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/CommandLine.h"
29 #include "llvm/Support/Debug.h"
30 #include "llvm/Support/raw_ostream.h"
34 #define DEBUG_TYPE "indvars"
36 STATISTIC(NumElimIdentity, "Number of IV identities eliminated");
37 STATISTIC(NumElimOperand, "Number of IV operands folded into a use");
38 STATISTIC(NumElimRem , "Number of IV remainder operations eliminated");
39 STATISTIC(NumElimCmp , "Number of IV comparisons eliminated");
42 /// This is a utility for simplifying induction variables
43 /// based on ScalarEvolution. It is the primary instrument of the
44 /// IndvarSimplify pass, but it may also be directly invoked to cleanup after
45 /// other loop passes that preserve SCEV.
46 class SimplifyIndvar {
52 SmallVectorImpl<WeakVH> &DeadInsts;
57 SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, DominatorTree *DT,
58 LoopInfo *LI,SmallVectorImpl<WeakVH> &Dead)
59 : L(Loop), LI(LI), SE(SE), DT(DT), DeadInsts(Dead), Changed(false) {
60 assert(LI && "IV simplification requires LoopInfo");
63 bool hasChanged() const { return Changed; }
65 /// Iteratively perform simplification on a worklist of users of the
66 /// specified induction variable. This is the top-level driver that applies
67 /// all simplifications to users of an IV.
68 void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr);
70 Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
72 bool eliminateIdentitySCEV(Instruction *UseInst, Instruction *IVOperand);
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);
80 Instruction *splitOverflowIntrinsic(Instruction *IVUser,
81 const DominatorTree *DT);
85 /// Fold an IV operand into its use. This removes increments of an
86 /// aligned IV when used by a instruction that ignores the low bits.
88 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
90 /// Return the operand of IVOperand for this induction variable if IVOperand can
91 /// be folded (in case more folding opportunities have been exposed).
92 /// Otherwise return null.
93 Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) {
94 Value *IVSrc = nullptr;
96 const SCEV *FoldedExpr = nullptr;
97 switch (UseInst->getOpcode()) {
100 case Instruction::UDiv:
101 case Instruction::LShr:
102 // We're only interested in the case where we know something about
103 // the numerator and have a constant denominator.
104 if (IVOperand != UseInst->getOperand(OperIdx) ||
105 !isa<ConstantInt>(UseInst->getOperand(1)))
108 // Attempt to fold a binary operator with constant operand.
109 // e.g. ((I + 1) >> 2) => I >> 2
110 if (!isa<BinaryOperator>(IVOperand)
111 || !isa<ConstantInt>(IVOperand->getOperand(1)))
114 IVSrc = IVOperand->getOperand(0);
115 // IVSrc must be the (SCEVable) IV, since the other operand is const.
116 assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand");
118 ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1));
119 if (UseInst->getOpcode() == Instruction::LShr) {
120 // Get a constant for the divisor. See createSCEV.
121 uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth();
122 if (D->getValue().uge(BitWidth))
125 D = ConstantInt::get(UseInst->getContext(),
126 APInt::getOneBitSet(BitWidth, D->getZExtValue()));
128 FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
130 // We have something that might fold it's operand. Compare SCEVs.
131 if (!SE->isSCEVable(UseInst->getType()))
134 // Bypass the operand if SCEV can prove it has no effect.
135 if (SE->getSCEV(UseInst) != FoldedExpr)
138 DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand
139 << " -> " << *UseInst << '\n');
141 UseInst->setOperand(OperIdx, IVSrc);
142 assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
146 if (IVOperand->use_empty())
147 DeadInsts.emplace_back(IVOperand);
151 /// SimplifyIVUsers helper for eliminating useless
152 /// comparisons against an induction variable.
153 void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
154 unsigned IVOperIdx = 0;
155 ICmpInst::Predicate Pred = ICmp->getPredicate();
156 if (IVOperand != ICmp->getOperand(0)) {
158 assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
160 Pred = ICmpInst::getSwappedPredicate(Pred);
163 // Get the SCEVs for the ICmp operands.
164 const SCEV *S = SE->getSCEV(ICmp->getOperand(IVOperIdx));
165 const SCEV *X = SE->getSCEV(ICmp->getOperand(1 - IVOperIdx));
167 // Simplify unnecessary loops away.
