.removeIncomingValue(BB: LoopBB, /*DeletePHIIfEmpty*/ false);
1351
1352 // Remove the cloned parent as a predecessor of any successor we ended up
1353 // cloning other than the unswitched one.
1354 auto *ClonedParentBB = cast<BasicBlock>(Val: VMap.lookup(Val: ParentBB));
1355 for (auto *SuccBB : successors(BB: ParentBB)) {
1356 if (SuccBB == UnswitchedSuccBB)
1357 continue;
1358
1359 auto *ClonedSuccBB = cast_or_null<BasicBlock>(Val: VMap.lookup(Val: SuccBB));
1360 if (!ClonedSuccBB)
1361 continue;
1362
1363 ClonedSuccBB->removePredecessor(Pred: ClonedParentBB,
1364 /*KeepOneInputPHIs*/ true);
1365 }
1366
1367 // Replace the cloned branch with an unconditional branch to the cloned
1368 // unswitched successor.
1369 auto *ClonedSuccBB = cast<BasicBlock>(Val: VMap.lookup(Val: UnswitchedSuccBB));
1370 Instruction *ClonedTerminator = ClonedParentBB->getTerminator();
1371 // Trivial Simplification. If Terminator is a conditional branch and
1372 // condition becomes dead - erase it.
1373 Value *ClonedConditionToErase = nullptr;
1374 if (auto *BI = dyn_cast<BranchInst>(Val: ClonedTerminator))
1375 ClonedConditionToErase = BI->getCondition();
1376 else if (auto *SI = dyn_cast<SwitchInst>(Val: ClonedTerminator))
1377 ClonedConditionToErase = SI->getCondition();
1378
1379 Instruction *BI = BranchInst::Create(IfTrue: ClonedSuccBB, InsertBefore: ClonedParentBB);
1380 BI->setDebugLoc(ClonedTerminator->getDebugLoc());
1381 ClonedTerminator->eraseFromParent();
1382
1383 if (ClonedConditionToErase)
1384 RecursivelyDeleteTriviallyDeadInstructions(V: ClonedConditionToErase, TLI: nullptr,
1385 MSSAU);
1386
1387 // If there are duplicate entries in the PHI nodes because of multiple edges
1388 // to the unswitched successor, we need to nuke all but one as we replaced it
1389 // with a direct branch.
1390 for (PHINode &PN : ClonedSuccBB->phis()) {
1391 bool Found = false;
1392 // Loop over the incoming operands backwards so we can easily delete as we
1393 // go without invalidating the index.
1394 for (int i = PN.getNumOperands() - 1; i >= 0; --i) {
1395 if (PN.getIncomingBlock(i) != ClonedParentBB)
1396 continue;
1397 if (!Found) {
1398 Found = true;
1399 continue;
1400 }
1401 PN.removeIncomingValue(Idx: i, /*DeletePHIIfEmpty*/ false);
1402 }
1403 }
1404
1405 // Record the domtree updates for the new blocks.
1406 SmallPtrSet<BasicBlock *, 4> SuccSet;
1407 for (auto *ClonedBB : NewBlocks) {
1408 for (auto *SuccBB : successors(BB: ClonedBB))
1409 if (SuccSet.insert(Ptr: SuccBB).second)
1410 DTUpdates.push_back(Elt: {DominatorTree::Insert, ClonedBB, SuccBB});
1411 SuccSet.clear();
1412 }
1413
1414 return ClonedPH;
1415}
1416
1417/// Recursively clone the specified loop and all of its children.
1418///
1419/// The target parent loop for the clone should be provided, or can be null if
1420/// the clone is a top-level loop. While cloning, all the blocks are mapped
1421/// with the provided value map. The entire original loop must be present in
1422/// the value map. The cloned loop is returned.
1423static Loop *cloneLoopNest(Loop &OrigRootL, Loop *RootParentL,
1424 const ValueToValueMapTy &VMap, LoopInfo &LI) {
1425 auto AddClonedBlocksToLoop = [&](Loop &OrigL, Loop &ClonedL) {
1426 assert(ClonedL.getBlocks().empty() && "Must start with an empty loop!");
1427 ClonedL.reserveBlocks(size: OrigL.getNumBlocks());
1428 for (auto *BB : OrigL.blocks()) {
1429 auto *ClonedBB = cast<BasicBlock>(Val: VMap.lookup(Val: BB));
1430 ClonedL.addBlockEntry(BB: ClonedBB);
1431 if (LI.getLoopFor(BB) == &OrigL)
1432 LI.changeLoopFor(BB: ClonedBB, L: &ClonedL);
1433 }
1434 };
1435
1436 // We specially handle the first loop because it may get cloned into
1437 // a different parent and because we most commonly are cloning leaf loops.
1438 Loop *ClonedRootL = LI.AllocateLoop();
1439 if (RootParentL)
1440 RootParentL->addChildLoop(NewChild: ClonedRootL);
1441 else
1442 LI.addTopLevelLoop(New: ClonedRootL);
1443 AddClonedBlocksToLoop(OrigRootL, *ClonedRootL);
1444
1445 if (OrigRootL.isInnermost())
1446 return ClonedRootL;
1447
1448 // If we have a nest, we can quickly clone the entire loop nest using an
1449 // iterative approach because it is a tree. We keep the cloned parent in the
1450 // data structure to avoid repeatedly querying through a map to find it.
1451 SmallVector<std::pair<Loop *, Loop *>, 16> LoopsToClone;
1452 // Build up the loops to clone in reverse order as we'll clone them from the
1453 // back.
1454 for (Loop *ChildL : llvm::reverse(C&: OrigRootL))
1455 LoopsToClone.push_back(Elt: {ClonedRootL, ChildL});
1456 do {
1457 Loop *ClonedParentL, *L;
1458 std::tie(args&: ClonedParentL, args&: L) = LoopsToClone.pop_back_val();
1459 Loop *ClonedL = LI.AllocateLoop();
1460 ClonedParentL->addChildLoop(NewChild: ClonedL);
1461 AddClonedBlocksToLoop(*L, *ClonedL);
1462 for (Loop *ChildL : llvm::reverse(C&: *L))
1463 LoopsToClone.push_back(Elt: {ClonedL, ChildL});
1464 } while (!LoopsToClone.empty());
1465
1466 return ClonedRootL;
1467}
1468
1469/// Build the cloned loops of an original loop from unswitching.
1470///
1471/// Because unswitching simplifies the CFG of the loop, this isn't a trivial
1472/// operation. We need to re-verify that there even is a loop (as the backedge
1473/// may not have been cloned), and even if there are remaining backedges the
1474/// backedge set may be different. However, we know that each child loop is
1475/// undisturbed, we only need to find where to place each child loop within
1476/// either any parent loop or within a cloned version of the original loop.
1477///
1478/// Because child loops may end up cloned outside of any cloned version of the
1479/// original loop, multiple cloned sibling loops may be created. All of them
1480/// are returned so that the newly introduced loop nest roots can be
1481/// identified.
1482static void buildClonedLoops(Loop &OrigL, ArrayRef<BasicBlock *> ExitBlocks,
1483 const ValueToValueMapTy &VMap, LoopInfo &LI,
1484 SmallVectorImpl<Loop *> &NonChildClonedLoops) {
1485 Loop *ClonedL = nullptr;
1486
1487 auto *OrigPH = OrigL.getLoopPreheader();
1488 auto *OrigHeader = OrigL.getHeader();
1489
1490 auto *ClonedPH = cast<BasicBlock>(Val: VMap.lookup(Val: OrigPH));
1491 auto *ClonedHeader = cast<BasicBlock>(Val: VMap.lookup(Val: OrigHeader));
1492
1493 // We need to know the loops of the cloned exit blocks to even compute the
1494 // accurate parent loop. If we only clone exits to some parent of the
1495 // original parent, we want to clone into that outer loop. We also keep track
1496 // of the loops that our cloned exit blocks participate in.
1497 Loop *ParentL = nullptr;
1498 SmallVector<BasicBlock *, 4> ClonedExitsInLoops;
1499 SmallDenseMap<BasicBlock *, Loop *, 16> ExitLoopMap;
1500 ClonedExitsInLoops.reserve(N: ExitBlocks.size());
1501 for (auto *ExitBB : ExitBlocks)
1502 if (auto *ClonedExitBB = cast_or_null<BasicBlock>(Val: VMap.lookup(Val: ExitBB)))
1503 if (Loop *ExitL = LI.getLoopFor(BB: ExitBB)) {
1504 ExitLoopMap[ClonedExitBB] = ExitL;
1505 ClonedExitsInLoops.push_back(Elt: ClonedExitBB);
1506 if (!ParentL || (ParentL != ExitL && ParentL->contains(L: ExitL)))
1507 ParentL = ExitL;
1508 }
1509 assert((!ParentL || ParentL == OrigL.getParentLoop() ||
1510 ParentL->contains(OrigL.getParentLoop())) &&
1511 "The computed parent loop should always contain (or be) the parent of "
1512 "the original loop.");
1513
1514 // We build the set of blocks dominated by the cloned header from the set of
1515 // cloned blocks out of the original loop. While not all of these will
1516 // necessarily be in the cloned loop, it is enough to establish that they
1517 // aren't in unreachable cycles, etc.
1518 SmallSetVector<BasicBlock *, 16> ClonedLoopBlocks;
1519 for (auto *BB : OrigL.blocks())
1520 if (auto *ClonedBB = cast_or_null<BasicBlock>(Val: VMap.lookup(Val: BB)))
1521 ClonedLoopBlocks.insert(X: ClonedBB);
1522
1523 // Rebuild the set of blocks that will end up in the cloned loop. We may have
1524 // skipped cloning some region of this loop which can in turn skip some of
1525 // the backedges so we have to rebuild the blocks in the loop based on the
1526 // backedges that remain after cloning.
1527 SmallVector<BasicBlock *, 16> Worklist;
1528 SmallPtrSet<BasicBlock *, 16> BlocksInClonedLoop;
1529 for (auto *Pred : predecessors(BB: ClonedHeader)) {
1530 // The only possible non-loop header predecessor is the preheader because
1531 // we know we cloned the loop in simplified form.
1532 if (Pred == ClonedPH)
1533 continue;
1534
1535 // Because the loop was in simplified form, the only non-loop predecessor
1536 // should be the preheader.
1537 assert(ClonedLoopBlocks.count(Pred) && "Found a predecessor of the loop "
1538 "header other than the preheader "
1539 "that is not part of the loop!");
1540
1541 // Insert this block into the loop set and on the first visit (and if it
1542 // isn't the header we're currently walking) put it into the worklist to
1543 // recurse through.
1544 if (BlocksInClonedLoop.insert(Ptr: Pred).second && Pred != ClonedHeader)
1545 Worklist.push_back(Elt: Pred);
1546 }
1547
1548 // If we had any backedges then there *is* a cloned loop. Put the header into
1549 // the loop set and then walk the worklist backwards to find all the blocks
1550 // that remain within the loop after cloning.
1551 if (!BlocksInClonedLoop.empty()) {
1552 BlocksInClonedLoop.insert(Ptr: ClonedHeader);
1553
1554 while (!Worklist.empty()) {
1555 BasicBlock *BB = Worklist.pop_back_val();
1556 assert(BlocksInClonedLoop.count(BB) &&
1557 "Didn't put block into the loop set!");
1558
1559 // Insert any predecessors that are in the possible set into the cloned
1560 // set, and if the insert is successful, add them to the worklist. Note
1561 // that we filter on the blocks that are definitely reachable via the
1562 // backedge to the loop header so we may prune out dead code within the
1563 // cloned loop.
1564 for (auto *Pred : predecessors(BB))
1565 if (ClonedLoopBlocks.count(key: Pred) &&
1566 BlocksInClonedLoop.insert(Ptr: Pred).second)
1567 Worklist.push_back(Elt: Pred);
1568 }
1569
1570 ClonedL = LI.AllocateLoop();
1571 if (ParentL) {
1572 ParentL->addBasicBlockToLoop(NewBB: ClonedPH, LI);
1573 ParentL->addChildLoop(NewChild: ClonedL);
1574 } else {
1575 LI.addTopLevelLoop(New: ClonedL);
1576 }
1577 NonChildClonedLoops.push_back(Elt: ClonedL);
1578
1579 ClonedL->reserveBlocks(size: BlocksInClonedLoop.size());
1580 // We don't want to just add the cloned loop blocks based on how we
1581 // discovered them. The original order of blocks was carefully built in
1582 // a way that doesn't rely on predecessor ordering. Rather than re-invent
1583 // that logic, we just re-walk the original blocks (and those of the child
1584 // loops) and filter them as we add them into the cloned loop.
1585 for (auto *BB : OrigL.blocks()) {
1586 auto *ClonedBB = cast_or_null<BasicBlock>(Val: VMap.lookup(Val: BB));
1587 if (!ClonedBB || !BlocksInClonedLoop.count(Ptr: ClonedBB))
1588 continue;
1589
1590 // Directly add the blocks that are only in this loop.
1591 if (LI.getLoopFor(BB) == &OrigL) {
1592 ClonedL->addBasicBlockToLoop(NewBB: ClonedBB, LI);
1593 continue;
1594 }
1595
1596 // We want to manually add it to this loop and parents.
1597 // Registering it with LoopInfo will happen when we clone the top
1598 // loop for this block.
1599 for (Loop *PL = ClonedL; PL; PL = PL->getParentLoop())
1600 PL->addBlockEntry(BB: ClonedBB);
1601 }
1602
1603 // Now add each child loop whose header remains within the cloned loop. All
1604 // of the blocks within the loop must satisfy the same constraints as the
1605 // header so once we pass the header checks we can just clone the entire
1606 // child loop nest.
1607 for (Loop *ChildL : OrigL) {
1608 auto *ClonedChildHeader =
1609 cast_or_null<BasicBlock>(Val: VMap.lookup(Val: ChildL->getHeader()));
1610 if (!ClonedChildHeader || !BlocksInClonedLoop.count(Ptr: ClonedChildHeader))
1611 continue;
1612
1613#ifndef NDEBUG
1614 // We should never have a cloned child loop header but fail to have
1615 // all of the blocks for that child loop.
1616 for (auto *ChildLoopBB : ChildL->blocks())
1617 assert(BlocksInClonedLoop.count(
1618 cast<BasicBlock>(VMap.lookup(ChildLoopBB))) &&
1619 "Child cloned loop has a header within the cloned outer "
1620 "loop but not all of its blocks!");
1621#endif
1622
1623 cloneLoopNest(OrigRootL&: *ChildL, RootParentL: ClonedL, VMap, LI);
1624 }
1625 }
1626
1627 // Now that we've handled all the components of the original loop that were
1628 // cloned into a new loop, we still need to handle anything from the original
1629 // loop that wasn't in a cloned loop.
1630
1631 // Figure out what blocks are left to place within any loop nest containing
1632 // the unswitched loop. If we never formed a loop, the cloned PH is one of
1633 // them.
1634 SmallPtrSet<BasicBlock *, 16> UnloopedBlockSet;
1635 if (BlocksInClonedLoop.empty())
1636 UnloopedBlockSet.insert(Ptr: ClonedPH);
1637 for (auto *ClonedBB : ClonedLoopBlocks)
1638 if (!BlocksInClonedLoop.count(Ptr: ClonedBB))
1639 UnloopedBlockSet.insert(Ptr: ClonedBB);
1640
1641 // Copy the cloned exits and sort them in ascending loop depth, we'll work
1642 // backwards across these to process them inside out. The order shouldn't
1643 // matter as we're just trying to build up the map from inside-out; we use
1644 // the map in a more stably ordered way below.
