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 {
51 SmallVectorImpl<WeakVH> &DeadInsts;
56 SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, LoopInfo *LI,
57 SmallVectorImpl<WeakVH> &Dead)
58 : L(Loop), LI(LI), SE(SE), 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 simplicitions to users of an IV.
67 void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr);
69 Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
71 bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
72 void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
73 void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
75 bool strengthenOverflowingOperation(BinaryOperator *OBO, Value *IVOperand);
77 Instruction *splitOverflowIntrinsic(Instruction *IVUser,
78 const DominatorTree *DT);
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 // If the condition is always true or always false, replace it with
171 if (SE->isKnownPredicate(Pred, S, X))
172 ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
173 else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X))
174 ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
178 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
181 DeadInsts.emplace_back(ICmp);
184 /// SimplifyIVUsers helper for eliminating useless
185 /// remainder operations operating on an induction variable.
186 void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
189 // We're only interested in the case where we know something about
191 if (IVOperand != Rem->getOperand(0))
194 // Get the SCEVs for the ICmp operands.
195 const SCEV *S = SE->getSCEV(Rem->getOperand(0));
196 const SCEV *X = SE->getSCEV(Rem->getOperand(1));
198 // Simplify unnecessary loops away.
199 const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
200 S = SE->getSCEVAtScope(S, ICmpLoop);
201 X = SE->getSCEVAtScope(X, ICmpLoop);
203 // i % n --> i if i is in [0,n).
204 if ((!IsSigned || SE->isKnownNonNegative(S)) &&
205 SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
207 Rem->replaceAllUsesWith(Rem->getOperand(0));
209 // (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n).
210 const SCEV *LessOne =
211 SE->getMinusSCEV(S, SE->getConstant(S->getType(), 1));
212 if (IsSigned && !SE->isKnownNonNegative(LessOne))
215 if (!SE->isKnownPredicate(IsSigned ?
216 ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
220 ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ,
221 Rem->getOperand(0), Rem->getOperand(1));
223 SelectInst::Create(ICmp,
224 ConstantInt::get(Rem->getType(), 0),
225 Rem->getOperand(0), "tmp", Rem);
226 Rem->replaceAllUsesWith(Sel);
229 DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
232 DeadInsts.emplace_back(Rem);
235 /// Eliminate an operation that consumes a simple IV and has
236 /// no observable side-effect given the range of IV values.
237 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
238 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
239 Instruction *IVOperand) {
240 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
241 eliminateIVComparison(ICmp, IVOperand);
244 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
245 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
246 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
247 eliminateIVRemainder(Rem, IVOperand, IsSigned);
252 // Eliminate any operation that SCEV can prove is an identity function.
253 if (!SE->isSCEVable(UseInst->getType()) ||
254 (UseInst->getType() != IVOperand->getType()) ||
255 (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
258 DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
260 UseInst->replaceAllUsesWith(IVOperand);
263 DeadInsts.emplace_back(UseInst);
267 /// Annotate BO with nsw / nuw if it provably does not signed-overflow /
268 /// unsigned-overflow. Returns true if anything changed, false otherwise.
269 bool SimplifyIndvar::strengthenOverflowingOperation(BinaryOperator *BO,
272 // Fastpath: we don't have any work to do if `BO` is `nuw` and `nsw`.
273 if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
276 const SCEV *(ScalarEvolution::*GetExprForBO)(const SCEV *, const SCEV *,
279 switch (BO->getOpcode()) {
283 case Instruction::Add:
284 GetExprForBO = &ScalarEvolution::getAddExpr;
287 case Instruction::Sub:
288 GetExprForBO = &ScalarEvolution::getMinusSCEV;
291 case Instruction::Mul:
292 GetExprForBO = &ScalarEvolution::getMulExpr;
296 unsigned BitWidth = cast<IntegerType>(BO->getType())->getBitWidth();
297 Type *WideTy = IntegerType::get(BO->getContext(), BitWidth * 2);
298 const SCEV *LHS = SE->getSCEV(BO->getOperand(0));
299 const SCEV *RHS = SE->getSCEV(BO->getOperand(1));
301 bool Changed = false;
303 if (!BO->hasNoUnsignedWrap()) {
304 const SCEV *ExtendAfterOp = SE->getZeroExtendExpr(SE->getSCEV(BO), WideTy);
305 const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
306 SE->getZeroExtendExpr(LHS, WideTy), SE->getZeroExtendExpr(RHS, WideTy),
308 if (ExtendAfterOp == OpAfterExtend) {
309 BO->setHasNoUnsignedWrap();
315 if (!BO->hasNoSignedWrap()) {
316 const SCEV *ExtendAfterOp = SE->getSignExtendExpr(SE->getSCEV(BO), WideTy);
317 const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
318 SE->getSignExtendExpr(LHS, WideTy), SE->getSignExtendExpr(RHS, WideTy),
320 if (ExtendAfterOp == OpAfterExtend) {
321 BO->setHasNoSignedWrap();
330 /// \brief Split sadd.with.overflow into add + sadd.with.overflow to allow
331 /// analysis and optimization.
333 /// \return A new value representing the non-overflowing add if possible,
334 /// otherwise return the original value.
335 Instruction *SimplifyIndvar::splitOverflowIntrinsic(Instruction *IVUser,
336 const DominatorTree *DT) {
337 IntrinsicInst *II = dyn_cast<IntrinsicInst>(IVUser);
338 if (!II || II->getIntrinsicID() != Intrinsic::sadd_with_overflow)
341 // Find a branch guarded by the overflow check.
