1 //===- NaryReassociate.cpp - Reassociate n-ary expressions ----------------===//
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 // This pass reassociates n-ary add expressions and eliminates the redundancy
10 // exposed by the reassociation.
11 //
12 // A motivating example:
13 //
14 // void foo(int a, int b) {
15 // bar(a + b);
16 // bar((a + 2) + b);
17 // }
18 //
19 // An ideal compiler should reassociate (a + 2) + b to (a + b) + 2 and simplify
20 // the above code to
21 //
22 // int t = a + b;
23 // bar(t);
24 // bar(t + 2);
25 //
26 // However, the Reassociate pass is unable to do that because it processes each
27 // instruction individually and believes (a + 2) + b is the best form according
28 // to its rank system.
29 //
30 // To address this limitation, NaryReassociate reassociates an expression in a
31 // form that reuses existing instructions. As a result, NaryReassociate can
32 // reassociate (a + 2) + b in the example to (a + b) + 2 because it detects that
33 // (a + b) is computed before.
34 //
35 // NaryReassociate works as follows. For every instruction in the form of (a +
36 // b) + c, it checks whether a + c or b + c is already computed by a dominating
37 // instruction. If so, it then reassociates (a + b) + c into (a + c) + b or (b +
38 // c) + a and removes the redundancy accordingly. To efficiently look up whether
39 // an expression is computed before, we store each instruction seen and its SCEV
40 // into an SCEV-to-instruction map.
41 //
42 // Although the algorithm pattern-matches only ternary additions, it
43 // automatically handles many >3-ary expressions by walking through the function
44 // in the depth-first order. For example, given
45 //
46 // (a + c) + d
47 // ((a + b) + c) + d
48 //
49 // NaryReassociate first rewrites (a + b) + c to (a + c) + b, and then rewrites
50 // ((a + c) + b) + d into ((a + c) + d) + b.
51 //
52 // Finally, the above dominator-based algorithm may need to be run multiple
53 // iterations before emitting optimal code. One source of this need is that we
54 // only split an operand when it is used only once. The above algorithm can
55 // eliminate an instruction and decrease the usage count of its operands. As a
56 // result, an instruction that previously had multiple uses may become a
57 // single-use instruction and thus eligible for split consideration. For
58 // example,
59 //
60 // ac = a + c
61 // ab = a + b
62 // abc = ab + c
63 // ab2 = ab + b
64 // ab2c = ab2 + c
65 //
66 // In the first iteration, we cannot reassociate abc to ac+b because ab is used
67 // twice. However, we can reassociate ab2c to abc+b in the first iteration. As a
68 // result, ab2 becomes dead and ab will be used only once in the second
69 // iteration.
70 //
71 // Limitations and TODO items:
72 //
73 // 1) We only considers n-ary adds and muls for now. This should be extended
74 // and generalized.
75 //
76 //===----------------------------------------------------------------------===//
77
78 #include "llvm/Transforms/Scalar/NaryReassociate.h"
79 #include "llvm/ADT/DepthFirstIterator.h"
80 #include "llvm/ADT/SmallVector.h"
81 #include "llvm/Analysis/AssumptionCache.h"
82 #include "llvm/Analysis/ScalarEvolution.h"
83 #include "llvm/Analysis/TargetLibraryInfo.h"
84 #include "llvm/Analysis/TargetTransformInfo.h"
85 #include "llvm/Transforms/Utils/Local.h"
86 #include "llvm/Analysis/ValueTracking.h"
87 #include "llvm/IR/BasicBlock.h"
88 #include "llvm/IR/Constants.h"
89 #include "llvm/IR/DataLayout.h"
90 #include "llvm/IR/DerivedTypes.h"
91 #include "llvm/IR/Dominators.h"
92 #include "llvm/IR/Function.h"
93 #include "llvm/IR/GetElementPtrTypeIterator.h"
94 #include "llvm/IR/IRBuilder.h"
95 #include "llvm/IR/InstrTypes.h"
96 #include "llvm/IR/Instruction.h"
97 #include "llvm/IR/Instructions.h"
98 #include "llvm/IR/Module.h"
99 #include "llvm/IR/Operator.h"
100 #include "llvm/IR/PatternMatch.h"
101 #include "llvm/IR/Type.h"
102 #include "llvm/IR/Value.h"