1645 auto OrderedClonedExitsInLoops = ClonedExitsInLoops;
1646 llvm::sort(C&: OrderedClonedExitsInLoops, Comp: [&](BasicBlock *LHS, BasicBlock *RHS) {
1647 return ExitLoopMap.lookup(Val: LHS)->getLoopDepth() <
1648 ExitLoopMap.lookup(Val: RHS)->getLoopDepth();
1649 });
1650
1651 // Populate the existing ExitLoopMap with everything reachable from each
1652 // exit, starting from the inner most exit.
1653 while (!UnloopedBlockSet.empty() && !OrderedClonedExitsInLoops.empty()) {
1654 assert(Worklist.empty() && "Didn't clear worklist!");
1655
1656 BasicBlock *ExitBB = OrderedClonedExitsInLoops.pop_back_val();
1657 Loop *ExitL = ExitLoopMap.lookup(Val: ExitBB);
1658
1659 // Walk the CFG back until we hit the cloned PH adding everything reachable
1660 // and in the unlooped set to this exit block's loop.
1661 Worklist.push_back(Elt: ExitBB);
1662 do {
1663 BasicBlock *BB = Worklist.pop_back_val();
1664 // We can stop recursing at the cloned preheader (if we get there).
1665 if (BB == ClonedPH)
1666 continue;
1667
1668 for (BasicBlock *PredBB : predecessors(BB)) {
1669 // If this pred has already been moved to our set or is part of some
1670 // (inner) loop, no update needed.
1671 if (!UnloopedBlockSet.erase(Ptr: PredBB)) {
1672 assert(
1673 (BlocksInClonedLoop.count(PredBB) || ExitLoopMap.count(PredBB)) &&
1674 "Predecessor not mapped to a loop!");
1675 continue;
1676 }
1677
1678 // We just insert into the loop set here. We'll add these blocks to the
1679 // exit loop after we build up the set in an order that doesn't rely on
1680 // predecessor order (which in turn relies on use list order).
1681 bool Inserted = ExitLoopMap.insert(KV: {PredBB, ExitL}).second;
1682 (void)Inserted;
1683 assert(Inserted && "Should only visit an unlooped block once!");
1684
1685 // And recurse through to its predecessors.
1686 Worklist.push_back(Elt: PredBB);
1687 }
1688 } while (!Worklist.empty());
1689 }
1690
1691 // Now that the ExitLoopMap gives as mapping for all the non-looping cloned
1692 // blocks to their outer loops, walk the cloned blocks and the cloned exits
1693 // in their original order adding them to the correct loop.
1694
1695 // We need a stable insertion order. We use the order of the original loop
1696 // order and map into the correct parent loop.
1697 for (auto *BB : llvm::concat<BasicBlock *const>(
1698 Ranges: ArrayRef(ClonedPH), Ranges&: ClonedLoopBlocks, Ranges&: ClonedExitsInLoops))
1699 if (Loop *OuterL = ExitLoopMap.lookup(Val: BB))
1700 OuterL->addBasicBlockToLoop(NewBB: BB, LI);
1701
1702#ifndef NDEBUG
1703 for (auto &BBAndL : ExitLoopMap) {
1704 auto *BB = BBAndL.first;
1705 auto *OuterL = BBAndL.second;
1706 assert(LI.getLoopFor(BB) == OuterL &&
1707 "Failed to put all blocks into outer loops!");
1708 }
1709#endif
1710
1711 // Now that all the blocks are placed into the correct containing loop in the
1712 // absence of child loops, find all the potentially cloned child loops and
1713 // clone them into whatever outer loop we placed their header into.
1714 for (Loop *ChildL : OrigL) {
1715 auto *ClonedChildHeader =
1716 cast_or_null<BasicBlock>(Val: VMap.lookup(Val: ChildL->getHeader()));
1717 if (!ClonedChildHeader || BlocksInClonedLoop.count(Ptr: ClonedChildHeader))
1718 continue;
1719
1720#ifndef NDEBUG
1721 for (auto *ChildLoopBB : ChildL->blocks())
1722 assert(VMap.count(ChildLoopBB) &&
1723 "Cloned a child loop header but not all of that loops blocks!");
1724#endif
1725
1726 NonChildClonedLoops.push_back(Elt: cloneLoopNest(
1727 OrigRootL&: *ChildL, RootParentL: ExitLoopMap.lookup(Val: ClonedChildHeader), VMap, LI));
1728 }
1729}
1730
1731static void
1732deleteDeadClonedBlocks(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
1733 ArrayRef<std::unique_ptr<ValueToValueMapTy>> VMaps,
1734 DominatorTree &DT, MemorySSAUpdater *MSSAU) {
1735 // Find all the dead clones, and remove them from their successors.
1736 SmallVector<BasicBlock *, 16> DeadBlocks;
1737 for (BasicBlock *BB : llvm::concat<BasicBlock *const>(Ranges: L.blocks(), Ranges&: ExitBlocks))
1738 for (const auto &VMap : VMaps)
1739 if (BasicBlock *ClonedBB = cast_or_null<BasicBlock>(Val: VMap->lookup(Val: BB)))
1740 if (!DT.isReachableFromEntry(A: ClonedBB)) {
1741 for (BasicBlock *SuccBB : successors(BB: ClonedBB))
1742 SuccBB->removePredecessor(Pred: ClonedBB);
1743 DeadBlocks.push_back(Elt: ClonedBB);
1744 }
1745
1746 // Remove all MemorySSA in the dead blocks
1747 if (MSSAU) {
1748 SmallSetVector<BasicBlock *, 8> DeadBlockSet(DeadBlocks.begin(),
1749 DeadBlocks.end());
1750 MSSAU->removeBlocks(DeadBlocks: DeadBlockSet);
1751 }
1752
1753 // Drop any remaining references to break cycles.
1754 for (BasicBlock *BB : DeadBlocks)
1755 BB->dropAllReferences();
1756 // Erase them from the IR.
1757 for (BasicBlock *BB : DeadBlocks)
1758 BB->eraseFromParent();
1759}
1760
1761static void deleteDeadBlocksFromLoop(Loop &L,
1762 SmallVectorImpl<BasicBlock *> &ExitBlocks,
1763 DominatorTree &DT, LoopInfo &LI,
1764 MemorySSAUpdater *MSSAU,
1765 ScalarEvolution *SE,
1766 LPMUpdater &LoopUpdater) {
1767 // Find all the dead blocks tied to this loop, and remove them from their
1768 // successors.
1769 SmallSetVector<BasicBlock *, 8> DeadBlockSet;
1770
1771 // Start with loop/exit blocks and get a transitive closure of reachable dead
1772 // blocks.
1773 SmallVector<BasicBlock *, 16> DeathCandidates(ExitBlocks.begin(),
1774 ExitBlocks.end());
1775 DeathCandidates.append(in_start: L.blocks().begin(), in_end: L.blocks().end());
1776 while (!DeathCandidates.empty()) {
1777 auto *BB = DeathCandidates.pop_back_val();
1778 if (!DeadBlockSet.count(key: BB) && !DT.isReachableFromEntry(A: BB)) {
1779 for (BasicBlock *SuccBB : successors(BB)) {
1780 SuccBB->removePredecessor(Pred: BB);
1781 DeathCandidates.push_back(Elt: SuccBB);
1782 }
1783 DeadBlockSet.insert(X: BB);
1784 }
1785 }
1786
1787 // Remove all MemorySSA in the dead blocks
1788 if (MSSAU)
1789 MSSAU->removeBlocks(DeadBlocks: DeadBlockSet);
1790
1791 // Filter out the dead blocks from the exit blocks list so that it can be
1792 // used in the caller.
1793 llvm::erase_if(C&: ExitBlocks,
1794 P: [&](BasicBlock *BB) { return DeadBlockSet.count(key: BB); });
1795
1796 // Walk from this loop up through its parents removing all of the dead blocks.
1797 for (Loop *ParentL = &L; ParentL; ParentL = ParentL->getParentLoop()) {
1798 for (auto *BB : DeadBlockSet)
1799 ParentL->getBlocksSet().erase(Ptr: BB);
1800 llvm::erase_if(C&: ParentL->getBlocksVector(),
1801 P: [&](BasicBlock *BB) { return DeadBlockSet.count(key: BB); });
1802 }
1803
1804 // Now delete the dead child loops. This raw delete will clear them
1805 // recursively.
1806 llvm::erase_if(C&: L.getSubLoopsVector(), P: [&](Loop *ChildL) {
1807 if (!DeadBlockSet.count(key: ChildL->getHeader()))
1808 return false;
1809
1810 assert(llvm::all_of(ChildL->blocks(),
1811 [&](BasicBlock *ChildBB) {
1812 return DeadBlockSet.count(ChildBB);
1813 }) &&
1814 "If the child loop header is dead all blocks in the child loop must "
1815 "be dead as well!");
1816 LoopUpdater.markLoopAsDeleted(L&: *ChildL, Name: ChildL->getName());
1817 if (SE)
1818 SE->forgetBlockAndLoopDispositions();
1819 LI.destroy(L: ChildL);
1820 return true;
1821 });
1822
1823 // Remove the loop mappings for the dead blocks and drop all the references
1824 // from these blocks to others to handle cyclic references as we start
1825 // deleting the blocks themselves.
1826 for (auto *BB : DeadBlockSet) {
1827 // Check that the dominator tree has already been updated.
1828 assert(!DT.getNode(BB) && "Should already have cleared domtree!");
1829 LI.changeLoopFor(BB, L: nullptr);
1830 // Drop all uses of the instructions to make sure we won't have dangling
1831 // uses in other blocks.
1832 for (auto &I : *BB)
1833 if (!I.use_empty())
1834 I.replaceAllUsesWith(V: PoisonValue::get(T: I.getType()));
1835 BB->dropAllReferences();
1836 }
1837
1838 // Actually delete the blocks now that they've been fully unhooked from the
1839 // IR.
1840 for (auto *BB : DeadBlockSet)
1841 BB->eraseFromParent();
1842}
1843
1844/// Recompute the set of blocks in a loop after unswitching.
1845///
1846/// This walks from the original headers predecessors to rebuild the loop. We
1847/// take advantage of the fact that new blocks can't have been added, and so we
1848/// filter by the original loop's blocks. This also handles potentially
1849/// unreachable code that we don't want to explore but might be found examining
1850/// the predecessors of the header.
1851///
1852/// If the original loop is no longer a loop, this will return an empty set. If
1853/// it remains a loop, all the blocks within it will be added to the set
1854/// (including those blocks in inner loops).
1855static SmallPtrSet<const BasicBlock *, 16> recomputeLoopBlockSet(Loop &L,
1856 LoopInfo &LI) {
1857 SmallPtrSet<const BasicBlock *, 16> LoopBlockSet;
1858
1859 auto *PH = L.getLoopPreheader();
1860 auto *Header = L.getHeader();
1861
1862 // A worklist to use while walking backwards from the header.
1863 SmallVector<BasicBlock *, 16> Worklist;
1864
1865 // First walk the predecessors of the header to find the backedges. This will
1866 // form the basis of our walk.
1867 for (auto *Pred : predecessors(BB: Header)) {
1868 // Skip the preheader.
1869 if (Pred == PH)
1870 continue;
1871
1872 // Because the loop was in simplified form, the only non-loop predecessor
1873 // is the preheader.
1874 assert(L.contains(Pred) && "Found a predecessor of the loop header other "
1875 "than the preheader that is not part of the "
1876 "loop!");
1877
1878 // Insert this block into the loop set and on the first visit and, if it
1879 // isn't the header we're currently walking, put it into the worklist to
1880 // recurse through.
1881 if (LoopBlockSet.insert(Ptr: Pred).second && Pred != Header)
1882 Worklist.push_back(Elt: Pred);
1883 }
1884
1885 // If no backedges were found, we're done.
1886 if (LoopBlockSet.empty())
1887 return LoopBlockSet;
1888
1889 // We found backedges, recurse through them to identify the loop blocks.
1890 while (!Worklist.empty()) {
1891 BasicBlock *BB = Worklist.pop_back_val();
1892 assert(LoopBlockSet.count(BB) && "Didn't put block into the loop set!");
1893
1894 // No need to walk past the header.
1895 if (BB == Header)
1896 continue;
1897
1898 // Because we know the inner loop structure remains valid we can use the
1899 // loop structure to jump immediately across the entire nested loop.
1900 // Further, because it is in loop simplified form, we can directly jump
1901 // to its preheader afterward.
1902 if (Loop *InnerL = LI.getLoopFor(BB))
1903 if (InnerL != &L) {
1904 assert(L.contains(InnerL) &&
1905 "Should not reach a loop *outside* this loop!");
1906 // The preheader is the only possible predecessor of the loop so
1907 // insert it into the set and check whether it was already handled.
1908 auto *InnerPH = InnerL->getLoopPreheader();
1909 assert(L.contains(InnerPH) && "Cannot contain an inner loop block "
1910 "but not contain the inner loop "
1911 "preheader!");
1912 if (!LoopBlockSet.insert(Ptr: InnerPH).second)
1913 // The only way to reach the preheader is through the loop body
1914 // itself so if it has been visited the loop is already handled.
1915 continue;
1916
1917 // Insert all of the blocks (other than those already present) into
1918 // the loop set. We expect at least the block that led us to find the
1919 // inner loop to be in the block set, but we may also have other loop
1920 // blocks if they were already enqueued as predecessors of some other
1921 // outer loop block.
1922 for (auto *InnerBB : InnerL->blocks()) {
1923 if (InnerBB == BB) {
1924 assert(LoopBlockSet.count(InnerBB) &&
1925 "Block should already be in the set!");
1926 continue;
1927 }
1928
1929 LoopBlockSet.insert(Ptr: InnerBB);
1930 }
1931
1932 // Add the preheader to the worklist so we will continue past the
1933 // loop body.
1934 Worklist.push_back(Elt: InnerPH);
1935 continue;
1936 }
1937
1938 // Insert any predecessors that were in the original loop into the new
1939 // set, and if the insert is successful, add them to the worklist.
1940 for (auto *Pred : predecessors(BB))
1941 if (L.contains(BB: Pred) && LoopBlockSet.insert(Ptr: Pred).second)
1942 Worklist.push_back(Elt: Pred);
1943 }
1944
1945 assert(LoopBlockSet.count(Header) && "Cannot fail to add the header!");
1946
1947 // We've found all the blocks participating in the loop, return our completed
1948 // set.
1949 return LoopBlockSet;
1950}
1951
1952/// Rebuild a loop after unswitching removes some subset of blocks and edges.
1953///
1954/// The removal may have removed some child loops entirely but cannot have
1955/// disturbed any remaining child loops. However, they may need to be hoisted
1956/// to the parent loop (or to be top-level loops). The original loop may be
1957/// completely removed.
1958///
1959/// The sibling loops resulting from this update are returned. If the original
1960/// loop remains a valid loop, it will be the first entry in this list with all
1961/// of the newly sibling loops following it.
1962///
1963/// Returns true if the loop remains a loop after unswitching, and false if it
1964/// is no longer a loop after unswitching (and should not continue to be
1965/// referenced).
1966static bool rebuildLoopAfterUnswitch(Loop &L, ArrayRef<BasicBlock *> ExitBlocks,
1967 LoopInfo &LI,
1968 SmallVectorImpl<Loop *> &HoistedLoops,
1969 ScalarEvolution *SE) {
1970 auto *PH = L.getLoopPreheader();
1971
1972 // Compute the actual parent loop from the exit blocks. Because we may have
1973 // pruned some exits the loop may be different from the original parent.
