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/IVUsers.h"
21 #include "llvm/Analysis/LoopInfo.h"
22 #include "llvm/Analysis/LoopPass.h"
23 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
24 #include "llvm/IR/DataLayout.h"
25 #include "llvm/IR/Dominators.h"
26 #include "llvm/IR/IRBuilder.h"
27 #include "llvm/IR/Instructions.h"
28 #include "llvm/IR/IntrinsicInst.h"
29 #include "llvm/Support/CommandLine.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/raw_ostream.h"
35 #define DEBUG_TYPE "indvars"
37 STATISTIC(NumElimIdentity, "Number of IV identities eliminated");
38 STATISTIC(NumElimOperand, "Number of IV operands folded into a use");
39 STATISTIC(NumElimRem , "Number of IV remainder operations eliminated");
40 STATISTIC(NumElimCmp , "Number of IV comparisons eliminated");
43 /// This is a utility for simplifying induction variables
44 /// based on ScalarEvolution. It is the primary instrument of the
45 /// IndvarSimplify pass, but it may also be directly invoked to cleanup after
46 /// other loop passes that preserve SCEV.
47 class SimplifyIndvar {
52 SmallVectorImpl<WeakVH> &DeadInsts;
57 SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, LoopInfo *LI,
58 SmallVectorImpl<WeakVH> &Dead, IVUsers *IVU = nullptr)
59 : L(Loop), LI(LI), SE(SE), 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 simplicitions to users of an IV.
68 void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr);
70 Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
72 bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
73 void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
74 void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
76 bool strengthenOverflowingOperation(BinaryOperator *OBO, Value *IVOperand);
78 Instruction *splitOverflowIntrinsic(Instruction *IVUser,
79 const DominatorTree *DT);
83 /// Fold an IV operand into its use. This removes increments of an
84 /// aligned IV when used by a instruction that ignores the low bits.
86 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
88 /// Return the operand of IVOperand for this induction variable if IVOperand can
89 /// be folded (in case more folding opportunities have been exposed).
90 /// Otherwise return null.
91 Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) {
92 Value *IVSrc = nullptr;
94 const SCEV *FoldedExpr = nullptr;
95 switch (UseInst->getOpcode()) {
98 case Instruction::UDiv:
99 case Instruction::LShr:
100 // We're only interested in the case where we know something about
101 // the numerator and have a constant denominator.
102 if (IVOperand != UseInst->getOperand(OperIdx) ||
103 !isa<ConstantInt>(UseInst->getOperand(1)))
106 // Attempt to fold a binary operator with constant operand.
107 // e.g. ((I + 1) >> 2) => I >> 2
108 if (!isa<BinaryOperator>(IVOperand)
109 || !isa<ConstantInt>(IVOperand->getOperand(1)))
112 IVSrc = IVOperand->getOperand(0);
113 // IVSrc must be the (SCEVable) IV, since the other operand is const.
114 assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand");
116 ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1));
117 if (UseInst->getOpcode() == Instruction::LShr) {
118 // Get a constant for the divisor. See createSCEV.
119 uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth();
120 if (D->getValue().uge(BitWidth))
123 D = ConstantInt::get(UseInst->getContext(),
124 APInt::getOneBitSet(BitWidth, D->getZExtValue()));
126 FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
128 // We have something that might fold it's operand. Compare SCEVs.
129 if (!SE->isSCEVable(UseInst->getType()))
132 // Bypass the operand if SCEV can prove it has no effect.
133 if (SE->getSCEV(UseInst) != FoldedExpr)
136 DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand
137 << " -> " << *UseInst << '\n');
139 UseInst->setOperand(OperIdx, IVSrc);
140 assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
144 if (IVOperand->use_empty())
145 DeadInsts.push_back(IVOperand);
149 /// SimplifyIVUsers helper for eliminating useless
150 /// comparisons against an induction variable.
151 void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
152 unsigned IVOperIdx = 0;
153 ICmpInst::Predicate Pred = ICmp->getPredicate();
154 if (IVOperand != ICmp->getOperand(0)) {
156 assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
158 Pred = ICmpInst::getSwappedPredicate(Pred);
161 // Get the SCEVs for the ICmp operands.
162 const SCEV *S = SE->getSCEV(ICmp->getOperand(IVOperIdx));
163 const SCEV *X = SE->getSCEV(ICmp->getOperand(1 - IVOperIdx));
165 // Simplify unnecessary loops away.
166 const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
167 S = SE->getSCEVAtScope(S, ICmpLoop);
168 X = SE->getSCEVAtScope(X, ICmpLoop);
170 // If the condition is always true or always false, replace it with
172 if (SE->isKnownPredicate(Pred, S, X))
173 ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
174 else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X))
175 ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
179 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
182 DeadInsts.push_back(ICmp);
185 /// SimplifyIVUsers helper for eliminating useless
186 /// remainder operations operating on an induction variable.
187 void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
190 // We're only interested in the case where we know something about
192 if (IVOperand != Rem->getOperand(0))
195 // Get the SCEVs for the ICmp operands.