103 #include "llvm/IR/ValueHandle.h"
104 #include "llvm/Pass.h"
105 #include "llvm/Support/Casting.h"
106 #include "llvm/Support/ErrorHandling.h"
107 #include "llvm/Transforms/Scalar.h"
108 #include <cassert>
109 #include <cstdint>
110
111 using namespace llvm;
112 using namespace PatternMatch;
113
114 #define DEBUG_TYPE "nary-reassociate"
115
116 namespace {
117
118 class NaryReassociateLegacyPass : public FunctionPass {
119 public:
120 static char ID;
121
NaryReassociateLegacyPass()122 NaryReassociateLegacyPass() : FunctionPass(ID) {
123 initializeNaryReassociateLegacyPassPass(*PassRegistry::getPassRegistry());
124 }
125
doInitialization(Module & M)126 bool doInitialization(Module &M) override {
127 return false;
128 }
129
130 bool runOnFunction(Function &F) override;
131
getAnalysisUsage(AnalysisUsage & AU) const132 void getAnalysisUsage(AnalysisUsage &AU) const override {
133 AU.addPreserved<DominatorTreeWrapperPass>();
134 AU.addPreserved<ScalarEvolutionWrapperPass>();
135 AU.addPreserved<TargetLibraryInfoWrapperPass>();
136 AU.addRequired<AssumptionCacheTracker>();
137 AU.addRequired<DominatorTreeWrapperPass>();
138 AU.addRequired<ScalarEvolutionWrapperPass>();
139 AU.addRequired<TargetLibraryInfoWrapperPass>();
140 AU.addRequired<TargetTransformInfoWrapperPass>();
141 AU.setPreservesCFG();
142 }
143
144 private:
145 NaryReassociatePass Impl;
146 };
147
148 } // end anonymous namespace
149
150 char NaryReassociateLegacyPass::ID = 0;
151
152 INITIALIZE_PASS_BEGIN(NaryReassociateLegacyPass, "nary-reassociate",
153 "Nary reassociation", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)154 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
155 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
156 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
157 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
158 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
159 INITIALIZE_PASS_END(NaryReassociateLegacyPass, "nary-reassociate",
160 "Nary reassociation", false, false)
161
162 FunctionPass *llvm::createNaryReassociatePass() {
163 return new NaryReassociateLegacyPass();
164 }
165
runOnFunction(Function & F)166 bool NaryReassociateLegacyPass::runOnFunction(Function &F) {
167 if (skipFunction(F))
168 return false;
169
170 auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
171 auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
172 auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
173 auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
174 auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
175
176 return Impl.runImpl(F, AC, DT, SE, TLI, TTI);
177 }
178
run(Function & F,FunctionAnalysisManager & AM)179 PreservedAnalyses NaryReassociatePass::run(Function &F,
180 FunctionAnalysisManager &AM) {
181 auto *AC = &AM.getResult<AssumptionAnalysis>(F);
182 auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
183 auto *SE = &AM.getResult<ScalarEvolutionAnalysis>(F);
184 auto *TLI = &AM.getResult<TargetLibraryAnalysis>(F);
185 auto *TTI = &AM.getResult<TargetIRAnalysis>(F);
186
187 if (!runImpl(F, AC, DT, SE, TLI, TTI))
188 return PreservedAnalyses::all();
189
190 PreservedAnalyses PA;
191 PA.preserveSet<CFGAnalyses>();
192 PA.preserve<ScalarEvolutionAnalysis>();
193 return PA;
194 }
195
runImpl(Function & F,AssumptionCache * AC_,DominatorTree * DT_,ScalarEvolution * SE_,TargetLibraryInfo * TLI_,TargetTransformInfo * TTI_)196 bool NaryReassociatePass::runImpl(Function &F, AssumptionCache *AC_,
197 DominatorTree *DT_, ScalarEvolution *SE_,
198 TargetLibraryInfo *TLI_,
199 TargetTransformInfo *TTI_) {
200 AC = AC_;
201 DT = DT_;
202 SE = SE_;
203 TLI = TLI_;
204 TTI = TTI_;
205 DL = &F.getParent()->getDataLayout();
206
207 bool Changed = false, ChangedInThisIteration;
208 do {
209 ChangedInThisIteration = doOneIteration(F);
210 Changed |= ChangedInThisIteration;
211 } while (ChangedInThisIteration);
212 return Changed;
213 }
214
215 // Whitelist the instruction types NaryReassociate handles for now.