1974 Loop *ParentL = nullptr;
1975 SmallVector<Loop *, 4> ExitLoops;
1976 SmallVector<BasicBlock *, 4> ExitsInLoops;
1977 ExitsInLoops.reserve(N: ExitBlocks.size());
1978 for (auto *ExitBB : ExitBlocks)
1979 if (Loop *ExitL = LI.getLoopFor(BB: ExitBB)) {
1980 ExitLoops.push_back(Elt: ExitL);
1981 ExitsInLoops.push_back(Elt: ExitBB);
1982 if (!ParentL || (ParentL != ExitL && ParentL->contains(L: ExitL)))
1983 ParentL = ExitL;
1984 }
1985
1986 // Recompute the blocks participating in this loop. This may be empty if it
1987 // is no longer a loop.
1988 auto LoopBlockSet = recomputeLoopBlockSet(L, LI);
1989
1990 // If we still have a loop, we need to re-set the loop's parent as the exit
1991 // block set changing may have moved it within the loop nest. Note that this
1992 // can only happen when this loop has a parent as it can only hoist the loop
1993 // *up* the nest.
1994 if (!LoopBlockSet.empty() && L.getParentLoop() != ParentL) {
1995 // Remove this loop's (original) blocks from all of the intervening loops.
1996 for (Loop *IL = L.getParentLoop(); IL != ParentL;
1997 IL = IL->getParentLoop()) {
1998 IL->getBlocksSet().erase(Ptr: PH);
1999 for (auto *BB : L.blocks())
2000 IL->getBlocksSet().erase(Ptr: BB);
2001 llvm::erase_if(C&: IL->getBlocksVector(), P: [&](BasicBlock *BB) {
2002 return BB == PH || L.contains(BB);
2003 });
2004 }
2005
2006 LI.changeLoopFor(BB: PH, L: ParentL);
2007 L.getParentLoop()->removeChildLoop(Child: &L);
2008 if (ParentL)
2009 ParentL->addChildLoop(NewChild: &L);
2010 else
2011 LI.addTopLevelLoop(New: &L);
2012 }
2013
2014 // Now we update all the blocks which are no longer within the loop.
2015 auto &Blocks = L.getBlocksVector();
2016 auto BlocksSplitI =
2017 LoopBlockSet.empty()
2018 ? Blocks.begin()
2019 : std::stable_partition(
2020 first: Blocks.begin(), last: Blocks.end(),
2021 pred: [&](BasicBlock *BB) { return LoopBlockSet.count(Ptr: BB); });
2022
2023 // Before we erase the list of unlooped blocks, build a set of them.
2024 SmallPtrSet<BasicBlock *, 16> UnloopedBlocks(BlocksSplitI, Blocks.end());
2025 if (LoopBlockSet.empty())
2026 UnloopedBlocks.insert(Ptr: PH);
2027
2028 // Now erase these blocks from the loop.
2029 for (auto *BB : make_range(x: BlocksSplitI, y: Blocks.end()))
2030 L.getBlocksSet().erase(Ptr: BB);
2031 Blocks.erase(first: BlocksSplitI, last: Blocks.end());
2032
2033 // Sort the exits in ascending loop depth, we'll work backwards across these
2034 // to process them inside out.
2035 llvm::stable_sort(Range&: ExitsInLoops, C: [&](BasicBlock *LHS, BasicBlock *RHS) {
2036 return LI.getLoopDepth(BB: LHS) < LI.getLoopDepth(BB: RHS);
2037 });
2038
2039 // We'll build up a set for each exit loop.
2040 SmallPtrSet<BasicBlock *, 16> NewExitLoopBlocks;
2041 Loop *PrevExitL = L.getParentLoop(); // The deepest possible exit loop.
2042
2043 auto RemoveUnloopedBlocksFromLoop =
2044 [](Loop &L, SmallPtrSetImpl<BasicBlock *> &UnloopedBlocks) {
2045 for (auto *BB : UnloopedBlocks)
2046 L.getBlocksSet().erase(Ptr: BB);
2047 llvm::erase_if(C&: L.getBlocksVector(), P: [&](BasicBlock *BB) {
2048 return UnloopedBlocks.count(Ptr: BB);
2049 });
2050 };
2051
2052 SmallVector<BasicBlock *, 16> Worklist;
2053 while (!UnloopedBlocks.empty() && !ExitsInLoops.empty()) {
2054 assert(Worklist.empty() && "Didn't clear worklist!");
2055 assert(NewExitLoopBlocks.empty() && "Didn't clear loop set!");
2056
2057 // Grab the next exit block, in decreasing loop depth order.
2058 BasicBlock *ExitBB = ExitsInLoops.pop_back_val();
2059 Loop &ExitL = *LI.getLoopFor(BB: ExitBB);
2060 assert(ExitL.contains(&L) && "Exit loop must contain the inner loop!");
2061
2062 // Erase all of the unlooped blocks from the loops between the previous
2063 // exit loop and this exit loop. This works because the ExitInLoops list is
2064 // sorted in increasing order of loop depth and thus we visit loops in
2065 // decreasing order of loop depth.
2066 for (; PrevExitL != &ExitL; PrevExitL = PrevExitL->getParentLoop())
2067 RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
2068
2069 // Walk the CFG back until we hit the cloned PH adding everything reachable
2070 // and in the unlooped set to this exit block's loop.
2071 Worklist.push_back(Elt: ExitBB);
2072 do {
2073 BasicBlock *BB = Worklist.pop_back_val();
2074 // We can stop recursing at the cloned preheader (if we get there).
2075 if (BB == PH)
2076 continue;
2077
2078 for (BasicBlock *PredBB : predecessors(BB)) {
2079 // If this pred has already been moved to our set or is part of some
2080 // (inner) loop, no update needed.
2081 if (!UnloopedBlocks.erase(Ptr: PredBB)) {
2082 assert((NewExitLoopBlocks.count(PredBB) ||
2083 ExitL.contains(LI.getLoopFor(PredBB))) &&
2084 "Predecessor not in a nested loop (or already visited)!");
2085 continue;
2086 }
2087
2088 // We just insert into the loop set here. We'll add these blocks to the
2089 // exit loop after we build up the set in a deterministic order rather
2090 // than the predecessor-influenced visit order.
2091 bool Inserted = NewExitLoopBlocks.insert(Ptr: PredBB).second;
2092 (void)Inserted;
2093 assert(Inserted && "Should only visit an unlooped block once!");
2094
2095 // And recurse through to its predecessors.
2096 Worklist.push_back(Elt: PredBB);
2097 }
2098 } while (!Worklist.empty());
2099
2100 // If blocks in this exit loop were directly part of the original loop (as
2101 // opposed to a child loop) update the map to point to this exit loop. This
2102 // just updates a map and so the fact that the order is unstable is fine.
2103 for (auto *BB : NewExitLoopBlocks)
2104 if (Loop *BBL = LI.getLoopFor(BB))
2105 if (BBL == &L || !L.contains(L: BBL))
2106 LI.changeLoopFor(BB, L: &ExitL);
2107
2108 // We will remove the remaining unlooped blocks from this loop in the next
2109 // iteration or below.
2110 NewExitLoopBlocks.clear();
2111 }
2112
2113 // Any remaining unlooped blocks are no longer part of any loop unless they
2114 // are part of some child loop.
2115 for (; PrevExitL; PrevExitL = PrevExitL->getParentLoop())
2116 RemoveUnloopedBlocksFromLoop(*PrevExitL, UnloopedBlocks);
2117 for (auto *BB : UnloopedBlocks)
2118 if (Loop *BBL = LI.getLoopFor(BB))
2119 if (BBL == &L || !L.contains(L: BBL))
2120 LI.changeLoopFor(BB, L: nullptr);
2121
2122 // Sink all the child loops whose headers are no longer in the loop set to
2123 // the parent (or to be top level loops). We reach into the loop and directly
2124 // update its subloop vector to make this batch update efficient.
2125 auto &SubLoops = L.getSubLoopsVector();
2126 auto SubLoopsSplitI =
2127 LoopBlockSet.empty()
2128 ? SubLoops.begin()
2129 : std::stable_partition(
2130 first: SubLoops.begin(), last: SubLoops.end(), pred: [&](Loop *SubL) {
2131 return LoopBlockSet.count(Ptr: SubL->getHeader());
2132 });
2133 for (auto *HoistedL : make_range(x: SubLoopsSplitI, y: SubLoops.end())) {
2134 HoistedLoops.push_back(Elt: HoistedL);
2135 HoistedL->setParentLoop(nullptr);
2136
2137 // To compute the new parent of this hoisted loop we look at where we
2138 // placed the preheader above. We can't lookup the header itself because we
2139 // retained the mapping from the header to the hoisted loop. But the
2140 // preheader and header should have the exact same new parent computed
2141 // based on the set of exit blocks from the original loop as the preheader
2142 // is a predecessor of the header and so reached in the reverse walk. And
2143 // because the loops were all in simplified form the preheader of the
2144 // hoisted loop can't be part of some *other* loop.
2145 if (auto *NewParentL = LI.getLoopFor(BB: HoistedL->getLoopPreheader()))
2146 NewParentL->addChildLoop(NewChild: HoistedL);
2147 else
2148 LI.addTopLevelLoop(New: HoistedL);
2149 }
2150 SubLoops.erase(first: SubLoopsSplitI, last: SubLoops.end());
2151
2152 // Actually delete the loop if nothing remained within it.
2153 if (Blocks.empty()) {
2154 assert(SubLoops.empty() &&
2155 "Failed to remove all subloops from the original loop!");
2156 if (Loop *ParentL = L.getParentLoop())
2157 ParentL->removeChildLoop(I: llvm::find(Range&: *ParentL, Val: &L));
2158 else
2159 LI.removeLoop(I: llvm::find(Range&: LI, Val: &L));
2160 // markLoopAsDeleted for L should be triggered by the caller (it is
2161 // typically done within postUnswitch).
2162 if (SE)
2163 SE->forgetBlockAndLoopDispositions();
2164 LI.destroy(L: &L);
2165 return false;
2166 }
2167
2168 return true;
2169}
2170
2171/// Helper to visit a dominator subtree, invoking a callable on each node.
2172///
2173/// Returning false at any point will stop walking past that node of the tree.
2174template <typename CallableT>
2175void visitDomSubTree(DominatorTree &DT, BasicBlock *BB, CallableT Callable) {
2176 SmallVector<DomTreeNode *, 4> DomWorklist;
2177 DomWorklist.push_back(Elt: DT[BB]);
2178#ifndef NDEBUG
2179 SmallPtrSet<DomTreeNode *, 4> Visited;
2180 Visited.insert(DT[BB]);
2181#endif
2182 do {
2183 DomTreeNode *N = DomWorklist.pop_back_val();
2184
2185 // Visit this node.
2186 if (!Callable(N->getBlock()))
2187 continue;
2188
2189 // Accumulate the child nodes.
2190 for (DomTreeNode *ChildN : *N) {
2191 assert(Visited.insert(ChildN).second &&
2192 "Cannot visit a node twice when walking a tree!");
2193 DomWorklist.push_back(Elt: ChildN);
2194 }
2195 } while (!DomWorklist.empty());
2196}
2197
2198void postUnswitch(Loop &L, LPMUpdater &U, StringRef LoopName,
2199 bool CurrentLoopValid, bool PartiallyInvariant,
2200 bool InjectedCondition, ArrayRef<Loop *> NewLoops) {
2201 // If we did a non-trivial unswitch, we have added new (cloned) loops.
2202 if (!NewLoops.empty())
2203 U.addSiblingLoops(NewSibLoops: NewLoops);
2204
2205 // If the current loop remains valid, we should revisit it to catch any
2206 // other unswitch opportunities. Otherwise, we need to mark it as deleted.
2207 if (CurrentLoopValid) {
2208 if (PartiallyInvariant) {
2209 // Mark the new loop as partially unswitched, to avoid unswitching on
2210 // the same condition again.
2211 auto &Context = L.getHeader()->getContext();
2212 MDNode *DisableUnswitchMD = MDNode::get(
2213 Context,
2214 MDs: MDString::get(Context, Str: "llvm.loop.unswitch.partial.disable"));
2215 MDNode *NewLoopID = makePostTransformationMetadata(
2216 Context, OrigLoopID: L.getLoopID(), RemovePrefixes: {"llvm.loop.unswitch.partial"},
2217 AddAttrs: {DisableUnswitchMD});
2218 L.setLoopID(NewLoopID);
2219 } else if (InjectedCondition) {
2220 // Do the same for injection of invariant conditions.
2221 auto &Context = L.getHeader()->getContext();
2222 MDNode *DisableUnswitchMD = MDNode::get(
2223 Context,
2224 MDs: MDString::get(Context, Str: "llvm.loop.unswitch.injection.disable"));
2225 MDNode *NewLoopID = makePostTransformationMetadata(
2226 Context, OrigLoopID: L.getLoopID(), RemovePrefixes: {"llvm.loop.unswitch.injection"},
2227 AddAttrs: {DisableUnswitchMD});
2228 L.setLoopID(NewLoopID);
2229 } else
2230 U.revisitCurrentLoop();
2231 } else
2232 U.markLoopAsDeleted(L, Name: LoopName);
2233}
2234
2235static void unswitchNontrivialInvariants(
2236 Loop &L, Instruction &TI, ArrayRef<Value *> Invariants,
2237 IVConditionInfo &PartialIVInfo, DominatorTree &DT, LoopInfo &LI,
2238 AssumptionCache &AC, ScalarEvolution *SE, MemorySSAUpdater *MSSAU,
2239 LPMUpdater &LoopUpdater, bool InsertFreeze, bool InjectedCondition) {
2240 auto *ParentBB = TI.getParent();
2241 BranchInst *BI = dyn_cast<BranchInst>(Val: &TI);
2242 SwitchInst *SI = BI ? nullptr : cast<SwitchInst>(Val: &TI);
2243
2244 // Save the current loop name in a variable so that we can report it even
2245 // after it has been deleted.
2246 std::string LoopName(L.getName());
2247
2248 // We can only unswitch switches, conditional branches with an invariant
2249 // condition, or combining invariant conditions with an instruction or
2250 // partially invariant instructions.
2251 assert((SI || (BI && BI->isConditional())) &&
2252 "Can only unswitch switches and conditional branch!");
2253 bool PartiallyInvariant = !PartialIVInfo.InstToDuplicate.empty();
2254 bool FullUnswitch =
2255 SI || (skipTrivialSelect(Cond: BI->getCondition()) == Invariants[0] &&
2256 !PartiallyInvariant);
2257 if (FullUnswitch)
2258 assert(Invariants.size() == 1 &&
2259 "Cannot have other invariants with full unswitching!");
2260 else
2261 assert(isa<Instruction>(skipTrivialSelect(BI->getCondition())) &&
2262 "Partial unswitching requires an instruction as the condition!");
2263
2264 if (MSSAU && VerifyMemorySSA)
2265 MSSAU->getMemorySSA()->verifyMemorySSA();
2266
2267 // Constant and BBs tracking the cloned and continuing successor. When we are
2268 // unswitching the entire condition, this can just be trivially chosen to
2269 // unswitch towards `true`. However, when we are unswitching a set of
2270 // invariants combined with `and` or `or` or partially invariant instructions,
2271 // the combining operation determines the best direction to unswitch: we want
2272 // to unswitch the direction that will collapse the branch.