168 const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
169 S = SE->getSCEVAtScope(S, ICmpLoop);
170 X = SE->getSCEVAtScope(X, ICmpLoop);
172 ICmpInst::Predicate InvariantPredicate;
173 const SCEV *InvariantLHS, *InvariantRHS;
175 // If the condition is always true or always false, replace it with
177 if (SE->isKnownPredicate(Pred, S, X)) {
178 ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
179 DeadInsts.emplace_back(ICmp);
180 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
181 } else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X)) {
182 ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
183 DeadInsts.emplace_back(ICmp);
184 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
185 } else if (isa<PHINode>(IVOperand) &&
186 SE->isLoopInvariantPredicate(Pred, S, X, ICmpLoop,
187 InvariantPredicate, InvariantLHS,
190 // Rewrite the comparison to a loop invariant comparison if it can be done
191 // cheaply, where cheaply means "we don't need to emit any new
194 Value *NewLHS = nullptr, *NewRHS = nullptr;
196 if (S == InvariantLHS || X == InvariantLHS)
198 ICmp->getOperand(S == InvariantLHS ? IVOperIdx : (1 - IVOperIdx));
200 if (S == InvariantRHS || X == InvariantRHS)
202 ICmp->getOperand(S == InvariantRHS ? IVOperIdx : (1 - IVOperIdx));
204 for (Value *Incoming : cast<PHINode>(IVOperand)->incoming_values()) {
205 if (NewLHS && NewRHS)
208 const SCEV *IncomingS = SE->getSCEV(Incoming);
210 if (!NewLHS && IncomingS == InvariantLHS)
212 if (!NewRHS && IncomingS == InvariantRHS)
216 if (!NewLHS || !NewRHS)
217 // We could not find an existing value to replace either LHS or RHS.
218 // Generating new instructions has subtler tradeoffs, so avoid doing that
222 DEBUG(dbgs() << "INDVARS: Simplified comparison: " << *ICmp << '\n');
223 ICmp->setPredicate(InvariantPredicate);
224 ICmp->setOperand(0, NewLHS);
225 ICmp->setOperand(1, NewRHS);
233 /// SimplifyIVUsers helper for eliminating useless
234 /// remainder operations operating on an induction variable.
235 void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
238 // We're only interested in the case where we know something about
240 if (IVOperand != Rem->getOperand(0))
243 // Get the SCEVs for the ICmp operands.
244 const SCEV *S = SE->getSCEV(Rem->getOperand(0));
245 const SCEV *X = SE->getSCEV(Rem->getOperand(1));
247 // Simplify unnecessary loops away.
248 const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
249 S = SE->getSCEVAtScope(S, ICmpLoop);
250 X = SE->getSCEVAtScope(X, ICmpLoop);
252 // i % n --> i if i is in [0,n).
253 if ((!IsSigned || SE->isKnownNonNegative(S)) &&
254 SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
256 Rem->replaceAllUsesWith(Rem->getOperand(0));
258 // (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n).
259 const SCEV *LessOne = SE->getMinusSCEV(S, SE->getOne(S->getType()));
260 if (IsSigned && !SE->isKnownNonNegative(LessOne))
263 if (!SE->isKnownPredicate(IsSigned ?
264 ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
268 ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ,
269 Rem->getOperand(0), Rem->getOperand(1));
271 SelectInst::Create(ICmp,
272 ConstantInt::get(Rem->getType(), 0),
273 Rem->getOperand(0), "tmp", Rem);
274 Rem->replaceAllUsesWith(Sel);
277 DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
280 DeadInsts.emplace_back(Rem);
283 /// Eliminate an operation that consumes a simple IV and has no observable
284 /// side-effect given the range of IV values. IVOperand is guaranteed SCEVable,
285 /// but UseInst may not be.
286 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
287 Instruction *IVOperand) {
288 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
289 eliminateIVComparison(ICmp, IVOperand);
292 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
293 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
294 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
295 eliminateIVRemainder(Rem, IVOperand, IsSigned);
300 if (eliminateIdentitySCEV(UseInst, IVOperand))
306 /// Eliminate any operation that SCEV can prove is an identity function.
307 bool SimplifyIndvar::eliminateIdentitySCEV(Instruction *UseInst,
308 Instruction *IVOperand) {
309 if (!SE->isSCEVable(UseInst->getType()) ||
310 (UseInst->getType() != IVOperand->getType()) ||
311 (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
314 // getSCEV(X) == getSCEV(Y) does not guarantee that X and Y are related in the
315 // dominator tree, even if X is an operand to Y. For instance, in
317 // %iv = phi i32 {0,+,1}
318 // br %cond, label %left, label %merge
321 // %X = add i32 %iv, 0
325 // %M = phi (%X, %iv)
327 // getSCEV(%M) == getSCEV(%X) == {0,+,1}, but %X does not dominate %M, and
328 // %M.replaceAllUsesWith(%X) would be incorrect.