isPotentiallyNaryReassociable(Instruction * I)216 static bool isPotentiallyNaryReassociable(Instruction *I) {
217 switch (I->getOpcode()) {
218 case Instruction::Add:
219 case Instruction::GetElementPtr:
220 case Instruction::Mul:
221 return true;
222 default:
223 return false;
224 }
225 }
226
doOneIteration(Function & F)227 bool NaryReassociatePass::doOneIteration(Function &F) {
228 bool Changed = false;
229 SeenExprs.clear();
230 // Process the basic blocks in a depth first traversal of the dominator
231 // tree. This order ensures that all bases of a candidate are in Candidates
232 // when we process it.
233 for (const auto Node : depth_first(DT)) {
234 BasicBlock *BB = Node->getBlock();
235 for (auto I = BB->begin(); I != BB->end(); ++I) {
236 if (SE->isSCEVable(I->getType()) && isPotentiallyNaryReassociable(&*I)) {
237 const SCEV *OldSCEV = SE->getSCEV(&*I);
238 if (Instruction *NewI = tryReassociate(&*I)) {
239 Changed = true;
240 SE->forgetValue(&*I);
241 I->replaceAllUsesWith(NewI);
242 WeakVH NewIExist = NewI;
243 // If SeenExprs/NewIExist contains I's WeakTrackingVH/WeakVH, that
244 // entry will be replaced with nullptr if deleted.
245 RecursivelyDeleteTriviallyDeadInstructions(&*I, TLI);
246 if (!NewIExist) {
247 // Rare occation where the new instruction (NewI) have been removed,
248 // probably due to parts of the input code was dead from the
249 // beginning, reset the iterator and start over from the beginning
250 I = BB->begin();
251 continue;
252 }
253 I = NewI->getIterator();
254 }
255 // Add the rewritten instruction to SeenExprs; the original instruction
256 // is deleted.
257 const SCEV *NewSCEV = SE->getSCEV(&*I);
258 SeenExprs[NewSCEV].push_back(WeakTrackingVH(&*I));
259 // Ideally, NewSCEV should equal OldSCEV because tryReassociate(I)
260 // is equivalent to I. However, ScalarEvolution::getSCEV may
261 // weaken nsw causing NewSCEV not to equal OldSCEV. For example, suppose
262 // we reassociate
263 // I = &a[sext(i +nsw j)] // assuming sizeof(a[0]) = 4
264 // to
265 // NewI = &a[sext(i)] + sext(j).
266 //
267 // ScalarEvolution computes
268 // getSCEV(I) = a + 4 * sext(i + j)
269 // getSCEV(newI) = a + 4 * sext(i) + 4 * sext(j)
270 // which are different SCEVs.
271 //
272 // To alleviate this issue of ScalarEvolution not always capturing
273 // equivalence, we add I to SeenExprs[OldSCEV] as well so that we can
274 // map both SCEV before and after tryReassociate(I) to I.
275 //
276 // This improvement is exercised in @reassociate_gep_nsw in nary-gep.ll.
277 if (NewSCEV != OldSCEV)
278 SeenExprs[OldSCEV].push_back(WeakTrackingVH(&*I));
279 }
280 }
281 }
282 return Changed;
283 }
284
tryReassociate(Instruction * I)285 Instruction *NaryReassociatePass::tryReassociate(Instruction *I) {
286 switch (I->getOpcode()) {
287 case Instruction::Add:
288 case Instruction::Mul:
289 return tryReassociateBinaryOp(cast<BinaryOperator>(I));
290 case Instruction::GetElementPtr:
291 return tryReassociateGEP(cast<GetElementPtrInst>(I));
292 default:
293 llvm_unreachable("should be filtered out by isPotentiallyNaryReassociable");
294 }
295 }
296
isGEPFoldable(GetElementPtrInst * GEP,const TargetTransformInfo * TTI)297 static bool isGEPFoldable(GetElementPtrInst *GEP,
298 const TargetTransformInfo *TTI) {
299 SmallVector<const Value*, 4> Indices;
300 for (auto I = GEP->idx_begin(); I != GEP->idx_end(); ++I)
301 Indices.push_back(*I);
302 return TTI->getGEPCost(GEP->getSourceElementType(), GEP->getPointerOperand(),
303 Indices) == TargetTransformInfo::TCC_Free;
304 }
305
tryReassociateGEP(GetElementPtrInst * GEP)306 Instruction *NaryReassociatePass::tryReassociateGEP(GetElementPtrInst *GEP) {
307 // Not worth reassociating GEP if it is foldable.