2273 bool Direction = true;
2274 int ClonedSucc = 0;
2275 if (!FullUnswitch) {
2276 Value *Cond = skipTrivialSelect(Cond: BI->getCondition());
2277 (void)Cond;
2278 assert(((match(Cond, m_LogicalAnd()) ^ match(Cond, m_LogicalOr())) ||
2279 PartiallyInvariant) &&
2280 "Only `or`, `and`, an `select`, partially invariant instructions "
2281 "can combine invariants being unswitched.");
2282 if (!match(V: Cond, P: m_LogicalOr())) {
2283 if (match(V: Cond, P: m_LogicalAnd()) ||
2284 (PartiallyInvariant && !PartialIVInfo.KnownValue->isOneValue())) {
2285 Direction = false;
2286 ClonedSucc = 1;
2287 }
2288 }
2289 }
2290
2291 BasicBlock *RetainedSuccBB =
2292 BI ? BI->getSuccessor(i: 1 - ClonedSucc) : SI->getDefaultDest();
2293 SmallSetVector<BasicBlock *, 4> UnswitchedSuccBBs;
2294 if (BI)
2295 UnswitchedSuccBBs.insert(X: BI->getSuccessor(i: ClonedSucc));
2296 else
2297 for (auto Case : SI->cases())
2298 if (Case.getCaseSuccessor() != RetainedSuccBB)
2299 UnswitchedSuccBBs.insert(X: Case.getCaseSuccessor());
2300
2301 assert(!UnswitchedSuccBBs.count(RetainedSuccBB) &&
2302 "Should not unswitch the same successor we are retaining!");
2303
2304 // The branch should be in this exact loop. Any inner loop's invariant branch
2305 // should be handled by unswitching that inner loop. The caller of this
2306 // routine should filter out any candidates that remain (but were skipped for
2307 // whatever reason).
2308 assert(LI.getLoopFor(ParentBB) == &L && "Branch in an inner loop!");
2309
2310 // Compute the parent loop now before we start hacking on things.
2311 Loop *ParentL = L.getParentLoop();
2312 // Get blocks in RPO order for MSSA update, before changing the CFG.
2313 LoopBlocksRPO LBRPO(&L);
2314 if (MSSAU)
2315 LBRPO.perform(LI: &LI);
2316
2317 // Compute the outer-most loop containing one of our exit blocks. This is the
2318 // furthest up our loopnest which can be mutated, which we will use below to
2319 // update things.
2320 Loop *OuterExitL = &L;
2321 SmallVector<BasicBlock *, 4> ExitBlocks;
2322 L.getUniqueExitBlocks(ExitBlocks);
2323 for (auto *ExitBB : ExitBlocks) {
2324 // ExitBB can be an exit block for several levels in the loop nest. Make
2325 // sure we find the top most.
2326 Loop *NewOuterExitL = getTopMostExitingLoop(ExitBB, LI);
2327 if (!NewOuterExitL) {
2328 // We exited the entire nest with this block, so we're done.
2329 OuterExitL = nullptr;
2330 break;
2331 }
2332 if (NewOuterExitL != OuterExitL && NewOuterExitL->contains(L: OuterExitL))
2333 OuterExitL = NewOuterExitL;
2334 }
2335
2336 // At this point, we're definitely going to unswitch something so invalidate
2337 // any cached information in ScalarEvolution for the outer most loop
2338 // containing an exit block and all nested loops.
2339 if (SE) {
2340 if (OuterExitL)
2341 SE->forgetLoop(L: OuterExitL);
2342 else
2343 SE->forgetTopmostLoop(L: &L);
2344 SE->forgetBlockAndLoopDispositions();
2345 }
2346
2347 // If the edge from this terminator to a successor dominates that successor,
2348 // store a map from each block in its dominator subtree to it. This lets us
2349 // tell when cloning for a particular successor if a block is dominated by
2350 // some *other* successor with a single data structure. We use this to
2351 // significantly reduce cloning.
2352 SmallDenseMap<BasicBlock *, BasicBlock *, 16> DominatingSucc;
2353 for (auto *SuccBB : llvm::concat<BasicBlock *const>(Ranges: ArrayRef(RetainedSuccBB),
2354 Ranges&: UnswitchedSuccBBs))
2355 if (SuccBB->getUniquePredecessor() ||
2356 llvm::all_of(Range: predecessors(BB: SuccBB), P: [&](BasicBlock *PredBB) {
2357 return PredBB == ParentBB || DT.dominates(A: SuccBB, B: PredBB);
2358 }))
2359 visitDomSubTree(DT, BB: SuccBB, Callable: [&](BasicBlock *BB) {
2360 DominatingSucc[BB] = SuccBB;
2361 return true;
2362 });
2363
2364 // Split the preheader, so that we know that there is a safe place to insert
2365 // the conditional branch. We will change the preheader to have a conditional
2366 // branch on LoopCond. The original preheader will become the split point
2367 // between the unswitched versions, and we will have a new preheader for the
2368 // original loop.
2369 BasicBlock *SplitBB = L.getLoopPreheader();
2370 BasicBlock *LoopPH = SplitEdge(From: SplitBB, To: L.getHeader(), DT: &DT, LI: &LI, MSSAU);
2371
2372 // Keep track of the dominator tree updates needed.
2373 SmallVector<DominatorTree::UpdateType, 4> DTUpdates;
2374
2375 // Clone the loop for each unswitched successor.
2376 SmallVector<std::unique_ptr<ValueToValueMapTy>, 4> VMaps;
2377 VMaps.reserve(N: UnswitchedSuccBBs.size());
2378 SmallDenseMap<BasicBlock *, BasicBlock *, 4> ClonedPHs;
2379 for (auto *SuccBB : UnswitchedSuccBBs) {
2380 VMaps.emplace_back(Args: new ValueToValueMapTy());
2381 ClonedPHs[SuccBB] = buildClonedLoopBlocks(
2382 L, LoopPH, SplitBB, ExitBlocks, ParentBB, UnswitchedSuccBB: SuccBB, ContinueSuccBB: RetainedSuccBB,
2383 DominatingSucc, VMap&: *VMaps.back(), DTUpdates, AC, DT, LI, MSSAU, SE);
2384 }
2385
2386 // Drop metadata if we may break its semantics by moving this instr into the
2387 // split block.
2388 if (TI.getMetadata(KindID: LLVMContext::MD_make_implicit)) {
2389 if (DropNonTrivialImplicitNullChecks)
2390 // Do not spend time trying to understand if we can keep it, just drop it
2391 // to save compile time.
2392 TI.setMetadata(KindID: LLVMContext::MD_make_implicit, Node: nullptr);
2393 else {
2394 // It is only legal to preserve make.implicit metadata if we are
2395 // guaranteed no reach implicit null check after following this branch.
2396 ICFLoopSafetyInfo SafetyInfo;
2397 SafetyInfo.computeLoopSafetyInfo(CurLoop: &L);
2398 if (!SafetyInfo.isGuaranteedToExecute(Inst: TI, DT: &DT, CurLoop: &L))
2399 TI.setMetadata(KindID: LLVMContext::MD_make_implicit, Node: nullptr);
2400 }
2401 }
2402
2403 // The stitching of the branched code back together depends on whether we're
2404 // doing full unswitching or not with the exception that we always want to
2405 // nuke the initial terminator placed in the split block.
2406 SplitBB->getTerminator()->eraseFromParent();
2407 if (FullUnswitch) {
2408 // Keep a clone of the terminator for MSSA updates.
2409 Instruction *NewTI = TI.clone();
2410 NewTI->insertInto(ParentBB, It: ParentBB->end());
2411
2412 // Splice the terminator from the original loop and rewrite its
2413 // successors.
2414 TI.moveBefore(BB&: *SplitBB, I: SplitBB->end());
2415 TI.dropLocation();
2416
2417 // First wire up the moved terminator to the preheaders.
2418 if (BI) {
2419 BasicBlock *ClonedPH = ClonedPHs.begin()->second;
2420 BI->setSuccessor(idx: ClonedSucc, NewSucc: ClonedPH);
2421 BI->setSuccessor(idx: 1 - ClonedSucc, NewSucc: LoopPH);
2422 Value *Cond = skipTrivialSelect(Cond: BI->getCondition());
2423 if (InsertFreeze) {
2424 // We don't give any debug location to the new freeze, because the
2425 // BI (`dyn_cast<BranchInst>(TI)`) is an in-loop instruction hoisted
2426 // out of the loop.
2427 Cond = new FreezeInst(Cond, Cond->getName() + ".fr", BI->getIterator());
2428 cast<Instruction>(Val: Cond)->setDebugLoc(DebugLoc::getDropped());
2429 }
2430 BI->setCondition(Cond);
2431 DTUpdates.push_back(Elt: {DominatorTree::Insert, SplitBB, ClonedPH});
2432 } else {
2433 assert(SI && "Must either be a branch or switch!");
2434
2435 // Walk the cases and directly update their successors.
2436 assert(SI->getDefaultDest() == RetainedSuccBB &&
2437 "Not retaining default successor!");
2438 SI->setDefaultDest(LoopPH);
2439 for (const auto &Case : SI->cases())
2440 if (Case.getCaseSuccessor() == RetainedSuccBB)
2441 Case.setSuccessor(LoopPH);
2442 else
2443 Case.setSuccessor(ClonedPHs.find(Val: Case.getCaseSuccessor())->second);
2444
2445 if (InsertFreeze)
2446 SI->setCondition(new FreezeInst(SI->getCondition(),
2447 SI->getCondition()->getName() + ".fr",
2448 SI->getIterator()));
2449
2450 // We need to use the set to populate domtree updates as even when there
2451 // are multiple cases pointing at the same successor we only want to
2452 // remove and insert one edge in the domtree.
2453 for (BasicBlock *SuccBB : UnswitchedSuccBBs)
2454 DTUpdates.push_back(
2455 Elt: {DominatorTree::Insert, SplitBB, ClonedPHs.find(Val: SuccBB)->second});
2456 }
2457
2458 if (MSSAU) {
2459 DT.applyUpdates(Updates: DTUpdates);
2460 DTUpdates.clear();
2461
2462 // Remove all but one edge to the retained block and all unswitched
2463 // blocks. This is to avoid having duplicate entries in the cloned Phis,
2464 // when we know we only keep a single edge for each case.
2465 MSSAU->removeDuplicatePhiEdgesBetween(From: ParentBB, To: RetainedSuccBB);
2466 for (BasicBlock *SuccBB : UnswitchedSuccBBs)
2467 MSSAU->removeDuplicatePhiEdgesBetween(From: ParentBB, To: SuccBB);
2468
2469 for (auto &VMap : VMaps)
2470 MSSAU->updateForClonedLoop(LoopBlocks: LBRPO, ExitBlocks, VM: *VMap,
2471 /*IgnoreIncomingWithNoClones=*/true);
2472 MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
2473
2474 // Remove all edges to unswitched blocks.
2475 for (BasicBlock *SuccBB : UnswitchedSuccBBs)
2476 MSSAU->removeEdge(From: ParentBB, To: SuccBB);
2477 }
2478
2479 // Now unhook the successor relationship as we'll be replacing
2480 // the terminator with a direct branch. This is much simpler for branches
2481 // than switches so we handle those first.
2482 if (BI) {
2483 // Remove the parent as a predecessor of the unswitched successor.
2484 assert(UnswitchedSuccBBs.size() == 1 &&
2485 "Only one possible unswitched block for a branch!");
2486 BasicBlock *UnswitchedSuccBB = *UnswitchedSuccBBs.begin();
2487 UnswitchedSuccBB->removePredecessor(Pred: ParentBB,
2488 /*KeepOneInputPHIs*/ true);
2489 DTUpdates.push_back(Elt: {DominatorTree::Delete, ParentBB, UnswitchedSuccBB});
2490 } else {
2491 // Note that we actually want to remove the parent block as a predecessor
2492 // of *every* case successor. The case successor is either unswitched,
2493 // completely eliminating an edge from the parent to that successor, or it
2494 // is a duplicate edge to the retained successor as the retained successor
2495 // is always the default successor and as we'll replace this with a direct
2496 // branch we no longer need the duplicate entries in the PHI nodes.
2497 SwitchInst *NewSI = cast<SwitchInst>(Val: NewTI);
2498 assert(NewSI->getDefaultDest() == RetainedSuccBB &&
2499 "Not retaining default successor!");
2500 for (const auto &Case : NewSI->cases())
2501 Case.getCaseSuccessor()->removePredecessor(
2502 Pred: ParentBB,
2503 /*KeepOneInputPHIs*/ true);
2504
2505 // We need to use the set to populate domtree updates as even when there
2506 // are multiple cases pointing at the same successor we only want to
2507 // remove and insert one edge in the domtree.
2508 for (BasicBlock *SuccBB : UnswitchedSuccBBs)
2509 DTUpdates.push_back(Elt: {DominatorTree::Delete, ParentBB, SuccBB});
2510 }
2511
2512 // Create a new unconditional branch to the continuing block (as opposed to
2513 // the one cloned).
2514 Instruction *NewBI = BranchInst::Create(IfTrue: RetainedSuccBB, InsertBefore: ParentBB);
2515 NewBI->setDebugLoc(NewTI->getDebugLoc());
2516
2517 // After MSSAU update, remove the cloned terminator instruction NewTI.
2518 NewTI->eraseFromParent();
2519 } else {
2520 assert(BI && "Only branches have partial unswitching.");
2521 assert(UnswitchedSuccBBs.size() == 1 &&
2522 "Only one possible unswitched block for a branch!");
2523 BasicBlock *ClonedPH = ClonedPHs.begin()->second;
2524 // When doing a partial unswitch, we have to do a bit more work to build up
2525 // the branch in the split block.
2526 if (PartiallyInvariant)
2527 buildPartialInvariantUnswitchConditionalBranch(
2528 BB&: *SplitBB, ToDuplicate: Invariants, Direction, UnswitchedSucc&: *ClonedPH, NormalSucc&: *LoopPH, L, MSSAU, OriginalBranch: *BI);
2529 else {
2530 buildPartialUnswitchConditionalBranch(
2531 BB&: *SplitBB, Invariants, Direction, UnswitchedSucc&: *ClonedPH, NormalSucc&: *LoopPH,
2532 InsertFreeze: FreezeLoopUnswitchCond, I: BI, AC: &AC, DT, ComputeProfFrom: *BI);
2533 }
2534 DTUpdates.push_back(Elt: {DominatorTree::Insert, SplitBB, ClonedPH});
2535
2536 if (MSSAU) {
2537 DT.applyUpdates(Updates: DTUpdates);
2538 DTUpdates.clear();
2539
2540 // Perform MSSA cloning updates.
2541 for (auto &VMap : VMaps)
2542 MSSAU->updateForClonedLoop(LoopBlocks: LBRPO, ExitBlocks, VM: *VMap,
2543 /*IgnoreIncomingWithNoClones=*/true);
2544 MSSAU->updateExitBlocksForClonedLoop(ExitBlocks, VMaps, DT);
2545 }
2546 }
2547
2548 // Apply the updates accumulated above to get an up-to-date dominator tree.
2549 DT.applyUpdates(Updates: DTUpdates);
2550
2551 // Now that we have an accurate dominator tree, first delete the dead cloned
2552 // blocks so that we can accurately build any cloned loops. It is important to
2553 // not delete the blocks from the original loop yet because we still want to
2554 // reference the original loop to understand the cloned loop's structure.
2555 deleteDeadClonedBlocks(L, ExitBlocks, VMaps, DT, MSSAU);
2556
2557 // Build the cloned loop structure itself. This may be substantially
2558 // different from the original structure due to the simplified CFG. This also
2559 // handles inserting all the cloned blocks into the correct loops.