330 if (isa<PHINode>(UseInst))
331 // If UseInst is not a PHI node then we know that IVOperand dominates
332 // UseInst directly from the legality of SSA.
333 if (!DT || !DT->dominates(IVOperand, UseInst))
336 if (!LI->replacementPreservesLCSSAForm(UseInst, IVOperand))
339 DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
341 UseInst->replaceAllUsesWith(IVOperand);
344 DeadInsts.emplace_back(UseInst);
348 /// Annotate BO with nsw / nuw if it provably does not signed-overflow /
349 /// unsigned-overflow. Returns true if anything changed, false otherwise.
350 bool SimplifyIndvar::strengthenOverflowingOperation(BinaryOperator *BO,
353 // Fastpath: we don't have any work to do if `BO` is `nuw` and `nsw`.
354 if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
357 const SCEV *(ScalarEvolution::*GetExprForBO)(const SCEV *, const SCEV *,
360 switch (BO->getOpcode()) {
364 case Instruction::Add:
365 GetExprForBO = &ScalarEvolution::getAddExpr;
368 case Instruction::Sub:
369 GetExprForBO = &ScalarEvolution::getMinusSCEV;
372 case Instruction::Mul:
373 GetExprForBO = &ScalarEvolution::getMulExpr;
377 unsigned BitWidth = cast<IntegerType>(BO->getType())->getBitWidth();
378 Type *WideTy = IntegerType::get(BO->getContext(), BitWidth * 2);
379 const SCEV *LHS = SE->getSCEV(BO->getOperand(0));
380 const SCEV *RHS = SE->getSCEV(BO->getOperand(1));
382 bool Changed = false;
384 if (!BO->hasNoUnsignedWrap()) {
385 const SCEV *ExtendAfterOp = SE->getZeroExtendExpr(SE->getSCEV(BO), WideTy);
386 const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
387 SE->getZeroExtendExpr(LHS, WideTy), SE->getZeroExtendExpr(RHS, WideTy),
389 if (ExtendAfterOp == OpAfterExtend) {
390 BO->setHasNoUnsignedWrap();
396 if (!BO->hasNoSignedWrap()) {
397 const SCEV *ExtendAfterOp = SE->getSignExtendExpr(SE->getSCEV(BO), WideTy);
398 const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
399 SE->getSignExtendExpr(LHS, WideTy), SE->getSignExtendExpr(RHS, WideTy),
401 if (ExtendAfterOp == OpAfterExtend) {
402 BO->setHasNoSignedWrap();
411 /// \brief Split sadd.with.overflow into add + sadd.with.overflow to allow
412 /// analysis and optimization.
414 /// \return A new value representing the non-overflowing add if possible,
415 /// otherwise return the original value.
416 Instruction *SimplifyIndvar::splitOverflowIntrinsic(Instruction *IVUser,
417 const DominatorTree *DT) {
418 IntrinsicInst *II = dyn_cast<IntrinsicInst>(IVUser);
419 if (!II || II->getIntrinsicID() != Intrinsic::sadd_with_overflow)
422 // Find a branch guarded by the overflow check.
423 BranchInst *Branch = nullptr;
424 Instruction *AddVal = nullptr;
425 for (User *U : II->users()) {
426 if (ExtractValueInst *ExtractInst = dyn_cast<ExtractValueInst>(U)) {
427 if (ExtractInst->getNumIndices() != 1)
429 if (ExtractInst->getIndices()[0] == 0)
430 AddVal = ExtractInst;
431 else if (ExtractInst->getIndices()[0] == 1 && ExtractInst->hasOneUse())
432 Branch = dyn_cast<BranchInst>(ExtractInst->user_back());
435 if (!AddVal || !Branch)
438 BasicBlock *ContinueBB = Branch->getSuccessor(1);
439 if (std::next(pred_begin(ContinueBB)) != pred_end(ContinueBB))
442 // Check if all users of the add are provably NSW.
444 for (Use &U : AddVal->uses()) {
445 if (Instruction *UseInst = dyn_cast<Instruction>(U.getUser())) {
446 BasicBlock *UseBB = UseInst->getParent();
447 if (PHINode *PHI = dyn_cast<PHINode>(UseInst))
448 UseBB = PHI->getIncomingBlock(U);
449 if (!DT->dominates(ContinueBB, UseBB)) {
459 IRBuilder<> Builder(IVUser);
460 Instruction *AddInst = dyn_cast<Instruction>(
461 Builder.CreateNSWAdd(II->getOperand(0), II->getOperand(1)));
463 // The caller expects the new add to have the same form as the intrinsic. The
464 // IV operand position must be the same.
