1 //===-- SimplifyIndVar.cpp - Induction variable simplification ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements induction variable simplification. It does
11 // not define any actual pass or policy, but provides a single function to
12 // simplify a loop's induction variables based on ScalarEvolution.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Transforms/Utils/SimplifyIndVar.h"
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallVector.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/LoopInfo.h"
21 #include "llvm/Analysis/LoopPass.h"
22 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
23 #include "llvm/IR/DataLayout.h"
24 #include "llvm/IR/Dominators.h"
25 #include "llvm/IR/IRBuilder.h"
26 #include "llvm/IR/Instructions.h"
27 #include "llvm/IR/IntrinsicInst.h"
28 #include "llvm/Support/Debug.h"
29 #include "llvm/Support/raw_ostream.h"
33 #define DEBUG_TYPE "indvars"
35 STATISTIC(NumElimIdentity, "Number of IV identities eliminated");
36 STATISTIC(NumElimOperand, "Number of IV operands folded into a use");
37 STATISTIC(NumElimRem , "Number of IV remainder operations eliminated");
38 STATISTIC(NumElimCmp , "Number of IV comparisons eliminated");
41 /// This is a utility for simplifying induction variables
42 /// based on ScalarEvolution. It is the primary instrument of the
43 /// IndvarSimplify pass, but it may also be directly invoked to cleanup after
44 /// other loop passes that preserve SCEV.
45 class SimplifyIndvar {
51 SmallVectorImpl<WeakVH> &DeadInsts;
56 SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, DominatorTree *DT,
57 LoopInfo *LI,SmallVectorImpl<WeakVH> &Dead)
58 : L(Loop), LI(LI), SE(SE), DT(DT), DeadInsts(Dead), Changed(false) {
59 assert(LI && "IV simplification requires LoopInfo");
62 bool hasChanged() const { return Changed; }
64 /// Iteratively perform simplification on a worklist of users of the
65 /// specified induction variable. This is the top-level driver that applies
66 /// all simplifications to users of an IV.
67 void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr);
69 Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
71 bool eliminateIdentitySCEV(Instruction *UseInst, Instruction *IVOperand);
73 bool eliminateOverflowIntrinsic(CallInst *CI);
74 bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
75 void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
76 void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
78 bool strengthenOverflowingOperation(BinaryOperator *OBO, Value *IVOperand);
82 /// Fold an IV operand into its use. This removes increments of an
83 /// aligned IV when used by a instruction that ignores the low bits.
85 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
87 /// Return the operand of IVOperand for this induction variable if IVOperand can
88 /// be folded (in case more folding opportunities have been exposed).
89 /// Otherwise return null.
90 Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) {
91 Value *IVSrc = nullptr;
93 const SCEV *FoldedExpr = nullptr;
94 switch (UseInst->getOpcode()) {
97 case Instruction::UDiv:
98 case Instruction::LShr:
99 // We're only interested in the case where we know something about
100 // the numerator and have a constant denominator.
101 if (IVOperand != UseInst->getOperand(OperIdx) ||
102 !isa<ConstantInt>(UseInst->getOperand(1)))
105 // Attempt to fold a binary operator with constant operand.
106 // e.g. ((I + 1) >> 2) => I >> 2
107 if (!isa<BinaryOperator>(IVOperand)
108 || !isa<ConstantInt>(IVOperand->getOperand(1)))
111 IVSrc = IVOperand->getOperand(0);
112 // IVSrc must be the (SCEVable) IV, since the other operand is const.
113 assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand");
115 ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1));
116 if (UseInst->getOpcode() == Instruction::LShr) {
117 // Get a constant for the divisor. See createSCEV.
118 uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth();
119 if (D->getValue().uge(BitWidth))
122 D = ConstantInt::get(UseInst->getContext(),
123 APInt::getOneBitSet(BitWidth, D->getZExtValue()));
125 FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
127 // We have something that might fold it's operand. Compare SCEVs.
128 if (!SE->isSCEVable(UseInst->getType()))
131 // Bypass the operand if SCEV can prove it has no effect.