2560 SmallVector<Loop *, 4> NonChildClonedLoops;
2561 for (std::unique_ptr<ValueToValueMapTy> &VMap : VMaps)
2562 buildClonedLoops(OrigL&: L, ExitBlocks, VMap: *VMap, LI, NonChildClonedLoops);
2563
2564 // Now that our cloned loops have been built, we can update the original loop.
2565 // First we delete the dead blocks from it and then we rebuild the loop
2566 // structure taking these deletions into account.
2567 deleteDeadBlocksFromLoop(L, ExitBlocks, DT, LI, MSSAU, SE, LoopUpdater);
2568
2569 if (MSSAU && VerifyMemorySSA)
2570 MSSAU->getMemorySSA()->verifyMemorySSA();
2571
2572 SmallVector<Loop *, 4> HoistedLoops;
2573 bool IsStillLoop =
2574 rebuildLoopAfterUnswitch(L, ExitBlocks, LI, HoistedLoops, SE);
2575
2576 if (MSSAU && VerifyMemorySSA)
2577 MSSAU->getMemorySSA()->verifyMemorySSA();
2578
2579 // This transformation has a high risk of corrupting the dominator tree, and
2580 // the below steps to rebuild loop structures will result in hard to debug
2581 // errors in that case so verify that the dominator tree is sane first.
2582 // FIXME: Remove this when the bugs stop showing up and rely on existing
2583 // verification steps.
2584 assert(DT.verify(DominatorTree::VerificationLevel::Fast));
2585
2586 if (BI && !PartiallyInvariant) {
2587 // If we unswitched a branch which collapses the condition to a known
2588 // constant we want to replace all the uses of the invariants within both
2589 // the original and cloned blocks. We do this here so that we can use the
2590 // now updated dominator tree to identify which side the users are on.
2591 assert(UnswitchedSuccBBs.size() == 1 &&
2592 "Only one possible unswitched block for a branch!");
2593 BasicBlock *ClonedPH = ClonedPHs.begin()->second;
2594
2595 // When considering multiple partially-unswitched invariants
2596 // we cant just go replace them with constants in both branches.
2597 //
2598 // For 'AND' we infer that true branch ("continue") means true
2599 // for each invariant operand.
2600 // For 'OR' we can infer that false branch ("continue") means false
2601 // for each invariant operand.
2602 // So it happens that for multiple-partial case we dont replace
2603 // in the unswitched branch.
2604 bool ReplaceUnswitched =
2605 FullUnswitch || (Invariants.size() == 1) || PartiallyInvariant;
2606
2607 ConstantInt *UnswitchedReplacement =
2608 Direction ? ConstantInt::getTrue(Context&: BI->getContext())
2609 : ConstantInt::getFalse(Context&: BI->getContext());
2610 ConstantInt *ContinueReplacement =
2611 Direction ? ConstantInt::getFalse(Context&: BI->getContext())
2612 : ConstantInt::getTrue(Context&: BI->getContext());
2613 for (Value *Invariant : Invariants) {
2614 assert(!isa<Constant>(Invariant) &&
2615 "Should not be replacing constant values!");
2616 // Use make_early_inc_range here as set invalidates the iterator.
2617 for (Use &U : llvm::make_early_inc_range(Range: Invariant->uses())) {
2618 Instruction *UserI = dyn_cast<Instruction>(Val: U.getUser());
2619 if (!UserI)
2620 continue;
2621
2622 // Replace it with the 'continue' side if in the main loop body, and the
2623 // unswitched if in the cloned blocks.
2624 if (DT.dominates(A: LoopPH, B: UserI->getParent()))
2625 U.set(ContinueReplacement);
2626 else if (ReplaceUnswitched &&
2627 DT.dominates(A: ClonedPH, B: UserI->getParent()))
2628 U.set(UnswitchedReplacement);
2629 }
2630 }
2631 }
2632
2633 // We can change which blocks are exit blocks of all the cloned sibling
2634 // loops, the current loop, and any parent loops which shared exit blocks
2635 // with the current loop. As a consequence, we need to re-form LCSSA for
2636 // them. But we shouldn't need to re-form LCSSA for any child loops.
2637 // FIXME: This could be made more efficient by tracking which exit blocks are
2638 // new, and focusing on them, but that isn't likely to be necessary.
2639 //
2640 // In order to reasonably rebuild LCSSA we need to walk inside-out across the
2641 // loop nest and update every loop that could have had its exits changed. We
2642 // also need to cover any intervening loops. We add all of these loops to
2643 // a list and sort them by loop depth to achieve this without updating
2644 // unnecessary loops.
2645 auto UpdateLoop = [&](Loop &UpdateL) {
2646#ifndef NDEBUG
2647 UpdateL.verifyLoop();
2648 for (Loop *ChildL : UpdateL) {
2649 ChildL->verifyLoop();
2650 assert(ChildL->isRecursivelyLCSSAForm(DT, LI) &&
2651 "Perturbed a child loop's LCSSA form!");
2652 }
2653#endif
2654 // First build LCSSA for this loop so that we can preserve it when
2655 // forming dedicated exits. We don't want to perturb some other loop's
2656 // LCSSA while doing that CFG edit.
2657 formLCSSA(L&: UpdateL, DT, LI: &LI, SE);
2658
2659 // For loops reached by this loop's original exit blocks we may
2660 // introduced new, non-dedicated exits. At least try to re-form dedicated
2661 // exits for these loops. This may fail if they couldn't have dedicated
2662 // exits to start with.
2663 formDedicatedExitBlocks(L: &UpdateL, DT: &DT, LI: &LI, MSSAU, /*PreserveLCSSA*/ true);
2664 };
2665
2666 // For non-child cloned loops and hoisted loops, we just need to update LCSSA
2667 // and we can do it in any order as they don't nest relative to each other.
2668 //
2669 // Also check if any of the loops we have updated have become top-level loops
2670 // as that will necessitate widening the outer loop scope.
2671 for (Loop *UpdatedL :
2672 llvm::concat<Loop *>(Ranges&: NonChildClonedLoops, Ranges&: HoistedLoops)) {
2673 UpdateLoop(*UpdatedL);
2674 if (UpdatedL->isOutermost())
2675 OuterExitL = nullptr;
2676 }
2677 if (IsStillLoop) {
2678 UpdateLoop(L);
2679 if (L.isOutermost())
2680 OuterExitL = nullptr;
2681 }
2682
2683 // If the original loop had exit blocks, walk up through the outer most loop
2684 // of those exit blocks to update LCSSA and form updated dedicated exits.
2685 if (OuterExitL != &L)
2686 for (Loop *OuterL = ParentL; OuterL != OuterExitL;
2687 OuterL = OuterL->getParentLoop())
2688 UpdateLoop(*OuterL);
2689
2690#ifndef NDEBUG
2691 // Verify the entire loop structure to catch any incorrect updates before we
2692 // progress in the pass pipeline.
2693 LI.verify(DT);
2694#endif
2695
2696 // Now that we've unswitched something, make callbacks to report the changes.
2697 // For that we need to merge together the updated loops and the cloned loops
2698 // and check whether the original loop survived.
2699 SmallVector<Loop *, 4> SibLoops;
2700 for (Loop *UpdatedL : llvm::concat<Loop *>(Ranges&: NonChildClonedLoops, Ranges&: HoistedLoops))
2701 if (UpdatedL->getParentLoop() == ParentL)
2702 SibLoops.push_back(Elt: UpdatedL);
2703 postUnswitch(L, U&: LoopUpdater, LoopName, CurrentLoopValid: IsStillLoop, PartiallyInvariant,
2704 InjectedCondition, NewLoops: SibLoops);
2705
2706 if (MSSAU && VerifyMemorySSA)
2707 MSSAU->getMemorySSA()->verifyMemorySSA();
2708
2709 if (BI)
2710 ++NumBranches;
2711 else
2712 ++NumSwitches;
2713}
2714
2715/// Recursively compute the cost of a dominator subtree based on the per-block
2716/// cost map provided.
2717///
2718/// The recursive computation is memozied into the provided DT-indexed cost map
2719/// to allow querying it for most nodes in the domtree without it becoming
2720/// quadratic.
2721static InstructionCost computeDomSubtreeCost(
2722 DomTreeNode &N,
2723 const SmallDenseMap<BasicBlock *, InstructionCost, 4> &BBCostMap,
2724 SmallDenseMap<DomTreeNode *, InstructionCost, 4> &DTCostMap) {
2725 // Don't accumulate cost (or recurse through) blocks not in our block cost
2726 // map and thus not part of the duplication cost being considered.
2727 auto BBCostIt = BBCostMap.find(Val: N.getBlock());
2728 if (BBCostIt == BBCostMap.end())
2729 return 0;
2730
2731 // Lookup this node to see if we already computed its cost.
2732 auto DTCostIt = DTCostMap.find(Val: &N);
2733 if (DTCostIt != DTCostMap.end())
2734 return DTCostIt->second;
2735
2736 // If not, we have to compute it. We can't use insert above and update
2737 // because computing the cost may insert more things into the map.
2738 InstructionCost Cost = std::accumulate(
2739 first: N.begin(), last: N.end(), init: BBCostIt->second,
2740 binary_op: [&](InstructionCost Sum, DomTreeNode *ChildN) -> InstructionCost {
2741 return Sum + computeDomSubtreeCost(N&: *ChildN, BBCostMap, DTCostMap);
2742 });
2743 bool Inserted = DTCostMap.insert(KV: {&N, Cost}).second;
2744 (void)Inserted;
2745 assert(Inserted && "Should not insert a node while visiting children!");
2746 return Cost;
2747}
2748
2749/// Turns a select instruction into implicit control flow branch,
2750/// making the following replacement:
2751///
2752/// head:
2753/// --code before select--
2754/// select %cond, %trueval, %falseval
2755/// --code after select--
2756///
2757/// into
2758///
2759/// head:
2760/// --code before select--
2761/// br i1 %cond, label %then, label %tail
2762///
2763/// then:
2764/// br %tail
2765///
2766/// tail:
2767/// phi [ %trueval, %then ], [ %falseval, %head]
2768/// unreachable
2769///
2770/// It also makes all relevant DT and LI updates, so that all structures are in
2771/// valid state after this transform.
2772static BranchInst *turnSelectIntoBranch(SelectInst *SI, DominatorTree &DT,
2773 LoopInfo &LI, MemorySSAUpdater *MSSAU,
2774 AssumptionCache *AC) {
2775 LLVM_DEBUG(dbgs() << "Turning " << *SI << " into a branch.\n");
2776 BasicBlock *HeadBB = SI->getParent();
2777
2778 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
2779 SplitBlockAndInsertIfThen(Cond: SI->getCondition(), SplitBefore: SI, Unreachable: false,
2780 BranchWeights: SI->getMetadata(KindID: LLVMContext::MD_prof), DTU: &DTU, LI: &LI);
2781 auto *CondBr = cast<BranchInst>(Val: HeadBB->getTerminator());
2782 BasicBlock *ThenBB = CondBr->getSuccessor(i: 0),
2783 *TailBB = CondBr->getSuccessor(i: 1);
2784 if (MSSAU)
2785 MSSAU->moveAllAfterSpliceBlocks(From: HeadBB, To: TailBB, Start: SI);
2786
2787 PHINode *Phi =
2788 PHINode::Create(Ty: SI->getType(), NumReservedValues: 2, NameStr: "unswitched.select", InsertBefore: SI->getIterator());
2789 Phi->addIncoming(V: SI->getTrueValue(), BB: ThenBB);
2790 Phi->addIncoming(V: SI->getFalseValue(), BB: HeadBB);
2791 Phi->setDebugLoc(SI->getDebugLoc());
2792 SI->replaceAllUsesWith(V: Phi);
2793 SI->eraseFromParent();
2794
2795 if (MSSAU && VerifyMemorySSA)
2796 MSSAU->getMemorySSA()->verifyMemorySSA();
2797
2798 ++NumSelects;
2799 return CondBr;
2800}
2801
2802/// Turns a llvm.experimental.guard intrinsic into implicit control flow branch,
2803/// making the following replacement:
2804///
2805/// --code before guard--
2806/// call void (i1, ...) @llvm.experimental.guard(i1 %cond) [ "deopt"() ]
2807/// --code after guard--
2808///
2809/// into
2810///
2811/// --code before guard--
2812/// br i1 %cond, label %guarded, label %deopt
2813///
2814/// guarded:
2815/// --code after guard--
2816///
2817/// deopt:
2818/// call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
2819/// unreachable
2820///
2821/// It also makes all relevant DT and LI updates, so that all structures are in
2822/// valid state after this transform.
2823static BranchInst *turnGuardIntoBranch(IntrinsicInst *GI, Loop &L,
2824 DominatorTree &DT, LoopInfo &LI,
2825 MemorySSAUpdater *MSSAU) {
2826 LLVM_DEBUG(dbgs() << "Turning " << *GI << " into a branch.\n");
2827 BasicBlock *CheckBB = GI->getParent();
2828
2829 if (MSSAU && VerifyMemorySSA)
2830 MSSAU->getMemorySSA()->verifyMemorySSA();
2831
2832 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
2833 // llvm.experimental.guard doesn't have branch weights. We can assume,
2834 // however, that the deopt path is unlikely.
2835 Instruction *DeoptBlockTerm = SplitBlockAndInsertIfThen(
2836 Cond: GI->getArgOperand(i: 0), SplitBefore: GI, Unreachable: true,
2837 BranchWeights: !ProfcheckDisableMetadataFixes && EstimateProfile
2838 ? MDBuilder(GI->getContext()).createUnlikelyBranchWeights()
2839 : nullptr,
2840 DTU: &DTU, LI: &LI);
2841 BranchInst *CheckBI = cast<BranchInst>(Val: CheckBB->getTerminator());
2842 // SplitBlockAndInsertIfThen inserts control flow that branches to
2843 // DeoptBlockTerm if the condition is true. We want the opposite.
2844 CheckBI->swapSuccessors();
2845
2846 BasicBlock *GuardedBlock = CheckBI->getSuccessor(i: 0);
2847 GuardedBlock->setName("guarded");
2848 CheckBI->getSuccessor(i: 1)->setName("deopt");
2849 BasicBlock *DeoptBlock = CheckBI->getSuccessor(i: 1);
2850
2851 if (MSSAU)
2852 MSSAU->moveAllAfterSpliceBlocks(From: CheckBB, To: GuardedBlock, Start: GI);
2853
2854 GI->moveBefore(InsertPos: DeoptBlockTerm->getIterator());
2855 GI->setArgOperand(i: 0, v: ConstantInt::getFalse(Context&: GI->getContext()));
2856
2857 if (MSSAU) {
2858 MemoryDef *MD = cast<MemoryDef>(Val: MSSAU->getMemorySSA()->getMemoryAccess(I: GI));
2859 MSSAU->moveToPlace(What: MD, BB: DeoptBlock, Where: MemorySSA::BeforeTerminator);
2860 if (VerifyMemorySSA)
2861 MSSAU->getMemorySSA()->verifyMemorySSA();
2862 }
2863
2864 if (VerifyLoopInfo)
2865 LI.verify(DomTree: DT);
2866 ++NumGuards;
2867 return CheckBI;
2868}
2869
2870/// Cost multiplier is a way to limit potentially exponential behavior
2871/// of loop-unswitch. Cost is multiplied in proportion of 2^number of unswitch
2872/// candidates available. Also consider the number of "sibling" loops with
2873/// the idea of accounting for previous unswitches that already happened on this
2874/// cluster of loops. There was an attempt to keep this formula simple,
2875/// just enough to limit the worst case behavior. Even if it is not that simple
2876/// now it is still not an attempt to provide a detailed heuristic size
2877/// prediction.