196 const SCEV *S = SE->getSCEV(Rem->getOperand(0));
197 const SCEV *X = SE->getSCEV(Rem->getOperand(1));
199 // Simplify unnecessary loops away.
200 const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
201 S = SE->getSCEVAtScope(S, ICmpLoop);
202 X = SE->getSCEVAtScope(X, ICmpLoop);
204 // i % n --> i if i is in [0,n).
205 if ((!IsSigned || SE->isKnownNonNegative(S)) &&
206 SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
208 Rem->replaceAllUsesWith(Rem->getOperand(0));
210 // (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n).
211 const SCEV *LessOne =
212 SE->getMinusSCEV(S, SE->getConstant(S->getType(), 1));
213 if (IsSigned && !SE->isKnownNonNegative(LessOne))
216 if (!SE->isKnownPredicate(IsSigned ?
217 ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
221 ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ,
222 Rem->getOperand(0), Rem->getOperand(1));
224 SelectInst::Create(ICmp,
225 ConstantInt::get(Rem->getType(), 0),
226 Rem->getOperand(0), "tmp", Rem);
227 Rem->replaceAllUsesWith(Sel);
230 DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
233 DeadInsts.push_back(Rem);
236 /// Eliminate an operation that consumes a simple IV and has
237 /// no observable side-effect given the range of IV values.
238 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
239 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
240 Instruction *IVOperand) {
241 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
242 eliminateIVComparison(ICmp, IVOperand);
245 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
246 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
247 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
248 eliminateIVRemainder(Rem, IVOperand, IsSigned);
253 // Eliminate any operation that SCEV can prove is an identity function.
254 if (!SE->isSCEVable(UseInst->getType()) ||
255 (UseInst->getType() != IVOperand->getType()) ||
256 (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
259 DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
261 UseInst->replaceAllUsesWith(IVOperand);
264 DeadInsts.push_back(UseInst);
268 /// Annotate BO with nsw / nuw if it provably does not signed-overflow /
269 /// unsigned-overflow. Returns true if anything changed, false otherwise.
270 bool SimplifyIndvar::strengthenOverflowingOperation(BinaryOperator *BO,
273 // Fastpath: we don't have any work to do if `BO` is `nuw` and `nsw`.
274 if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
277 const SCEV *(ScalarEvolution::*GetExprForBO)(const SCEV *, const SCEV *,
280 switch (BO->getOpcode()) {
284 case Instruction::Add:
285 GetExprForBO = &ScalarEvolution::getAddExpr;
288 case Instruction::Sub:
289 GetExprForBO = &ScalarEvolution::getMinusSCEV;
292 case Instruction::Mul:
293 GetExprForBO = &ScalarEvolution::getMulExpr;
297 unsigned BitWidth = cast<IntegerType>(BO->getType())->getBitWidth();
298 Type *WideTy = IntegerType::get(BO->getContext(), BitWidth * 2);
299 const SCEV *LHS = SE->getSCEV(BO->getOperand(0));
300 const SCEV *RHS = SE->getSCEV(BO->getOperand(1));
302 bool Changed = false;
304 if (!BO->hasNoUnsignedWrap()) {
305 const SCEV *ExtendAfterOp = SE->getZeroExtendExpr(SE->getSCEV(BO), WideTy);
306 const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
307 SE->getZeroExtendExpr(LHS, WideTy), SE->getZeroExtendExpr(RHS, WideTy),
309 if (ExtendAfterOp == OpAfterExtend) {
310 BO->setHasNoUnsignedWrap();
316 if (!BO->hasNoSignedWrap()) {
317 const SCEV *ExtendAfterOp = SE->getSignExtendExpr(SE->getSCEV(BO), WideTy);
318 const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
319 SE->getSignExtendExpr(LHS, WideTy), SE->getSignExtendExpr(RHS, WideTy),
321 if (ExtendAfterOp == OpAfterExtend) {
322 BO->setHasNoSignedWrap();
331 /// \brief Split sadd.with.overflow into add + sadd.with.overflow to allow
332 /// analysis and optimization.
334 /// \return A new value representing the non-overflowing add if possible,
335 /// otherwise return the original value.
336 Instruction *SimplifyIndvar::splitOverflowIntrinsic(Instruction *IVUser,
337 const DominatorTree *DT) {
338 IntrinsicInst *II = dyn_cast<IntrinsicInst>(IVUser);
339 if (!II || II->getIntrinsicID() != Intrinsic::sadd_with_overflow)
342 // Find a branch guarded by the overflow check.
343 BranchInst *Branch = nullptr;
344 Instruction *AddVal = nullptr;
345 for (User *U : II->users()) {
346 if (ExtractValueInst *ExtractInst = dyn_cast<ExtractValueInst>(U)) {
347 if (ExtractInst->getNumIndices() != 1)
349 if (ExtractInst->getIndices()[0] == 0)
350 AddVal = ExtractInst;
351 else if (ExtractInst->getIndices()[0] == 1 && ExtractInst->hasOneUse())
352 Branch = dyn_cast<BranchInst>(ExtractInst->user_back());
355 if (!AddVal || !Branch)
358 BasicBlock *ContinueBB = Branch->getSuccessor(1);
359 if (std::next(pred_begin(ContinueBB)) != pred_end(ContinueBB))
362 // Check if all users of the add are provably NSW.