308 if (isGEPFoldable(GEP, TTI))
309 return nullptr;
310
311 gep_type_iterator GTI = gep_type_begin(*GEP);
312 for (unsigned I = 1, E = GEP->getNumOperands(); I != E; ++I, ++GTI) {
313 if (GTI.isSequential()) {
314 if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I - 1,
315 GTI.getIndexedType())) {
316 return NewGEP;
317 }
318 }
319 }
320 return nullptr;
321 }
322
requiresSignExtension(Value * Index,GetElementPtrInst * GEP)323 bool NaryReassociatePass::requiresSignExtension(Value *Index,
324 GetElementPtrInst *GEP) {
325 unsigned PointerSizeInBits =
326 DL->getPointerSizeInBits(GEP->getType()->getPointerAddressSpace());
327 return cast<IntegerType>(Index->getType())->getBitWidth() < PointerSizeInBits;
328 }
329
330 GetElementPtrInst *
tryReassociateGEPAtIndex(GetElementPtrInst * GEP,unsigned I,Type * IndexedType)331 NaryReassociatePass::tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
332 unsigned I, Type *IndexedType) {
333 Value *IndexToSplit = GEP->getOperand(I + 1);
334 if (SExtInst *SExt = dyn_cast<SExtInst>(IndexToSplit)) {
335 IndexToSplit = SExt->getOperand(0);
336 } else if (ZExtInst *ZExt = dyn_cast<ZExtInst>(IndexToSplit)) {
337 // zext can be treated as sext if the source is non-negative.
338 if (isKnownNonNegative(ZExt->getOperand(0), *DL, 0, AC, GEP, DT))
339 IndexToSplit = ZExt->getOperand(0);
340 }
341
342 if (AddOperator *AO = dyn_cast<AddOperator>(IndexToSplit)) {
343 // If the I-th index needs sext and the underlying add is not equipped with
344 // nsw, we cannot split the add because
345 // sext(LHS + RHS) != sext(LHS) + sext(RHS).
346 if (requiresSignExtension(IndexToSplit, GEP) &&
347 computeOverflowForSignedAdd(AO, *DL, AC, GEP, DT) !=
348 OverflowResult::NeverOverflows)
349 return nullptr;
350
351 Value *LHS = AO->getOperand(0), *RHS = AO->getOperand(1);
352 // IndexToSplit = LHS + RHS.
353 if (auto *NewGEP = tryReassociateGEPAtIndex(GEP, I, LHS, RHS, IndexedType))
354 return NewGEP;
355 // Symmetrically, try IndexToSplit = RHS + LHS.
356 if (LHS != RHS) {
357 if (auto *NewGEP =
358 tryReassociateGEPAtIndex(GEP, I, RHS, LHS, IndexedType))
359 return NewGEP;
360 }
361 }
362 return nullptr;
363 }
364
365 GetElementPtrInst *
tryReassociateGEPAtIndex(GetElementPtrInst * GEP,unsigned I,Value * LHS,Value * RHS,Type * IndexedType)366 NaryReassociatePass::tryReassociateGEPAtIndex(GetElementPtrInst *GEP,
367 unsigned I, Value *LHS,
368 Value *RHS, Type *IndexedType) {
369 // Look for GEP's closest dominator that has the same SCEV as GEP except that
370 // the I-th index is replaced with LHS.
371 SmallVector<const SCEV *, 4> IndexExprs;
372 for (auto Index = GEP->idx_begin(); Index != GEP->idx_end(); ++Index)
373 IndexExprs.push_back(SE->getSCEV(*Index));
374 // Replace the I-th index with LHS.