132 if (SE->getSCEV(UseInst) != FoldedExpr)
135 DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand
136 << " -> " << *UseInst << '\n');
138 UseInst->setOperand(OperIdx, IVSrc);
139 assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
143 if (IVOperand->use_empty())
144 DeadInsts.emplace_back(IVOperand);
148 /// SimplifyIVUsers helper for eliminating useless
149 /// comparisons against an induction variable.
150 void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
151 unsigned IVOperIdx = 0;
152 ICmpInst::Predicate Pred = ICmp->getPredicate();
153 if (IVOperand != ICmp->getOperand(0)) {
155 assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
157 Pred = ICmpInst::getSwappedPredicate(Pred);
160 // Get the SCEVs for the ICmp operands.
161 const SCEV *S = SE->getSCEV(ICmp->getOperand(IVOperIdx));
162 const SCEV *X = SE->getSCEV(ICmp->getOperand(1 - IVOperIdx));
164 // Simplify unnecessary loops away.
165 const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
166 S = SE->getSCEVAtScope(S, ICmpLoop);
167 X = SE->getSCEVAtScope(X, ICmpLoop);
169 ICmpInst::Predicate InvariantPredicate;
170 const SCEV *InvariantLHS, *InvariantRHS;
172 // If the condition is always true or always false, replace it with
174 if (SE->isKnownPredicate(Pred, S, X)) {
175 ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
176 DeadInsts.emplace_back(ICmp);
177 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
178 } else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X)) {
179 ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
180 DeadInsts.emplace_back(ICmp);
181 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
182 } else if (isa<PHINode>(IVOperand) &&
183 SE->isLoopInvariantPredicate(Pred, S, X, L, InvariantPredicate,
184 InvariantLHS, InvariantRHS)) {
186 // Rewrite the comparison to a loop invariant comparison if it can be done
187 // cheaply, where cheaply means "we don't need to emit any new
190 Value *NewLHS = nullptr, *NewRHS = nullptr;
192 if (S == InvariantLHS || X == InvariantLHS)
194 ICmp->getOperand(S == InvariantLHS ? IVOperIdx : (1 - IVOperIdx));
196 if (S == InvariantRHS || X == InvariantRHS)
198 ICmp->getOperand(S == InvariantRHS ? IVOperIdx : (1 - IVOperIdx));
200 auto *PN = cast<PHINode>(IVOperand);
201 for (unsigned i = 0, e = PN->getNumIncomingValues();
202 i != e && (!NewLHS || !NewRHS);
205 // If this is a value incoming from the backedge, then it cannot be a loop
206 // invariant value (since we know that IVOperand is an induction variable).
207 if (L->contains(PN->getIncomingBlock(i)))
210 // NB! This following assert does not fundamentally have to be true, but
211 // it is true today given how SCEV analyzes induction variables.
212 // Specifically, today SCEV will *not* recognize %iv as an induction
213 // variable in the following case:
215 // define void @f(i32 %k) {
217 // br i1 undef, label %r, label %l
220 // %k.inc.l = add i32 %k, 1
224 // %k.inc.r = add i32 %k, 1
228 // %iv = phi i32 [ %k.inc.l, %l ], [ %k.inc.r, %r ], [ %iv.inc, %loop ]
229 // %iv.inc = add i32 %iv, 1
233 // but if it starts to, at some point, then the assertion below will have
234 // to be changed to a runtime check.
236 Value *Incoming = PN->getIncomingValue(i);
239 if (auto *I = dyn_cast<Instruction>(Incoming))
240 assert(DT->dominates(I, ICmp) && "Should be a unique loop dominating value!");
243 const SCEV *IncomingS = SE->getSCEV(Incoming);
245 if (!NewLHS && IncomingS == InvariantLHS)
247 if (!NewRHS && IncomingS == InvariantRHS)
251 if (!NewLHS || !NewRHS)
252 // We could not find an existing value to replace either LHS or RHS.
253 // Generating new instructions has subtler tradeoffs, so avoid doing that
257 DEBUG(dbgs() << "INDVARS: Simplified comparison: " << *ICmp << '\n');
258 ICmp->setPredicate(InvariantPredicate);
259 ICmp->setOperand(0, NewLHS);
260 ICmp->setOperand(1, NewRHS);
268 /// SimplifyIVUsers helper for eliminating useless
269 /// remainder operations operating on an induction variable.
