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