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 {
51 const DataLayout *DL; // May be NULL
53 SmallVectorImpl<WeakVH> &DeadInsts;
58 SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, LPPassManager *LPM,
59 SmallVectorImpl<WeakVH> &Dead, IVUsers *IVU = nullptr) :
61 LI(LPM->getAnalysisIfAvailable<LoopInfo>()),
65 DataLayoutPass *DLP = LPM->getAnalysisIfAvailable<DataLayoutPass>();
66 DL = DLP ? &DLP->getDataLayout() : nullptr;
67 assert(LI && "IV simplification requires LoopInfo");
70 bool hasChanged() const { return Changed; }
72 /// Iteratively perform simplification on a worklist of users of the
73 /// specified induction variable. This is the top-level driver that applies
74 /// all simplicitions to users of an IV.
75 void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr);
77 Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
79 bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
80 void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
81 void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
84 Instruction *splitOverflowIntrinsic(Instruction *IVUser,
85 const DominatorTree *DT);
89 /// Fold an IV operand into its use. This removes increments of an
90 /// aligned IV when used by a instruction that ignores the low bits.
92 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
94 /// Return the operand of IVOperand for this induction variable if IVOperand can
95 /// be folded (in case more folding opportunities have been exposed).
96 /// Otherwise return null.
97 Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) {
98 Value *IVSrc = nullptr;
100 const SCEV *FoldedExpr = nullptr;
101 switch (UseInst->getOpcode()) {
104 case Instruction::UDiv:
105 case Instruction::LShr:
106 // We're only interested in the case where we know something about
107 // the numerator and have a constant denominator.
108 if (IVOperand != UseInst->getOperand(OperIdx) ||
109 !isa<ConstantInt>(UseInst->getOperand(1)))
112 // Attempt to fold a binary operator with constant operand.
113 // e.g. ((I + 1) >> 2) => I >> 2
114 if (!isa<BinaryOperator>(IVOperand)
115 || !isa<ConstantInt>(IVOperand->getOperand(1)))
118 IVSrc = IVOperand->getOperand(0);
119 // IVSrc must be the (SCEVable) IV, since the other operand is const.
120 assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand");
122 ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1));
123 if (UseInst->getOpcode() == Instruction::LShr) {
124 // Get a constant for the divisor. See createSCEV.
125 uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth();
126 if (D->getValue().uge(BitWidth))
129 D = ConstantInt::get(UseInst->getContext(),
130 APInt::getOneBitSet(BitWidth, D->getZExtValue()));
132 FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
134 // We have something that might fold it's operand. Compare SCEVs.
135 if (!SE->isSCEVable(UseInst->getType()))
138 // Bypass the operand if SCEV can prove it has no effect.
139 if (SE->getSCEV(UseInst) != FoldedExpr)
142 DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand
143 << " -> " << *UseInst << '\n');
145 UseInst->setOperand(OperIdx, IVSrc);
146 assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
150 if (IVOperand->use_empty())
151 DeadInsts.push_back(IVOperand);
155 /// SimplifyIVUsers helper for eliminating useless
156 /// comparisons against an induction variable.
157 void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
158 unsigned IVOperIdx = 0;
159 ICmpInst::Predicate Pred = ICmp->getPredicate();
160 if (IVOperand != ICmp->getOperand(0)) {
162 assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
164 Pred = ICmpInst::getSwappedPredicate(Pred);
167 // Get the SCEVs for the ICmp operands.
168 const SCEV *S = SE->getSCEV(ICmp->getOperand(IVOperIdx));
169 const SCEV *X = SE->getSCEV(ICmp->getOperand(1 - IVOperIdx));
171 // Simplify unnecessary loops away.
172 const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
173 S = SE->getSCEVAtScope(S, ICmpLoop);
174 X = SE->getSCEVAtScope(X, ICmpLoop);
176 // If the condition is always true or always false, replace it with
178 if (SE->isKnownPredicate(Pred, S, X))
179 ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
180 else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X))
181 ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
185 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
188 DeadInsts.push_back(ICmp);
191 /// SimplifyIVUsers helper for eliminating useless
192 /// remainder operations operating on an induction variable.