270 void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
273 // We're only interested in the case where we know something about
275 if (IVOperand != Rem->getOperand(0))
278 // Get the SCEVs for the ICmp operands.
279 const SCEV *S = SE->getSCEV(Rem->getOperand(0));
280 const SCEV *X = SE->getSCEV(Rem->getOperand(1));
282 // Simplify unnecessary loops away.
283 const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
284 S = SE->getSCEVAtScope(S, ICmpLoop);
285 X = SE->getSCEVAtScope(X, ICmpLoop);
287 // i % n --> i if i is in [0,n).
288 if ((!IsSigned || SE->isKnownNonNegative(S)) &&
289 SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
291 Rem->replaceAllUsesWith(Rem->getOperand(0));
293 // (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n).
294 const SCEV *LessOne = SE->getMinusSCEV(S, SE->getOne(S->getType()));
295 if (IsSigned && !SE->isKnownNonNegative(LessOne))
298 if (!SE->isKnownPredicate(IsSigned ?
299 ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
303 ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ,
304 Rem->getOperand(0), Rem->getOperand(1));
306 SelectInst::Create(ICmp,
307 ConstantInt::get(Rem->getType(), 0),
308 Rem->getOperand(0), "tmp", Rem);
309 Rem->replaceAllUsesWith(Sel);
312 DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
315 DeadInsts.emplace_back(Rem);
318 bool SimplifyIndvar::eliminateOverflowIntrinsic(CallInst *CI) {
319 auto *F = CI->getCalledFunction();
323 typedef const SCEV *(ScalarEvolution::*OperationFunctionTy)(
324 const SCEV *, const SCEV *, SCEV::NoWrapFlags);
325 typedef const SCEV *(ScalarEvolution::*ExtensionFunctionTy)(
326 const SCEV *, Type *);
328 OperationFunctionTy Operation;
329 ExtensionFunctionTy Extension;
331 Instruction::BinaryOps RawOp;
333 // We always have exactly one of nsw or nuw. If NoSignedOverflow is false, we
335 bool NoSignedOverflow;
337 switch (F->getIntrinsicID()) {
341 case Intrinsic::sadd_with_overflow:
342 Operation = &ScalarEvolution::getAddExpr;
343 Extension = &ScalarEvolution::getSignExtendExpr;
344 RawOp = Instruction::Add;
345 NoSignedOverflow = true;
348 case Intrinsic::uadd_with_overflow:
349 Operation = &ScalarEvolution::getAddExpr;
350 Extension = &ScalarEvolution::getZeroExtendExpr;
351 RawOp = Instruction::Add;
352 NoSignedOverflow = false;
355 case Intrinsic::ssub_with_overflow:
356 Operation = &ScalarEvolution::getMinusSCEV;
357 Extension = &ScalarEvolution::getSignExtendExpr;
358 RawOp = Instruction::Sub;
359 NoSignedOverflow = true;
362 case Intrinsic::usub_with_overflow:
363 Operation = &ScalarEvolution::getMinusSCEV;
364 Extension = &ScalarEvolution::getZeroExtendExpr;
365 RawOp = Instruction::Sub;
366 NoSignedOverflow = false;
370 const SCEV *LHS = SE->getSCEV(CI->getArgOperand(0));
371 const SCEV *RHS = SE->getSCEV(CI->getArgOperand(1));
373 auto *NarrowTy = cast<IntegerType>(LHS->getType());
375 IntegerType::get(NarrowTy->getContext(), NarrowTy->getBitWidth() * 2);
378 (SE->*Extension)((SE->*Operation)(LHS, RHS, SCEV::FlagAnyWrap), WideTy);
380 (SE->*Operation)((SE->*Extension)(LHS, WideTy),
381 (SE->*Extension)(RHS, WideTy), SCEV::FlagAnyWrap);
386 // Proved no overflow, nuke the overflow check and, if possible, the overflow
387 // intrinsic as well.