375 IndexExprs[I] = SE->getSCEV(LHS);
376 if (isKnownNonNegative(LHS, *DL, 0, AC, GEP, DT) &&
377 DL->getTypeSizeInBits(LHS->getType()) <
378 DL->getTypeSizeInBits(GEP->getOperand(I)->getType())) {
379 // Zero-extend LHS if it is non-negative. InstCombine canonicalizes sext to
380 // zext if the source operand is proved non-negative. We should do that
381 // consistently so that CandidateExpr more likely appears before. See
382 // @reassociate_gep_assume for an example of this canonicalization.
383 IndexExprs[I] =
384 SE->getZeroExtendExpr(IndexExprs[I], GEP->getOperand(I)->getType());
385 }
386 const SCEV *CandidateExpr = SE->getGEPExpr(cast<GEPOperator>(GEP),
387 IndexExprs);
388
389 Value *Candidate = findClosestMatchingDominator(CandidateExpr, GEP);
390 if (Candidate == nullptr)
391 return nullptr;
392
393 IRBuilder<> Builder(GEP);
394 // Candidate does not necessarily have the same pointer type as GEP. Use
395 // bitcast or pointer cast to make sure they have the same type, so that the
396 // later RAUW doesn't complain.
397 Candidate = Builder.CreateBitOrPointerCast(Candidate, GEP->getType());
398 assert(Candidate->getType() == GEP->getType());
399
400 // NewGEP = (char *)Candidate + RHS * sizeof(IndexedType)
401 uint64_t IndexedSize = DL->getTypeAllocSize(IndexedType);
402 Type *ElementType = GEP->getResultElementType();
403 uint64_t ElementSize = DL->getTypeAllocSize(ElementType);
404 // Another less rare case: because I is not necessarily the last index of the
405 // GEP, the size of the type at the I-th index (IndexedSize) is not
406 // necessarily divisible by ElementSize. For example,
407 //
408 // #pragma pack(1)
409 // struct S {
410 // int a[3];
411 // int64 b[8];
412 // };
413 // #pragma pack()
414 //
415 // sizeof(S) = 100 is indivisible by sizeof(int64) = 8.
416 //
417 // TODO: bail out on this case for now. We could emit uglygep.
418 if (IndexedSize % ElementSize != 0)
419 return nullptr;
420
421 // NewGEP = &Candidate[RHS * (sizeof(IndexedType) / sizeof(Candidate[0])));
422 Type *IntPtrTy = DL->getIntPtrType(GEP->getType());
423 if (RHS->getType() != IntPtrTy)
424 RHS = Builder.CreateSExtOrTrunc(RHS, IntPtrTy);
425 if (IndexedSize != ElementSize) {
426 RHS = Builder.CreateMul(
427 RHS, ConstantInt::get(IntPtrTy, IndexedSize / ElementSize));
428 }
429 GetElementPtrInst *NewGEP = cast<GetElementPtrInst>(
430 Builder.CreateGEP(GEP->getResultElementType(), Candidate, RHS));
431 NewGEP->setIsInBounds(GEP->isInBounds());
432 NewGEP->takeName(GEP);
433 return NewGEP;
434 }
435
tryReassociateBinaryOp(BinaryOperator * I)436 Instruction *NaryReassociatePass::tryReassociateBinaryOp(BinaryOperator *I) {
437 Value *LHS = I->getOperand(0), *RHS = I->getOperand(1);
438 // There is no need to reassociate 0.
439 if (SE->getSCEV(I)->isZero())
440 return nullptr;
441 if (auto *NewI = tryReassociateBinaryOp(LHS, RHS, I))
442 return NewI;
443 if (auto *NewI = tryReassociateBinaryOp(RHS, LHS, I))
444 return NewI;
445 return nullptr;
446 }
447
tryReassociateBinaryOp(Value * LHS,Value * RHS,BinaryOperator * I)448 Instruction *NaryReassociatePass::tryReassociateBinaryOp(Value *LHS, Value *RHS,
449 BinaryOperator *I) {
450 Value *A = nullptr, *B = nullptr;
451 // To be conservative, we reassociate I only when it is the only user of (A op
452 // B).
