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