389 BinaryOperator *NewResult = BinaryOperator::Create(
390 RawOp, CI->getArgOperand(0), CI->getArgOperand(1), "", CI);
392 if (NoSignedOverflow)
393 NewResult->setHasNoSignedWrap(true);
395 NewResult->setHasNoUnsignedWrap(true);
397 SmallVector<ExtractValueInst *, 4> ToDelete;
399 for (auto *U : CI->users()) {
400 if (auto *EVI = dyn_cast<ExtractValueInst>(U)) {
401 if (EVI->getIndices()[0] == 1)
402 EVI->replaceAllUsesWith(ConstantInt::getFalse(CI->getContext()));
404 assert(EVI->getIndices()[0] == 0 && "Only two possibilities!");
405 EVI->replaceAllUsesWith(NewResult);
407 ToDelete.push_back(EVI);
411 for (auto *EVI : ToDelete)
412 EVI->eraseFromParent();
415 CI->eraseFromParent();
420 /// Eliminate an operation that consumes a simple IV and has no observable
421 /// side-effect given the range of IV values. IVOperand is guaranteed SCEVable,
422 /// but UseInst may not be.
423 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
424 Instruction *IVOperand) {
425 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
426 eliminateIVComparison(ICmp, IVOperand);
429 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
430 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
431 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
432 eliminateIVRemainder(Rem, IVOperand, IsSigned);
437 if (auto *CI = dyn_cast<CallInst>(UseInst))
438 if (eliminateOverflowIntrinsic(CI))
441 if (eliminateIdentitySCEV(UseInst, IVOperand))
447 /// Eliminate any operation that SCEV can prove is an identity function.
448 bool SimplifyIndvar::eliminateIdentitySCEV(Instruction *UseInst,
449 Instruction *IVOperand) {
450 if (!SE->isSCEVable(UseInst->getType()) ||
451 (UseInst->getType() != IVOperand->getType()) ||
452 (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
455 // getSCEV(X) == getSCEV(Y) does not guarantee that X and Y are related in the
456 // dominator tree, even if X is an operand to Y. For instance, in
458 // %iv = phi i32 {0,+,1}
459 // br %cond, label %left, label %merge
462 // %X = add i32 %iv, 0
466 // %M = phi (%X, %iv)
468 // getSCEV(%M) == getSCEV(%X) == {0,+,1}, but %X does not dominate %M, and
469 // %M.replaceAllUsesWith(%X) would be incorrect.
471 if (isa<PHINode>(UseInst))
472 // If UseInst is not a PHI node then we know that IVOperand dominates
473 // UseInst directly from the legality of SSA.
474 if (!DT || !DT->dominates(IVOperand, UseInst))
477 if (!LI->replacementPreservesLCSSAForm(UseInst, IVOperand))
480 DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
482 UseInst->replaceAllUsesWith(IVOperand);
485 DeadInsts.emplace_back(UseInst);
489 /// Annotate BO with nsw / nuw if it provably does not signed-overflow /
490 /// unsigned-overflow. Returns true if anything changed, false otherwise.
491 bool SimplifyIndvar::strengthenOverflowingOperation(BinaryOperator *BO,
494 // Fastpath: we don't have any work to do if `BO` is `nuw` and `nsw`.
495 if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
498 const SCEV *(ScalarEvolution::*GetExprForBO)(const SCEV *, const SCEV *,
501 switch (BO->getOpcode()) {
505 case Instruction::Add:
506 GetExprForBO = &ScalarEvolution::getAddExpr;
509 case Instruction::Sub:
510 GetExprForBO = &ScalarEvolution::getMinusSCEV;
513 case Instruction::Mul:
514 GetExprForBO = &ScalarEvolution::getMulExpr;
518 unsigned BitWidth = cast<IntegerType>(BO->getType())->getBitWidth();
519 Type *WideTy = IntegerType::get(BO->getContext(), BitWidth * 2);
520 const SCEV *LHS = SE->getSCEV(BO->getOperand(0));
521 const SCEV *RHS = SE->getSCEV(BO->getOperand(1));
523 bool Changed = false;
525 if (!BO->hasNoUnsignedWrap()) {
526 const SCEV *ExtendAfterOp = SE->getZeroExtendExpr(SE->getSCEV(BO), WideTy);
527 const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
528 SE->getZeroExtendExpr(LHS, WideTy), SE->getZeroExtendExpr(RHS, WideTy),
530 if (ExtendAfterOp == OpAfterExtend) {
531 BO->setHasNoUnsignedWrap();
537 if (!BO->hasNoSignedWrap()) {
538 const SCEV *ExtendAfterOp = SE->getSignExtendExpr(SE->getSCEV(BO), WideTy);
539 const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
540 SE->getSignExtendExpr(LHS, WideTy), SE->getSignExtendExpr(RHS, WideTy),
542 if (ExtendAfterOp == OpAfterExtend) {
543 BO->setHasNoSignedWrap();
552 /// Add all uses of Def to the current IV's worklist.