342 BranchInst *Branch = nullptr;
343 Instruction *AddVal = nullptr;
344 for (User *U : II->users()) {
345 if (ExtractValueInst *ExtractInst = dyn_cast<ExtractValueInst>(U)) {
346 if (ExtractInst->getNumIndices() != 1)
348 if (ExtractInst->getIndices()[0] == 0)
349 AddVal = ExtractInst;
350 else if (ExtractInst->getIndices()[0] == 1 && ExtractInst->hasOneUse())
351 Branch = dyn_cast<BranchInst>(ExtractInst->user_back());
354 if (!AddVal || !Branch)
357 BasicBlock *ContinueBB = Branch->getSuccessor(1);
358 if (std::next(pred_begin(ContinueBB)) != pred_end(ContinueBB))
361 // Check if all users of the add are provably NSW.
363 for (Use &U : AddVal->uses()) {
364 if (Instruction *UseInst = dyn_cast<Instruction>(U.getUser())) {
365 BasicBlock *UseBB = UseInst->getParent();
366 if (PHINode *PHI = dyn_cast<PHINode>(UseInst))
367 UseBB = PHI->getIncomingBlock(U);
368 if (!DT->dominates(ContinueBB, UseBB)) {
378 IRBuilder<> Builder(IVUser);
379 Instruction *AddInst = dyn_cast<Instruction>(
380 Builder.CreateNSWAdd(II->getOperand(0), II->getOperand(1)));
382 // The caller expects the new add to have the same form as the intrinsic. The
383 // IV operand position must be the same.
384 assert((AddInst->getOpcode() == Instruction::Add &&
385 AddInst->getOperand(0) == II->getOperand(0)) &&
386 "Bad add instruction created from overflow intrinsic.");
388 AddVal->replaceAllUsesWith(AddInst);
389 DeadInsts.emplace_back(AddVal);
393 /// Add all uses of Def to the current IV's worklist.
394 static void pushIVUsers(
396 SmallPtrSet<Instruction*,16> &Simplified,
397 SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
399 for (User *U : Def->users()) {
400 Instruction *UI = cast<Instruction>(U);
402 // Avoid infinite or exponential worklist processing.
403 // Also ensure unique worklist users.
404 // If Def is a LoopPhi, it may not be in the Simplified set, so check for
406 if (UI != Def && Simplified.insert(UI).second)
407 SimpleIVUsers.push_back(std::make_pair(UI, Def));
411 /// Return true if this instruction generates a simple SCEV
412 /// expression in terms of that IV.
414 /// This is similar to IVUsers' isInteresting() but processes each instruction
415 /// non-recursively when the operand is already known to be a simpleIVUser.
417 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
418 if (!SE->isSCEVable(I->getType()))
421 // Get the symbolic expression for this instruction.
422 const SCEV *S = SE->getSCEV(I);
424 // Only consider affine recurrences.
425 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
426 if (AR && AR->getLoop() == L)
432 /// Iteratively perform simplification on a worklist of users
433 /// of the specified induction variable. Each successive simplification may push
434 /// more users which may themselves be candidates for simplification.
436 /// This algorithm does not require IVUsers analysis. Instead, it simplifies
437 /// instructions in-place during analysis. Rather than rewriting induction
438 /// variables bottom-up from their users, it transforms a chain of IVUsers
439 /// top-down, updating the IR only when it encouters a clear optimization
442 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
444 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
445 if (!SE->isSCEVable(CurrIV->getType()))
448 // Instructions processed by SimplifyIndvar for CurrIV.
449 SmallPtrSet<Instruction*,16> Simplified;
451 // Use-def pairs if IV users waiting to be processed for CurrIV.
452 SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
454 // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
455 // called multiple times for the same LoopPhi. This is the proper thing to
456 // do for loop header phis that use each other.
457 pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
459 while (!SimpleIVUsers.empty()) {
460 std::pair<Instruction*, Instruction*> UseOper =
461 SimpleIVUsers.pop_back_val();
462 Instruction *UseInst = UseOper.first;
464 // Bypass back edges to avoid extra work.
465 if (UseInst == CurrIV) continue;
467 if (V && V->shouldSplitOverflowInstrinsics()) {
468 UseInst = splitOverflowIntrinsic(UseInst, V->getDomTree());
473 Instruction *IVOperand = UseOper.second;
474 for (unsigned N = 0; IVOperand; ++N) {
475 assert(N <= Simplified.size() && "runaway iteration");
477 Value *NewOper = foldIVUser(UseOper.first, IVOperand);
479 break; // done folding
480 IVOperand = dyn_cast<Instruction>(NewOper);
485 if (eliminateIVUser(UseOper.first, IVOperand)) {
486 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
490 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(UseOper.first)) {
491 if (isa<OverflowingBinaryOperator>(BO) &&
492 strengthenOverflowingOperation(BO, IVOperand)) {
493 // re-queue uses of the now modified binary operator and fall
494 // through to the checks that remain.
495 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
499 CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
504 if (isSimpleIVUser(UseOper.first, L, SE)) {
505 pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
512 void IVVisitor::anchor() { }
514 /// Simplify instructions that use this induction variable
515 /// by using ScalarEvolution to analyze the IV's recurrence.
516 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, LPPassManager *LPM,
517 SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
519 LoopInfo *LI = &LPM->getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
520 SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, LI, Dead);
521 SIV.simplifyUsers(CurrIV, V);
522 return SIV.hasChanged();
525 /// Simplify users of induction variables within this
526 /// loop. This does not actually change or add IVs.
527 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, LPPassManager *LPM,
528 SmallVectorImpl<WeakVH> &Dead) {
529 bool Changed = false;
530 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
531 Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, LPM, Dead);