465 assert((AddInst->getOpcode() == Instruction::Add &&
466 AddInst->getOperand(0) == II->getOperand(0)) &&
467 "Bad add instruction created from overflow intrinsic.");
469 AddVal->replaceAllUsesWith(AddInst);
470 DeadInsts.emplace_back(AddVal);
474 /// Add all uses of Def to the current IV's worklist.
475 static void pushIVUsers(
477 SmallPtrSet<Instruction*,16> &Simplified,
478 SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
480 for (User *U : Def->users()) {
481 Instruction *UI = cast<Instruction>(U);
483 // Avoid infinite or exponential worklist processing.
484 // Also ensure unique worklist users.
485 // If Def is a LoopPhi, it may not be in the Simplified set, so check for
487 if (UI != Def && Simplified.insert(UI).second)
488 SimpleIVUsers.push_back(std::make_pair(UI, Def));
492 /// Return true if this instruction generates a simple SCEV
493 /// expression in terms of that IV.
495 /// This is similar to IVUsers' isInteresting() but processes each instruction
496 /// non-recursively when the operand is already known to be a simpleIVUser.
498 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
499 if (!SE->isSCEVable(I->getType()))
502 // Get the symbolic expression for this instruction.
503 const SCEV *S = SE->getSCEV(I);
505 // Only consider affine recurrences.
506 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
507 if (AR && AR->getLoop() == L)
513 /// Iteratively perform simplification on a worklist of users
514 /// of the specified induction variable. Each successive simplification may push
515 /// more users which may themselves be candidates for simplification.
517 /// This algorithm does not require IVUsers analysis. Instead, it simplifies
518 /// instructions in-place during analysis. Rather than rewriting induction
519 /// variables bottom-up from their users, it transforms a chain of IVUsers
520 /// top-down, updating the IR only when it encounters a clear optimization
523 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
525 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
526 if (!SE->isSCEVable(CurrIV->getType()))
529 // Instructions processed by SimplifyIndvar for CurrIV.
530 SmallPtrSet<Instruction*,16> Simplified;
532 // Use-def pairs if IV users waiting to be processed for CurrIV.
533 SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
535 // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
536 // called multiple times for the same LoopPhi. This is the proper thing to
537 // do for loop header phis that use each other.
538 pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
540 while (!SimpleIVUsers.empty()) {
541 std::pair<Instruction*, Instruction*> UseOper =
542 SimpleIVUsers.pop_back_val();
543 Instruction *UseInst = UseOper.first;
545 // Bypass back edges to avoid extra work.
546 if (UseInst == CurrIV) continue;
548 if (V && V->shouldSplitOverflowInstrinsics()) {
549 UseInst = splitOverflowIntrinsic(UseInst, V->getDomTree());
554 Instruction *IVOperand = UseOper.second;
555 for (unsigned N = 0; IVOperand; ++N) {
556 assert(N <= Simplified.size() && "runaway iteration");
558 Value *NewOper = foldIVUser(UseOper.first, IVOperand);
560 break; // done folding
561 IVOperand = dyn_cast<Instruction>(NewOper);
566 if (eliminateIVUser(UseOper.first, IVOperand)) {
567 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
571 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(UseOper.first)) {
572 if (isa<OverflowingBinaryOperator>(BO) &&
573 strengthenOverflowingOperation(BO, IVOperand)) {
574 // re-queue uses of the now modified binary operator and fall
575 // through to the checks that remain.
576 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
580 CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
585 if (isSimpleIVUser(UseOper.first, L, SE)) {
586 pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
593 void IVVisitor::anchor() { }
595 /// Simplify instructions that use this induction variable
596 /// by using ScalarEvolution to analyze the IV's recurrence.
597 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, DominatorTree *DT,
598 LPPassManager *LPM, SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
600 LoopInfo *LI = &LPM->getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
602 SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, DT, LI, Dead);
603 SIV.simplifyUsers(CurrIV, V);
604 return SIV.hasChanged();
607 /// Simplify users of induction variables within this
608 /// loop. This does not actually change or add IVs.
609 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, DominatorTree *DT,
610 LPPassManager *LPM, SmallVectorImpl<WeakVH> &Dead) {
611 bool Changed = false;
612 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
613 Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, DT, LPM, Dead);