453 if (LHS->hasOneUse() && matchTernaryOp(I, LHS, A, B)) {
454 // I = (A op B) op RHS
455 // = (A op RHS) op B or (B op RHS) op A
456 const SCEV *AExpr = SE->getSCEV(A), *BExpr = SE->getSCEV(B);
457 const SCEV *RHSExpr = SE->getSCEV(RHS);
458 if (BExpr != RHSExpr) {
459 if (auto *NewI =
460 tryReassociatedBinaryOp(getBinarySCEV(I, AExpr, RHSExpr), B, I))
461 return NewI;
462 }
463 if (AExpr != RHSExpr) {
464 if (auto *NewI =
465 tryReassociatedBinaryOp(getBinarySCEV(I, BExpr, RHSExpr), A, I))
466 return NewI;
467 }
468 }
469 return nullptr;
470 }
471
tryReassociatedBinaryOp(const SCEV * LHSExpr,Value * RHS,BinaryOperator * I)472 Instruction *NaryReassociatePass::tryReassociatedBinaryOp(const SCEV *LHSExpr,
473 Value *RHS,
474 BinaryOperator *I) {
475 // Look for the closest dominator LHS of I that computes LHSExpr, and replace
476 // I with LHS op RHS.
477 auto *LHS = findClosestMatchingDominator(LHSExpr, I);
478 if (LHS == nullptr)
479 return nullptr;
480
481 Instruction *NewI = nullptr;
482 switch (I->getOpcode()) {
483 case Instruction::Add:
484 NewI = BinaryOperator::CreateAdd(LHS, RHS, "", I);
485 break;
486 case Instruction::Mul:
487 NewI = BinaryOperator::CreateMul(LHS, RHS, "", I);
488 break;
489 default:
490 llvm_unreachable("Unexpected instruction.");
491 }
492 NewI->takeName(I);
493 return NewI;
494 }
495
matchTernaryOp(BinaryOperator * I,Value * V,Value * & Op1,Value * & Op2)496 bool NaryReassociatePass::matchTernaryOp(BinaryOperator *I, Value *V,
497 Value *&Op1, Value *&Op2) {
498 switch (I->getOpcode()) {
499 case Instruction::Add:
500 return match(V, m_Add(m_Value(Op1), m_Value(Op2)));
501 case Instruction::Mul:
502 return match(V, m_Mul(m_Value(Op1), m_Value(Op2)));
503 default:
504 llvm_unreachable("Unexpected instruction.");
505 }
506 return false;
507 }
508
getBinarySCEV(BinaryOperator * I,const SCEV * LHS,const SCEV * RHS)509 const SCEV *NaryReassociatePass::getBinarySCEV(BinaryOperator *I,
510 const SCEV *LHS,
511 const SCEV *RHS) {
512 switch (I->getOpcode()) {
513 case Instruction::Add:
514 return SE->getAddExpr(LHS, RHS);
515 case Instruction::Mul:
516 return SE->getMulExpr(LHS, RHS);
517 default:
518 llvm_unreachable("Unexpected instruction.");
519 }
520 return nullptr;
521 }
522
523 Instruction *
findClosestMatchingDominator(const SCEV * CandidateExpr,Instruction * Dominatee)524 NaryReassociatePass::findClosestMatchingDominator(const SCEV *CandidateExpr,
525 Instruction *Dominatee) {
526 auto Pos = SeenExprs.find(CandidateExpr);
527 if (Pos == SeenExprs.end())
528 return nullptr;
529
530 auto &Candidates = Pos->second;
531 // Because we process the basic blocks in pre-order of the dominator tree, a
532 // candidate that doesn't dominate the current instruction won't dominate any
533 // future instruction either. Therefore, we pop it out of the stack. This
534 // optimization makes the algorithm O(n).
535 while (!Candidates.empty()) {
536 // Candidates stores WeakTrackingVHs, so a candidate can be nullptr if it's
537 // removed
538 // during rewriting.
539 if (Value *Candidate = Candidates.back()) {
540 Instruction *CandidateInstruction = cast<Instruction>(Candidate);
541 if (DT->dominates(CandidateInstruction, Dominatee))
542 return CandidateInstruction;
543 }
544 Candidates.pop_back();
545 }
546 return nullptr;
547 }
548