364 for (Use &U : AddVal->uses()) {
365 if (Instruction *UseInst = dyn_cast<Instruction>(U.getUser())) {
366 BasicBlock *UseBB = UseInst->getParent();
367 if (PHINode *PHI = dyn_cast<PHINode>(UseInst))
368 UseBB = PHI->getIncomingBlock(U);
369 if (!DT->dominates(ContinueBB, UseBB)) {
379 IRBuilder<> Builder(IVUser);
380 Instruction *AddInst = dyn_cast<Instruction>(
381 Builder.CreateNSWAdd(II->getOperand(0), II->getOperand(1)));
383 // The caller expects the new add to have the same form as the intrinsic. The
384 // IV operand position must be the same.
385 assert((AddInst->getOpcode() == Instruction::Add &&
386 AddInst->getOperand(0) == II->getOperand(0)) &&
387 "Bad add instruction created from overflow intrinsic.");
389 AddVal->replaceAllUsesWith(AddInst);
390 DeadInsts.push_back(AddVal);
394 /// Add all uses of Def to the current IV's worklist.
395 static void pushIVUsers(
397 SmallPtrSet<Instruction*,16> &Simplified,
398 SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
400 for (User *U : Def->users()) {
401 Instruction *UI = cast<Instruction>(U);
403 // Avoid infinite or exponential worklist processing.
404 // Also ensure unique worklist users.
405 // If Def is a LoopPhi, it may not be in the Simplified set, so check for
407 if (UI != Def && Simplified.insert(UI).second)
408 SimpleIVUsers.push_back(std::make_pair(UI, Def));
412 /// Return true if this instruction generates a simple SCEV
413 /// expression in terms of that IV.
415 /// This is similar to IVUsers' isInteresting() but processes each instruction
416 /// non-recursively when the operand is already known to be a simpleIVUser.
418 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
419 if (!SE->isSCEVable(I->getType()))
422 // Get the symbolic expression for this instruction.
423 const SCEV *S = SE->getSCEV(I);
425 // Only consider affine recurrences.
426 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
427 if (AR && AR->getLoop() == L)
433 /// Iteratively perform simplification on a worklist of users
434 /// of the specified induction variable. Each successive simplification may push
435 /// more users which may themselves be candidates for simplification.
437 /// This algorithm does not require IVUsers analysis. Instead, it simplifies
438 /// instructions in-place during analysis. Rather than rewriting induction
439 /// variables bottom-up from their users, it transforms a chain of IVUsers
440 /// top-down, updating the IR only when it encouters a clear optimization
443 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
445 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
446 if (!SE->isSCEVable(CurrIV->getType()))
449 // Instructions processed by SimplifyIndvar for CurrIV.
450 SmallPtrSet<Instruction*,16> Simplified;
452 // Use-def pairs if IV users waiting to be processed for CurrIV.
453 SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
455 // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
456 // called multiple times for the same LoopPhi. This is the proper thing to
457 // do for loop header phis that use each other.
458 pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
460 while (!SimpleIVUsers.empty()) {
461 std::pair<Instruction*, Instruction*> UseOper =
462 SimpleIVUsers.pop_back_val();
463 Instruction *UseInst = UseOper.first;
465 // Bypass back edges to avoid extra work.
466 if (UseInst == CurrIV) continue;
468 if (V && V->shouldSplitOverflowInstrinsics()) {
469 UseInst = splitOverflowIntrinsic(UseInst, V->getDomTree());
474 Instruction *IVOperand = UseOper.second;
475 for (unsigned N = 0; IVOperand; ++N) {
476 assert(N <= Simplified.size() && "runaway iteration");
478 Value *NewOper = foldIVUser(UseOper.first, IVOperand);
480 break; // done folding
481 IVOperand = dyn_cast<Instruction>(NewOper);
486 if (eliminateIVUser(UseOper.first, IVOperand)) {
487 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
491 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(UseOper.first)) {
492 if (isa<OverflowingBinaryOperator>(BO) &&
493 strengthenOverflowingOperation(BO, IVOperand)) {
494 // re-queue uses of the now modified binary operator and fall
495 // through to the checks that remain.
496 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
500 CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
505 if (isSimpleIVUser(UseOper.first, L, SE)) {
506 pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
513 void IVVisitor::anchor() { }
515 /// Simplify instructions that use this induction variable
516 /// by using ScalarEvolution to analyze the IV's recurrence.
517 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, LPPassManager *LPM,
518 SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
520 LoopInfo *LI = &LPM->getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
521 SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, LI, Dead);
522 SIV.simplifyUsers(CurrIV, V);
523 return SIV.hasChanged();
526 /// Simplify users of induction variables within this
527 /// loop. This does not actually change or add IVs.
528 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, LPPassManager *LPM,
529 SmallVectorImpl<WeakVH> &Dead) {
530 bool Changed = false;
531 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
532 Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, LPM, Dead);