2878///
2879/// TODO: Make a proper accounting of "explosion" effect for all kinds of
2880/// unswitch candidates, making adequate predictions instead of wild guesses.
2881/// That requires knowing not just the number of "remaining" candidates but
2882/// also costs of unswitching for each of these candidates.
2883static int CalculateUnswitchCostMultiplier(
2884 const Instruction &TI, const Loop &L, const LoopInfo &LI,
2885 const DominatorTree &DT,
2886 ArrayRef<NonTrivialUnswitchCandidate> UnswitchCandidates) {
2887
2888 // Guards and other exiting conditions do not contribute to exponential
2889 // explosion as soon as they dominate the latch (otherwise there might be
2890 // another path to the latch remaining that does not allow to eliminate the
2891 // loop copy on unswitch).
2892 const BasicBlock *Latch = L.getLoopLatch();
2893 const BasicBlock *CondBlock = TI.getParent();
2894 if (DT.dominates(A: CondBlock, B: Latch) &&
2895 (isGuard(U: &TI) ||
2896 (TI.isTerminator() &&
2897 llvm::count_if(Range: successors(I: &TI), P: [&L](const BasicBlock *SuccBB) {
2898 return L.contains(BB: SuccBB);
2899 }) <= 1))) {
2900 NumCostMultiplierSkipped++;
2901 return 1;
2902 }
2903
2904 // Each invariant non-trivial condition, after being unswitched, is supposed
2905 // to have its own specialized sibling loop (the invariant condition has been
2906 // hoisted out of the child loop into a newly-cloned loop). When unswitching
2907 // conditions in nested loops, the basic block size of the outer loop should
2908 // not be altered. If such a size significantly increases across unswitching
2909 // invocations, something may be wrong; so adjust the final cost taking this
2910 // into account.
2911 auto *ParentL = L.getParentLoop();
2912 int ParentLoopSizeMultiplier = 1;
2913 if (ParentL)
2914 ParentLoopSizeMultiplier =
2915 std::max<int>(a: ParentL->getNumBlocks() / UnswitchParentBlocksDiv, b: 1);
2916
2917 int SiblingsCount =
2918 (ParentL ? ParentL->getSubLoopsVector().size() : llvm::size(Range: LI));
2919 // Count amount of clones that all the candidates might cause during
2920 // unswitching. Branch/guard/select counts as 1, switch counts as log2 of its
2921 // cases.
2922 int UnswitchedClones = 0;
2923 for (const auto &Candidate : UnswitchCandidates) {
2924 const Instruction *CI = Candidate.TI;
2925 const BasicBlock *CondBlock = CI->getParent();
2926 bool SkipExitingSuccessors = DT.dominates(A: CondBlock, B: Latch);
2927 if (isa<SelectInst>(Val: CI)) {
2928 UnswitchedClones++;
2929 continue;
2930 }
2931 if (isGuard(U: CI)) {
2932 if (!SkipExitingSuccessors)
2933 UnswitchedClones++;
2934 continue;
2935 }
2936 int NonExitingSuccessors =
2937 llvm::count_if(Range: successors(BB: CondBlock),
2938 P: [SkipExitingSuccessors, &L](const BasicBlock *SuccBB) {
2939 return !SkipExitingSuccessors || L.contains(BB: SuccBB);
2940 });
2941 UnswitchedClones += Log2_32(Value: NonExitingSuccessors);
2942 }
2943
2944 // Ignore up to the "unscaled candidates" number of unswitch candidates
2945 // when calculating the power-of-two scaling of the cost. The main idea
2946 // with this control is to allow a small number of unswitches to happen
2947 // and rely more on siblings multiplier (see below) when the number
2948 // of candidates is small.
2949 unsigned ClonesPower =
2950 std::max(a: UnswitchedClones - (int)UnswitchNumInitialUnscaledCandidates, b: 0);
2951
2952 // Allowing top-level loops to spread a bit more than nested ones.
2953 int SiblingsMultiplier =
2954 std::max(a: (ParentL ? SiblingsCount
2955 : SiblingsCount / (int)UnswitchSiblingsToplevelDiv),
2956 b: 1);
2957 // Compute the cost multiplier in a way that won't overflow by saturating
2958 // at an upper bound.
2959 int CostMultiplier;
2960 if (ClonesPower > Log2_32(Value: UnswitchThreshold) ||
2961 SiblingsMultiplier > UnswitchThreshold ||
2962 ParentLoopSizeMultiplier > UnswitchThreshold)
2963 CostMultiplier = UnswitchThreshold;
2964 else
2965 CostMultiplier = std::min(a: SiblingsMultiplier * (1 << ClonesPower),
2966 b: (int)UnswitchThreshold);
2967
2968 LLVM_DEBUG(dbgs() << " Computed multiplier " << CostMultiplier
2969 << " (siblings " << SiblingsMultiplier << " * parent size "
2970 << ParentLoopSizeMultiplier << " * clones "
2971 << (1 << ClonesPower) << ")"
2972 << " for unswitch candidate: " << TI << "\n");
2973 return CostMultiplier;
2974}
2975
2976static bool collectUnswitchCandidates(
2977 SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates,
2978 IVConditionInfo &PartialIVInfo, Instruction *&PartialIVCondBranch,
2979 const Loop &L, const LoopInfo &LI, AAResults &AA,
2980 const MemorySSAUpdater *MSSAU) {
2981 assert(UnswitchCandidates.empty() && "Should be!");
2982
2983 auto AddUnswitchCandidatesForInst = [&](Instruction *I, Value *Cond) {
2984 Cond = skipTrivialSelect(Cond);
2985 if (isa<Constant>(Val: Cond))
2986 return;
2987 if (L.isLoopInvariant(V: Cond)) {
2988 UnswitchCandidates.push_back(Elt: {I, {Cond}});
2989 return;
2990 }
2991 if (match(V: Cond, P: m_CombineOr(L: m_LogicalAnd(), R: m_LogicalOr()))) {
2992 TinyPtrVector<Value *> Invariants =
2993 collectHomogenousInstGraphLoopInvariants(
2994 L, Root&: *static_cast<Instruction *>(Cond), LI);
2995 if (!Invariants.empty())
2996 UnswitchCandidates.push_back(Elt: {I, std::move(Invariants)});
2997 }
2998 };
2999
3000 // Whether or not we should also collect guards in the loop.
3001 bool CollectGuards = false;
3002 if (UnswitchGuards) {
3003 auto *GuardDecl = Intrinsic::getDeclarationIfExists(
3004 M: L.getHeader()->getParent()->getParent(), id: Intrinsic::experimental_guard);
3005 if (GuardDecl && !GuardDecl->use_empty())
3006 CollectGuards = true;
3007 }
3008
3009 for (auto *BB : L.blocks()) {
3010 if (LI.getLoopFor(BB) != &L)
3011 continue;
3012
3013 for (auto &I : *BB) {
3014 if (auto *SI = dyn_cast<SelectInst>(Val: &I)) {
3015 auto *Cond = SI->getCondition();
3016 // Do not unswitch vector selects and logical and/or selects
3017 if (Cond->getType()->isIntegerTy(Bitwidth: 1) && !SI->getType()->isIntegerTy(Bitwidth: 1))
3018 AddUnswitchCandidatesForInst(SI, Cond);
3019 } else if (CollectGuards && isGuard(U: &I)) {
3020 auto *Cond =
3021 skipTrivialSelect(Cond: cast<IntrinsicInst>(Val: &I)->getArgOperand(i: 0));
3022 // TODO: Support AND, OR conditions and partial unswitching.
3023 if (!isa<Constant>(Val: Cond) && L.isLoopInvariant(V: Cond))
3024 UnswitchCandidates.push_back(Elt: {&I, {Cond}});
3025 }
3026 }
3027
3028 if (auto *SI = dyn_cast<SwitchInst>(Val: BB->getTerminator())) {
3029 // We can only consider fully loop-invariant switch conditions as we need
3030 // to completely eliminate the switch after unswitching.
3031 if (!isa<Constant>(Val: SI->getCondition()) &&
3032 L.isLoopInvariant(V: SI->getCondition()) && !BB->getUniqueSuccessor())
3033 UnswitchCandidates.push_back(Elt: {SI, {SI->getCondition()}});
3034 continue;
3035 }
3036
3037 auto *BI = dyn_cast<BranchInst>(Val: BB->getTerminator());
3038 if (!BI || !BI->isConditional() ||
3039 BI->getSuccessor(i: 0) == BI->getSuccessor(i: 1))
3040 continue;
3041
3042 AddUnswitchCandidatesForInst(BI, BI->getCondition());
3043 }
3044
3045 if (MSSAU && !findOptionMDForLoop(TheLoop: &L, Name: "llvm.loop.unswitch.partial.disable") &&
3046 !any_of(Range&: UnswitchCandidates, P: [&L](auto &TerminatorAndInvariants) {
3047 return TerminatorAndInvariants.TI == L.getHeader()->getTerminator();
3048 })) {
3049 MemorySSA *MSSA = MSSAU->getMemorySSA();
3050 if (auto Info = hasPartialIVCondition(L, MSSAThreshold, MSSA: *MSSA, AA)) {
3051 LLVM_DEBUG(
3052 dbgs() << "simple-loop-unswitch: Found partially invariant condition "
3053 << *Info->InstToDuplicate[0] << "\n");
3054 PartialIVInfo = *Info;
3055 PartialIVCondBranch = L.getHeader()->getTerminator();
3056 TinyPtrVector<Value *> ValsToDuplicate;
3057 llvm::append_range(C&: ValsToDuplicate, R&: Info->InstToDuplicate);
3058 UnswitchCandidates.push_back(
3059 Elt: {L.getHeader()->getTerminator(), std::move(ValsToDuplicate)});
3060 }
3061 }
3062 return !UnswitchCandidates.empty();
3063}
3064
3065/// Tries to canonicalize condition described by:
3066///
3067/// br (LHS pred RHS), label IfTrue, label IfFalse
3068///
3069/// into its equivalent where `Pred` is something that we support for injected
3070/// invariants (so far it is limited to ult), LHS in canonicalized form is
3071/// non-invariant and RHS is an invariant.
3072static void canonicalizeForInvariantConditionInjection(CmpPredicate &Pred,
3073 Value *&LHS, Value *&RHS,
3074 BasicBlock *&IfTrue,
3075 BasicBlock *&IfFalse,
3076 const Loop &L) {
3077 if (!L.contains(BB: IfTrue)) {
3078 Pred = ICmpInst::getInversePredicate(pred: Pred);
3079 std::swap(a&: IfTrue, b&: IfFalse);
3080 }
3081
3082 // Move loop-invariant argument to RHS position.
3083 if (L.isLoopInvariant(V: LHS)) {
3084 Pred = ICmpInst::getSwappedPredicate(pred: Pred);
3085 std::swap(a&: LHS, b&: RHS);
3086 }
3087
3088 if (Pred == ICmpInst::ICMP_SGE && match(V: RHS, P: m_Zero())) {
3089 // Turn "x >=s 0" into "x <u UMIN_INT"
3090 Pred = ICmpInst::ICMP_ULT;
3091 RHS = ConstantInt::get(
3092 Context&: RHS->getContext(),
3093 V: APInt::getSignedMinValue(numBits: RHS->getType()->getIntegerBitWidth()));
3094 }
3095}
3096
3097/// Returns true, if predicate described by ( \p Pred, \p LHS, \p RHS )
3098/// succeeding into blocks ( \p IfTrue, \p IfFalse) can be optimized by
3099/// injecting a loop-invariant condition.
3100static bool shouldTryInjectInvariantCondition(
3101 const ICmpInst::Predicate Pred, const Value *LHS, const Value *RHS,
3102 const BasicBlock *IfTrue, const BasicBlock *IfFalse, const Loop &L) {
3103 if (L.isLoopInvariant(V: LHS) || !L.isLoopInvariant(V: RHS))
3104 return false;
3105 // TODO: Support other predicates.
3106 if (Pred != ICmpInst::ICMP_ULT)
3107 return false;
3108 // TODO: Support non-loop-exiting branches?
3109 if (!L.contains(BB: IfTrue) || L.contains(BB: IfFalse))
3110 return false;
3111 // FIXME: For some reason this causes problems with MSSA updates, need to
3112 // investigate why. So far, just don't unswitch latch.
3113 if (L.getHeader() == IfTrue)
3114 return false;
3115 return true;
3116}
3117
3118/// Returns true, if metadata on \p BI allows us to optimize branching into \p
3119/// TakenSucc via injection of invariant conditions. The branch should be not
3120/// enough and not previously unswitched, the information about this comes from
3121/// the metadata.
3122bool shouldTryInjectBasingOnMetadata(const BranchInst *BI,
3123 const BasicBlock *TakenSucc) {
3124 SmallVector<uint32_t> Weights;
3125 if (!extractBranchWeights(I: *BI, Weights))
3126 return false;
3127 unsigned T = InjectInvariantConditionHotnesThreshold;
3128 BranchProbability LikelyTaken(T - 1, T);
3129
3130 assert(Weights.size() == 2 && "Unexpected profile data!");
3131 size_t Idx = BI->getSuccessor(i: 0) == TakenSucc ? 0 : 1;
3132 auto Num = Weights[Idx];
3133 auto Denom = Weights[0] + Weights[1];
3134 // Degenerate or overflowed metadata.
3135 if (Denom == 0 || Num > Denom)
3136 return false;
3137 BranchProbability ActualTaken(Num, Denom);
3138 if (LikelyTaken > ActualTaken)
3139 return false;
3140 return true;
3141}
3142
3143/// Materialize pending invariant condition of the given candidate into IR. The
3144/// injected loop-invariant condition implies the original loop-variant branch
3145/// condition, so the materialization turns
3146///
3147/// loop_block:
3148/// ...
3149/// br i1 %variant_cond, label InLoopSucc, label OutOfLoopSucc
3150///
3151/// into
3152///
3153/// preheader:
3154/// %invariant_cond = LHS pred RHS
3155/// ...
3156/// loop_block:
3157/// br i1 %invariant_cond, label InLoopSucc, label OriginalCheck
3158/// OriginalCheck:
3159/// br i1 %variant_cond, label InLoopSucc, label OutOfLoopSucc
3160/// ...
3161static NonTrivialUnswitchCandidate
3162injectPendingInvariantConditions(NonTrivialUnswitchCandidate Candidate, Loop &L,
3163 DominatorTree &DT, LoopInfo &LI,
3164 AssumptionCache &AC, MemorySSAUpdater *MSSAU) {
3165 assert(Candidate.hasPendingInjection() && "Nothing to inject!");
3166 BasicBlock *Preheader = L.getLoopPreheader();
3167 assert(Preheader && "Loop is not in simplified form?");
3168 assert(LI.getLoopFor(Candidate.TI->getParent()) == &L &&
3169 "Unswitching branch of inner loop!");
3170
3171 auto Pred = Candidate.PendingInjection->Pred;
3172 auto *LHS = Candidate.PendingInjection->LHS;
3173 auto *RHS = Candidate.PendingInjection->RHS;
3174 auto *InLoopSucc = Candidate.PendingInjection->InLoopSucc;
3175 auto *TI = cast<BranchInst>(Val: Candidate.TI);
3176 auto *BB = Candidate.TI->getParent();
3177 auto *OutOfLoopSucc = InLoopSucc == TI->getSuccessor(i: 0) ? TI->getSuccessor(i: 1)
3178 : TI->getSuccessor(i: 0);
3179 // FIXME: Remove this once limitation on successors is lifted.