553 static void pushIVUsers(
555 SmallPtrSet<Instruction*,16> &Simplified,
556 SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
558 for (User *U : Def->users()) {
559 Instruction *UI = cast<Instruction>(U);
561 // Avoid infinite or exponential worklist processing.
562 // Also ensure unique worklist users.
563 // If Def is a LoopPhi, it may not be in the Simplified set, so check for
565 if (UI != Def && Simplified.insert(UI).second)
566 SimpleIVUsers.push_back(std::make_pair(UI, Def));
570 /// Return true if this instruction generates a simple SCEV
571 /// expression in terms of that IV.
573 /// This is similar to IVUsers' isInteresting() but processes each instruction
574 /// non-recursively when the operand is already known to be a simpleIVUser.
576 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
577 if (!SE->isSCEVable(I->getType()))
580 // Get the symbolic expression for this instruction.
581 const SCEV *S = SE->getSCEV(I);
583 // Only consider affine recurrences.
584 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
585 if (AR && AR->getLoop() == L)
591 /// Iteratively perform simplification on a worklist of users
592 /// of the specified induction variable. Each successive simplification may push
593 /// more users which may themselves be candidates for simplification.
595 /// This algorithm does not require IVUsers analysis. Instead, it simplifies
596 /// instructions in-place during analysis. Rather than rewriting induction
597 /// variables bottom-up from their users, it transforms a chain of IVUsers
598 /// top-down, updating the IR only when it encounters a clear optimization
601 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
603 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
604 if (!SE->isSCEVable(CurrIV->getType()))
607 // Instructions processed by SimplifyIndvar for CurrIV.
608 SmallPtrSet<Instruction*,16> Simplified;
610 // Use-def pairs if IV users waiting to be processed for CurrIV.
611 SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
613 // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
614 // called multiple times for the same LoopPhi. This is the proper thing to
615 // do for loop header phis that use each other.
616 pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
618 while (!SimpleIVUsers.empty()) {
619 std::pair<Instruction*, Instruction*> UseOper =
620 SimpleIVUsers.pop_back_val();
621 Instruction *UseInst = UseOper.first;
623 // Bypass back edges to avoid extra work.
624 if (UseInst == CurrIV) continue;
626 Instruction *IVOperand = UseOper.second;
627 for (unsigned N = 0; IVOperand; ++N) {
628 assert(N <= Simplified.size() && "runaway iteration");
630 Value *NewOper = foldIVUser(UseOper.first, IVOperand);
632 break; // done folding
633 IVOperand = dyn_cast<Instruction>(NewOper);
638 if (eliminateIVUser(UseOper.first, IVOperand)) {
639 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
643 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(UseOper.first)) {
644 if (isa<OverflowingBinaryOperator>(BO) &&
645 strengthenOverflowingOperation(BO, IVOperand)) {
646 // re-queue uses of the now modified binary operator and fall
647 // through to the checks that remain.
648 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
652 CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
657 if (isSimpleIVUser(UseOper.first, L, SE)) {
658 pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
665 void IVVisitor::anchor() { }
667 /// Simplify instructions that use this induction variable
668 /// by using ScalarEvolution to analyze the IV's recurrence.
669 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, DominatorTree *DT,
670 LoopInfo *LI, SmallVectorImpl<WeakVH> &Dead,
672 SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, DT, LI, Dead);
673 SIV.simplifyUsers(CurrIV, V);
674 return SIV.hasChanged();
677 /// Simplify users of induction variables within this
678 /// loop. This does not actually change or add IVs.
679 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, DominatorTree *DT,
680 LoopInfo *LI, SmallVectorImpl<WeakVH> &Dead) {
681 bool Changed = false;
682 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
683 Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, DT, LI, Dead);