3180 assert(L.contains(InLoopSucc) && "Not supported yet!");
3181 assert(!L.contains(OutOfLoopSucc) && "Not supported yet!");
3182 auto &Ctx = BB->getContext();
3183
3184 IRBuilder<> Builder(Preheader->getTerminator());
3185 assert(ICmpInst::isUnsigned(Pred) && "Not supported yet!");
3186 if (LHS->getType() != RHS->getType()) {
3187 if (LHS->getType()->getIntegerBitWidth() <
3188 RHS->getType()->getIntegerBitWidth())
3189 LHS = Builder.CreateZExt(V: LHS, DestTy: RHS->getType(), Name: LHS->getName() + ".wide");
3190 else
3191 RHS = Builder.CreateZExt(V: RHS, DestTy: LHS->getType(), Name: RHS->getName() + ".wide");
3192 }
3193 // Do not use builder here: CreateICmp may simplify this into a constant and
3194 // unswitching will break. Better optimize it away later.
3195 auto *InjectedCond =
3196 ICmpInst::Create(Op: Instruction::ICmp, Pred, S1: LHS, S2: RHS, Name: "injected.cond",
3197 InsertBefore: Preheader->getTerminator()->getIterator());
3198
3199 BasicBlock *CheckBlock = BasicBlock::Create(Context&: Ctx, Name: BB->getName() + ".check",
3200 Parent: BB->getParent(), InsertBefore: InLoopSucc);
3201 Builder.SetInsertPoint(TI);
3202 auto *InvariantBr =
3203 Builder.CreateCondBr(Cond: InjectedCond, True: InLoopSucc, False: CheckBlock);
3204 // We don't know anything about the relation between the limits.
3205 setExplicitlyUnknownBranchWeightsIfProfiled(I&: *InvariantBr, DEBUG_TYPE);
3206
3207 Builder.SetInsertPoint(CheckBlock);
3208 Builder.CreateCondBr(
3209 Cond: TI->getCondition(), True: TI->getSuccessor(i: 0), False: TI->getSuccessor(i: 1),
3210 BranchWeights: !ProfcheckDisableMetadataFixes ? TI->getMetadata(KindID: LLVMContext::MD_prof)
3211 : nullptr);
3212 TI->eraseFromParent();
3213
3214 // Fixup phis.
3215 for (auto &I : *InLoopSucc) {
3216 auto *PN = dyn_cast<PHINode>(Val: &I);
3217 if (!PN)
3218 break;
3219 auto *Inc = PN->getIncomingValueForBlock(BB);
3220 PN->addIncoming(V: Inc, BB: CheckBlock);
3221 }
3222 OutOfLoopSucc->replacePhiUsesWith(Old: BB, New: CheckBlock);
3223
3224 SmallVector<DominatorTree::UpdateType, 4> DTUpdates = {
3225 { DominatorTree::Insert, BB, CheckBlock },
3226 { DominatorTree::Insert, CheckBlock, InLoopSucc },
3227 { DominatorTree::Insert, CheckBlock, OutOfLoopSucc },
3228 { DominatorTree::Delete, BB, OutOfLoopSucc }
3229 };
3230
3231 DT.applyUpdates(Updates: DTUpdates);
3232 if (MSSAU)
3233 MSSAU->applyUpdates(Updates: DTUpdates, DT);
3234 L.addBasicBlockToLoop(NewBB: CheckBlock, LI);
3235
3236#ifndef NDEBUG
3237 DT.verify();
3238 LI.verify(DT);
3239 if (MSSAU && VerifyMemorySSA)
3240 MSSAU->getMemorySSA()->verifyMemorySSA();
3241#endif
3242
3243 // TODO: In fact, cost of unswitching a new invariant candidate is *slightly*
3244 // higher because we have just inserted a new block. Need to think how to
3245 // adjust the cost of injected candidates when it was first computed.
3246 LLVM_DEBUG(dbgs() << "Injected a new loop-invariant branch " << *InvariantBr
3247 << " and considering it for unswitching.");
3248 ++NumInvariantConditionsInjected;
3249 return NonTrivialUnswitchCandidate(InvariantBr, { InjectedCond },
3250 Candidate.Cost);
3251}
3252
3253/// Given chain of loop branch conditions looking like:
3254/// br (Variant < Invariant1)
3255/// br (Variant < Invariant2)
3256/// br (Variant < Invariant3)
3257/// ...
3258/// collect set of invariant conditions on which we want to unswitch, which
3259/// look like:
3260/// Invariant1 <= Invariant2
3261/// Invariant2 <= Invariant3
3262/// ...
3263/// Though they might not immediately exist in the IR, we can still inject them.
3264static bool insertCandidatesWithPendingInjections(
3265 SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates, Loop &L,
3266 ICmpInst::Predicate Pred, ArrayRef<CompareDesc> Compares,
3267 const DominatorTree &DT) {
3268
3269 assert(ICmpInst::isRelational(Pred));
3270 assert(ICmpInst::isStrictPredicate(Pred));
3271 if (Compares.size() < 2)
3272 return false;
3273 ICmpInst::Predicate NonStrictPred = ICmpInst::getNonStrictPredicate(pred: Pred);
3274 for (auto Prev = Compares.begin(), Next = Compares.begin() + 1;
3275 Next != Compares.end(); ++Prev, ++Next) {
3276 Value *LHS = Next->Invariant;
3277 Value *RHS = Prev->Invariant;
3278 BasicBlock *InLoopSucc = Prev->InLoopSucc;
3279 InjectedInvariant ToInject(NonStrictPred, LHS, RHS, InLoopSucc);
3280 NonTrivialUnswitchCandidate Candidate(Prev->Term, { LHS, RHS },
3281 std::nullopt, std::move(ToInject));
3282 UnswitchCandidates.push_back(Elt: std::move(Candidate));
3283 }
3284 return true;
3285}
3286
3287/// Collect unswitch candidates by invariant conditions that are not immediately
3288/// present in the loop. However, they can be injected into the code if we
3289/// decide it's profitable.
3290/// An example of such conditions is following:
3291///
3292/// for (...) {
3293/// x = load ...
3294/// if (! x <u C1) break;
3295/// if (! x <u C2) break;
3296/// <do something>
3297/// }
3298///
3299/// We can unswitch by condition "C1 <=u C2". If that is true, then "x <u C1 <=
3300/// C2" automatically implies "x <u C2", so we can get rid of one of
3301/// loop-variant checks in unswitched loop version.
3302static bool collectUnswitchCandidatesWithInjections(
3303 SmallVectorImpl<NonTrivialUnswitchCandidate> &UnswitchCandidates,
3304 IVConditionInfo &PartialIVInfo, Instruction *&PartialIVCondBranch, Loop &L,
3305 const DominatorTree &DT, const LoopInfo &LI, AAResults &AA,
3306 const MemorySSAUpdater *MSSAU) {
3307 if (!InjectInvariantConditions)
3308 return false;
3309
3310 if (!DT.isReachableFromEntry(A: L.getHeader()))
3311 return false;
3312 auto *Latch = L.getLoopLatch();
3313 // Need to have a single latch and a preheader.
3314 if (!Latch)
3315 return false;
3316 assert(L.getLoopPreheader() && "Must have a preheader!");
3317
3318 DenseMap<Value *, SmallVector<CompareDesc, 4> > CandidatesULT;
3319 // Traverse the conditions that dominate latch (and therefore dominate each
3320 // other).
3321 for (auto *DTN = DT.getNode(BB: Latch); L.contains(BB: DTN->getBlock());
3322 DTN = DTN->getIDom()) {
3323 CmpPredicate Pred;
3324 Value *LHS = nullptr, *RHS = nullptr;
3325 BasicBlock *IfTrue = nullptr, *IfFalse = nullptr;
3326 auto *BB = DTN->getBlock();
3327 // Ignore inner loops.
3328 if (LI.getLoopFor(BB) != &L)
3329 continue;
3330 auto *Term = BB->getTerminator();
3331 if (!match(V: Term, P: m_Br(C: m_ICmp(Pred, L: m_Value(V&: LHS), R: m_Value(V&: RHS)),
3332 T: m_BasicBlock(V&: IfTrue), F: m_BasicBlock(V&: IfFalse))))
3333 continue;
3334 if (!LHS->getType()->isIntegerTy())
3335 continue;
3336 canonicalizeForInvariantConditionInjection(Pred, LHS, RHS, IfTrue, IfFalse,
3337 L);
3338 if (!shouldTryInjectInvariantCondition(Pred, LHS, RHS, IfTrue, IfFalse, L))
3339 continue;
3340 if (!shouldTryInjectBasingOnMetadata(BI: cast<BranchInst>(Val: Term), TakenSucc: IfTrue))
3341 continue;
3342 // Strip ZEXT for unsigned predicate.
3343 // TODO: once signed predicates are supported, also strip SEXT.
3344 CompareDesc Desc(cast<BranchInst>(Val: Term), RHS, IfTrue);
3345 while (auto *Zext = dyn_cast<ZExtInst>(Val: LHS))
3346 LHS = Zext->getOperand(i_nocapture: 0);
3347 CandidatesULT[LHS].push_back(Elt: Desc);
3348 }
3349
3350 bool Found = false;
3351 for (auto &It : CandidatesULT)
3352 Found |= insertCandidatesWithPendingInjections(
3353 UnswitchCandidates, L, Pred: ICmpInst::ICMP_ULT, Compares: It.second, DT);
3354 return Found;
3355}
3356
3357static bool isSafeForNoNTrivialUnswitching(Loop &L, LoopInfo &LI) {
3358 if (!L.isSafeToClone())
3359 return false;
3360 for (auto *BB : L.blocks())
3361 for (auto &I : *BB) {
3362 if (I.getType()->isTokenTy() && I.isUsedOutsideOfBlock(BB))
3363 return false;
3364 if (auto *CB = dyn_cast<CallBase>(Val: &I)) {
3365 assert(!CB->cannotDuplicate() && "Checked by L.isSafeToClone().");
3366 if (CB->isConvergent())
3367 return false;
3368 }
3369 }
3370
3371 // Check if there are irreducible CFG cycles in this loop. If so, we cannot
3372 // easily unswitch non-trivial edges out of the loop. Doing so might turn the
3373 // irreducible control flow into reducible control flow and introduce new
3374 // loops "out of thin air". If we ever discover important use cases for doing
3375 // this, we can add support to loop unswitch, but it is a lot of complexity
3376 // for what seems little or no real world benefit.
3377 LoopBlocksRPO RPOT(&L);
3378 RPOT.perform(LI: &LI);
3379 if (containsIrreducibleCFG<const BasicBlock *>(RPOTraversal&: RPOT, LI))
3380 return false;
3381
3382 SmallVector<BasicBlock *, 4> ExitBlocks;
3383 L.getUniqueExitBlocks(ExitBlocks);
3384 // We cannot unswitch if exit blocks contain a cleanuppad/catchswitch
3385 // instruction as we don't know how to split those exit blocks.
3386 // FIXME: We should teach SplitBlock to handle this and remove this
3387 // restriction.
3388 for (auto *ExitBB : ExitBlocks) {
3389 auto It = ExitBB->getFirstNonPHIIt();
3390 if (isa<CleanupPadInst>(Val: It) || isa<CatchSwitchInst>(Val: It)) {
3391 LLVM_DEBUG(dbgs() << "Cannot unswitch because of cleanuppad/catchswitch "
3392 "in exit block\n");
3393 return false;
3394 }
3395 }
3396
3397 return true;
3398}
3399
3400static NonTrivialUnswitchCandidate findBestNonTrivialUnswitchCandidate(
3401 ArrayRef<NonTrivialUnswitchCandidate> UnswitchCandidates, const Loop &L,
3402 const DominatorTree &DT, const LoopInfo &LI, AssumptionCache &AC,
3403 const TargetTransformInfo &TTI, const IVConditionInfo &PartialIVInfo) {
3404 // Given that unswitching these terminators will require duplicating parts of
3405 // the loop, so we need to be able to model that cost. Compute the ephemeral
3406 // values and set up a data structure to hold per-BB costs. We cache each
3407 // block's cost so that we don't recompute this when considering different
3408 // subsets of the loop for duplication during unswitching.
3409 SmallPtrSet<const Value *, 4> EphValues;
3410 CodeMetrics::collectEphemeralValues(L: &L, AC: &AC, EphValues);
3411 SmallDenseMap<BasicBlock *, InstructionCost, 4> BBCostMap;
3412
3413 // Compute the cost of each block, as well as the total loop cost. Also, bail
3414 // out if we see instructions which are incompatible with loop unswitching
3415 // (convergent, noduplicate, or cross-basic-block tokens).
3416 // FIXME: We might be able to safely handle some of these in non-duplicated
3417 // regions.
3418 TargetTransformInfo::TargetCostKind CostKind =
3419 L.getHeader()->getParent()->hasMinSize()
3420 ? TargetTransformInfo::TCK_CodeSize
3421 : TargetTransformInfo::TCK_SizeAndLatency;
3422 InstructionCost LoopCost = 0;
3423 for (auto *BB : L.blocks()) {
3424 InstructionCost Cost = 0;
3425 for (auto &I : *BB) {
3426 if (EphValues.count(Ptr: &I))
3427 continue;
3428 Cost += TTI.getInstructionCost(U: &I, CostKind);
3429 }
3430 assert(Cost >= 0 && "Must not have negative costs!");
3431 LoopCost += Cost;
3432 assert(LoopCost >= 0 && "Must not have negative loop costs!");
3433 BBCostMap[BB] = Cost;
3434 }
3435 LLVM_DEBUG(dbgs() << " Total loop cost: " << LoopCost << "\n");
3436
3437 // Now we find the best candidate by searching for the one with the following
3438 // properties in order:
3439 //
3440 // 1) An unswitching cost below the threshold
3441 // 2) The smallest number of duplicated unswitch candidates (to avoid
3442 // creating redundant subsequent unswitching)
3443 // 3) The smallest cost after unswitching.
3444 //
3445 // We prioritize reducing fanout of unswitch candidates provided the cost
3446 // remains below the threshold because this has a multiplicative effect.
3447 //
3448 // This requires memoizing each dominator subtree to avoid redundant work.
3449 //
3450 // FIXME: Need to actually do the number of candidates part above.
3451 SmallDenseMap<DomTreeNode *, InstructionCost, 4> DTCostMap;
3452 // Given a terminator which might be unswitched, computes the non-duplicated
3453 // cost for that terminator.
3454 auto ComputeUnswitchedCost = [&](Instruction &TI,
3455 bool FullUnswitch) -> InstructionCost {
3456 // Unswitching selects unswitches the entire loop.
3457 if (isa<SelectInst>(Val: TI))
3458 return LoopCost;
3459
3460 BasicBlock &BB = *TI.getParent();
3461 SmallPtrSet<BasicBlock *, 4> Visited;
3462
3463 InstructionCost Cost = 0;
3464 for (BasicBlock *SuccBB : successors(BB: &BB)) {
3465 // Don't count successors more than once.
3466 if (!Visited.insert(Ptr: SuccBB).second)
3467 continue;
3468
3469 // If this is a partial unswitch candidate, then it must be a conditional
3470 // branch with a condition of either `or`, `and`, their corresponding
3471 // select forms or partially invariant instructions. In that case, one of
3472 // the successors is necessarily duplicated, so don't even try to remove
3473 // its cost.
3474 if (!FullUnswitch) {
3475 auto &BI = cast<BranchInst>(Val&: TI);
3476 Value *Cond = skipTrivialSelect(Cond: BI.getCondition());
3477 if (match(V: Cond, P: m_LogicalAnd())) {
3478 if (SuccBB == BI.getSuccessor(i: 1))
3479 continue;
3480 } else if (match(V: Cond, P: m_LogicalOr())) {
3481 if (SuccBB == BI.getSuccessor(i: 0))
3482 continue;
3483 } else if ((PartialIVInfo.KnownValue->isOneValue() &&
3484 SuccBB == BI.getSuccessor(i: 0)) ||
3485 (!PartialIVInfo.KnownValue->isOneValue() &&
3486 SuccBB == BI.getSuccessor(i: 1)))
3487 continue;
3488 }
3489
3490 // This successor's domtree will not need to be duplicated after
3491 // unswitching if the edge to the successor dominates it (and thus the
3492 // entire tree). This essentially means there is no other path into this
3493 // subtree and so it will end up live in only one clone of the loop.
3494 if (SuccBB->getUniquePredecessor() ||
3495 llvm::all_of(Range: predecessors(BB: SuccBB), P: [&](BasicBlock *PredBB) {
3496 return PredBB == &BB || DT.dominates(A: SuccBB, B: PredBB);
3497 })) {
3498 Cost += computeDomSubtreeCost(N&: *DT[SuccBB], BBCostMap, DTCostMap);
3499 assert(Cost <= LoopCost &&
3500 "Non-duplicated cost should never exceed total loop cost!");
3501 }
3502 }
3503
3504 // Now scale the cost by the number of unique successors minus one. We
3505 // subtract one because there is already at least one copy of the entire
3506 // loop. This is computing the new cost of unswitching a condition.
3507 // Note that guards always have 2 unique successors that are implicit and
3508 // will be materialized if we decide to unswitch it.
3509 int SuccessorsCount = isGuard(U: &TI) ? 2 : Visited.size();
3510 assert(SuccessorsCount > 1 &&
3511 "Cannot unswitch a condition without multiple distinct successors!");
3512 return (LoopCost - Cost) * (SuccessorsCount - 1);
3513 };
3514
3515 std::optional<NonTrivialUnswitchCandidate> Best;
3516 for (auto &Candidate : UnswitchCandidates) {
3517 Instruction &TI = *Candidate.TI;
3518 ArrayRef<Value *> Invariants = Candidate.Invariants;
3519 BranchInst *BI = dyn_cast<BranchInst>(Val: &TI);
3520 bool FullUnswitch =
3521 !BI || Candidate.hasPendingInjection() ||
3522 (Invariants.size() == 1 &&
3523 Invariants[0] == skipTrivialSelect(Cond: BI->getCondition()));
3524 InstructionCost CandidateCost = ComputeUnswitchedCost(TI, FullUnswitch);
3525 // Calculate cost multiplier which is a tool to limit potentially
3526 // exponential behavior of loop-unswitch.
3527 if (EnableUnswitchCostMultiplier) {
3528 int CostMultiplier =
3529 CalculateUnswitchCostMultiplier(TI, L, LI, DT, UnswitchCandidates);
3530 assert(
3531 (CostMultiplier > 0 && CostMultiplier <= UnswitchThreshold) &&
3532 "cost multiplier needs to be in the range of 1..UnswitchThreshold");
3533 CandidateCost *= CostMultiplier;
3534 LLVM_DEBUG(dbgs() << " Computed cost of " << CandidateCost
3535 << " (multiplier: " << CostMultiplier << ")"
3536 << " for unswitch candidate: " << TI << "\n");
3537 } else {
3538 LLVM_DEBUG(dbgs() << " Computed cost of " << CandidateCost
3539 << " for unswitch candidate: " << TI << "\n");
3540 }
3541
3542 if (!Best || CandidateCost < Best->Cost) {
3543 Best = Candidate;
3544 Best->Cost = CandidateCost;
3545 }
3546 }
3547 assert(Best && "Must be!");
3548 return *Best;
3549}
3550
3551// Insert a freeze on an unswitched branch if all is true:
3552// 1. freeze-loop-unswitch-cond option is true
3553// 2. The branch may not execute in the loop pre-transformation. If a branch may
3554// not execute and could cause UB, it would always cause UB if it is hoisted outside
3555// of the loop. Insert a freeze to prevent this case.
3556// 3. The branch condition may be poison or undef
3557static bool shouldInsertFreeze(Loop &L, Instruction &TI, DominatorTree &DT,
3558 AssumptionCache &AC) {
3559 assert(isa<BranchInst>(TI) || isa<SwitchInst>(TI));
3560 if (!FreezeLoopUnswitchCond)
3561 return false;
3562
3563 ICFLoopSafetyInfo SafetyInfo;
3564 SafetyInfo.computeLoopSafetyInfo(CurLoop: &L);
3565 if (SafetyInfo.isGuaranteedToExecute(Inst: TI, DT: &DT, CurLoop: &L))
3566 return false;
3567
3568 Value *Cond;
3569 if (BranchInst *BI = dyn_cast<BranchInst>(Val: &TI))
3570 Cond = skipTrivialSelect(Cond: BI->getCondition());
3571 else
3572 Cond = skipTrivialSelect(Cond: cast<SwitchInst>(Val: &TI)->getCondition());
3573 return !isGuaranteedNotToBeUndefOrPoison(
3574 V: Cond, AC: &AC, CtxI: L.getLoopPreheader()->getTerminator(), DT: &DT);
3575}
3576
3577static bool unswitchBestCondition(Loop &L, DominatorTree &DT, LoopInfo &LI,
3578 AssumptionCache &AC, AAResults &AA,
3579 TargetTransformInfo &TTI, ScalarEvolution *SE,
3580 MemorySSAUpdater *MSSAU,
3581 LPMUpdater &LoopUpdater) {
3582 // Collect all invariant conditions within this loop (as opposed to an inner
3583 // loop which would be handled when visiting that inner loop).
3584 SmallVector<NonTrivialUnswitchCandidate, 4> UnswitchCandidates;
3585 IVConditionInfo PartialIVInfo;
3586 Instruction *PartialIVCondBranch = nullptr;
3587 collectUnswitchCandidates(UnswitchCandidates, PartialIVInfo,
3588 PartialIVCondBranch, L, LI, AA, MSSAU);
3589 if (!findOptionMDForLoop(TheLoop: &L, Name: "llvm.loop.unswitch.injection.disable"))
3590 collectUnswitchCandidatesWithInjections(UnswitchCandidates, PartialIVInfo,
3591 PartialIVCondBranch, L, DT, LI, AA,
3592 MSSAU);
3593 // If we didn't find any candidates, we're done.
3594 if (UnswitchCandidates.empty())
3595 return false;
3596
3597 LLVM_DEBUG(
3598 dbgs() << "Considering " << UnswitchCandidates.size()
3599 << " non-trivial loop invariant conditions for unswitching.\n");
3600
3601 NonTrivialUnswitchCandidate Best = findBestNonTrivialUnswitchCandidate(
3602 UnswitchCandidates, L, DT, LI, AC, TTI, PartialIVInfo);
3603
3604 assert(Best.TI && "Failed to find loop unswitch candidate");
3605 assert(Best.Cost && "Failed to compute cost");
3606
3607 if (*Best.Cost >= UnswitchThreshold) {
3608 LLVM_DEBUG(dbgs() << "Cannot unswitch, lowest cost found: " << *Best.Cost
3609 << "\n");
3610 return false;
3611 }
3612
3613 bool InjectedCondition = false;
3614 if (Best.hasPendingInjection()) {
3615 Best = injectPendingInvariantConditions(Candidate: Best, L, DT, LI, AC, MSSAU);
3616 InjectedCondition = true;
3617 }
3618 assert(!Best.hasPendingInjection() &&
3619 "All injections should have been done by now!");
3620
3621 if (Best.TI != PartialIVCondBranch)
3622 PartialIVInfo.InstToDuplicate.clear();
3623
3624 bool InsertFreeze;
3625 if (auto *SI = dyn_cast<SelectInst>(Val: Best.TI)) {
3626 // If the best candidate is a select, turn it into a branch. Select
3627 // instructions with a poison conditional do not propagate poison, but
3628 // branching on poison causes UB. Insert a freeze on the select
3629 // conditional to prevent UB after turning the select into a branch.
3630 InsertFreeze = !isGuaranteedNotToBeUndefOrPoison(
3631 V: SI->getCondition(), AC: &AC, CtxI: L.getLoopPreheader()->getTerminator(), DT: &DT);
3632 Best.TI = turnSelectIntoBranch(SI, DT, LI, MSSAU, AC: &AC);
3633 } else {
3634 // If the best candidate is a guard, turn it into a branch.
3635 if (isGuard(U: Best.TI))
3636 Best.TI =
3637 turnGuardIntoBranch(GI: cast<IntrinsicInst>(Val: Best.TI), L, DT, LI, MSSAU);
3638 InsertFreeze = shouldInsertFreeze(L, TI&: *Best.TI, DT, AC);
3639 }
3640
3641 LLVM_DEBUG(dbgs() << " Unswitching non-trivial (cost = " << Best.Cost
3642 << ") terminator: " << *Best.TI << "\n");
3643 unswitchNontrivialInvariants(L, TI&: *Best.TI, Invariants: Best.Invariants, PartialIVInfo, DT,
3644 LI, AC, SE, MSSAU, LoopUpdater, InsertFreeze,
3645 InjectedCondition);
3646 return true;
3647}
3648
3649/// Unswitch control flow predicated on loop invariant conditions.
3650///
3651/// This first hoists all branches or switches which are trivial (IE, do not
3652/// require duplicating any part of the loop) out of the loop body. It then
3653/// looks at other loop invariant control flows and tries to unswitch those as
3654/// well by cloning the loop if the result is small enough.
3655///
3656/// The `DT`, `LI`, `AC`, `AA`, `TTI` parameters are required analyses that are
3657/// also updated based on the unswitch. The `MSSA` analysis is also updated if
3658/// valid (i.e. its use is enabled).
3659///
3660/// If either `NonTrivial` is true or the flag `EnableNonTrivialUnswitch` is
3661/// true, we will attempt to do non-trivial unswitching as well as trivial
3662/// unswitching.
3663///
3664/// The `postUnswitch` function will be run after unswitching is complete
3665/// with information on whether or not the provided loop remains a loop and
3666/// a list of new sibling loops created.
3667///
3668/// If `SE` is non-null, we will update that analysis based on the unswitching
3669/// done.
3670static bool unswitchLoop(Loop &L, DominatorTree &DT, LoopInfo &LI,
3671 AssumptionCache &AC, AAResults &AA,
3672 TargetTransformInfo &TTI, bool Trivial,
3673 bool NonTrivial, ScalarEvolution *SE,
3674 MemorySSAUpdater *MSSAU, LPMUpdater &LoopUpdater) {
3675 assert(L.isRecursivelyLCSSAForm(DT, LI) &&
3676 "Loops must be in LCSSA form before unswitching.");
3677
3678 // Must be in loop simplified form: we need a preheader and dedicated exits.
3679 if (!L.isLoopSimplifyForm())
3680 return false;
3681
3682 // Try trivial unswitch first before loop over other basic blocks in the loop.
3683 if (Trivial && unswitchAllTrivialConditions(L, DT, LI, SE, MSSAU)) {
3684 // If we unswitched successfully we will want to clean up the loop before
3685 // processing it further so just mark it as unswitched and return.
3686 postUnswitch(L, U&: LoopUpdater, LoopName: L.getName(),
3687 /*CurrentLoopValid*/ true, /*PartiallyInvariant*/ false,
3688 /*InjectedCondition*/ false, NewLoops: {});
3689 return true;
3690 }
3691
3692 const Function *F = L.getHeader()->getParent();
3693
3694 // Check whether we should continue with non-trivial conditions.
3695 // EnableNonTrivialUnswitch: Global variable that forces non-trivial
3696 // unswitching for testing and debugging.
3697 // NonTrivial: Parameter that enables non-trivial unswitching for this
3698 // invocation of the transform. But this should be allowed only
3699 // for targets without branch divergence.
3700 //
3701 // FIXME: If divergence analysis becomes available to a loop
3702 // transform, we should allow unswitching for non-trivial uniform
3703 // branches even on targets that have divergence.
3704 // https://bugs.llvm.org/show_bug.cgi?id=48819
3705 bool ContinueWithNonTrivial =
3706 EnableNonTrivialUnswitch || (NonTrivial && !TTI.hasBranchDivergence(F));
3707 if (!ContinueWithNonTrivial)
3708 return false;
3709
3710 // Skip non-trivial unswitching for optsize functions.
3711 if (F->hasOptSize())
3712 return false;
3713
3714 // Perform legality checks.
3715 if (!isSafeForNoNTrivialUnswitching(L, LI))
3716 return false;
3717
3718 // For non-trivial unswitching, because it often creates new loops, we rely on
3719 // the pass manager to iterate on the loops rather than trying to immediately
3720 // reach a fixed point. There is no substantial advantage to iterating
3721 // internally, and if any of the new loops are simplified enough to contain
3722 // trivial unswitching we want to prefer those.
3723
3724 // Try to unswitch the best invariant condition. We prefer this full unswitch to
3725 // a partial unswitch when possible below the threshold.
3726 if (unswitchBestCondition(L, DT, LI, AC, AA, TTI, SE, MSSAU, LoopUpdater))
3727 return true;
3728
3729 // No other opportunities to unswitch.
3730 return false;
3731}
3732
3733PreservedAnalyses SimpleLoopUnswitchPass::run(Loop &L, LoopAnalysisManager &AM,
3734 LoopStandardAnalysisResults &AR,
3735 LPMUpdater &U) {
3736 Function &F = *L.getHeader()->getParent();
3737 (void)F;
3738 LLVM_DEBUG(dbgs() << "Unswitching loop in " << F.getName() << ": " << L
3739 << "\n");
3740
3741 std::optional<MemorySSAUpdater> MSSAU;
3742 if (AR.MSSA) {
3743 MSSAU = MemorySSAUpdater(AR.MSSA);
3744 if (VerifyMemorySSA)
3745 AR.MSSA->verifyMemorySSA();
3746 }
3747 if (!unswitchLoop(L, DT&: AR.DT, LI&: AR.LI, AC&: AR.AC, AA&: AR.AA, TTI&: AR.TTI, Trivial, NonTrivial,
3748 SE: &AR.SE, MSSAU: MSSAU ? &*MSSAU : nullptr, LoopUpdater&: U))
3749 return PreservedAnalyses::all();
3750
3751 if (AR.MSSA && VerifyMemorySSA)
3752 AR.MSSA->verifyMemorySSA();
3753
3754 // Historically this pass has had issues with the dominator tree so verify it
3755 // in asserts builds.
3756 assert(AR.DT.verify(DominatorTree::VerificationLevel::Fast));
3757
3758 auto PA = getLoopPassPreservedAnalyses();
3759 if (AR.MSSA)
3760 PA.preserve<MemorySSAAnalysis>();
3761 return PA;
3762}
3763
3764void SimpleLoopUnswitchPass::printPipeline(
3765 raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
3766 static_cast<PassInfoMixin<SimpleLoopUnswitchPass> *>(this)->printPipeline(
3767 OS, MapClassName2PassName);
3768
3769 OS << '<';
3770 OS << (NonTrivial ? "" : "no-") << "nontrivial;";
3771 OS << (Trivial ? "" : "no-") << "trivial";
3772 OS << '>';
3773}
3774