193 void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
196 // We're only interested in the case where we know something about
198 if (IVOperand != Rem->getOperand(0))
201 // Get the SCEVs for the ICmp operands.
202 const SCEV *S = SE->getSCEV(Rem->getOperand(0));
203 const SCEV *X = SE->getSCEV(Rem->getOperand(1));
205 // Simplify unnecessary loops away.
206 const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
207 S = SE->getSCEVAtScope(S, ICmpLoop);
208 X = SE->getSCEVAtScope(X, ICmpLoop);
210 // i % n --> i if i is in [0,n).
211 if ((!IsSigned || SE->isKnownNonNegative(S)) &&
212 SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
214 Rem->replaceAllUsesWith(Rem->getOperand(0));
216 // (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n).
217 const SCEV *LessOne =
218 SE->getMinusSCEV(S, SE->getConstant(S->getType(), 1));
219 if (IsSigned && !SE->isKnownNonNegative(LessOne))
222 if (!SE->isKnownPredicate(IsSigned ?
223 ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
227 ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ,
228 Rem->getOperand(0), Rem->getOperand(1));
230 SelectInst::Create(ICmp,
231 ConstantInt::get(Rem->getType(), 0),
232 Rem->getOperand(0), "tmp", Rem);
233 Rem->replaceAllUsesWith(Sel);
236 DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
239 DeadInsts.push_back(Rem);
242 /// Eliminate an operation that consumes a simple IV and has
243 /// no observable side-effect given the range of IV values.
244 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
245 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
246 Instruction *IVOperand) {
247 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
248 eliminateIVComparison(ICmp, IVOperand);
251 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
252 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
253 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
254 eliminateIVRemainder(Rem, IVOperand, IsSigned);
259 // Eliminate any operation that SCEV can prove is an identity function.
260 if (!SE->isSCEVable(UseInst->getType()) ||
261 (UseInst->getType() != IVOperand->getType()) ||
262 (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
265 DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
267 UseInst->replaceAllUsesWith(IVOperand);
270 DeadInsts.push_back(UseInst);
274 /// \brief Split sadd.with.overflow into add + sadd.with.overflow to allow
275 /// analysis and optimization.
277 /// \return A new value representing the non-overflowing add if possible,
278 /// otherwise return the original value.
279 Instruction *SimplifyIndvar::splitOverflowIntrinsic(Instruction *IVUser,
280 const DominatorTree *DT) {
281 IntrinsicInst *II = dyn_cast<IntrinsicInst>(IVUser);
282 if (!II || II->getIntrinsicID() != Intrinsic::sadd_with_overflow)
285 // Find a branch guarded by the overflow check.
286 BranchInst *Branch = nullptr;
287 Instruction *AddVal = nullptr;
288 for (User *U : II->users()) {
289 if (ExtractValueInst *ExtractInst = dyn_cast<ExtractValueInst>(U)) {
290 if (ExtractInst->getNumIndices() != 1)
292 if (ExtractInst->getIndices()[0] == 0)
293 AddVal = ExtractInst;
294 else if (ExtractInst->getIndices()[0] == 1 && ExtractInst->hasOneUse())
295 Branch = dyn_cast<BranchInst>(ExtractInst->user_back());
298 if (!AddVal || !Branch)
301 BasicBlock *ContinueBB = Branch->getSuccessor(1);
302 if (std::next(pred_begin(ContinueBB)) != pred_end(ContinueBB))
305 // Check if all users of the add are provably NSW.
307 for (Use &U : AddVal->uses()) {
308 if (Instruction *UseInst = dyn_cast<Instruction>(U.getUser())) {
309 BasicBlock *UseBB = UseInst->getParent();
310 if (PHINode *PHI = dyn_cast<PHINode>(UseInst))
311 UseBB = PHI->getIncomingBlock(U);
312 if (!DT->dominates(ContinueBB, UseBB)) {
322 IRBuilder<> Builder(IVUser);
323 Instruction *AddInst = dyn_cast<Instruction>(
324 Builder.CreateNSWAdd(II->getOperand(0), II->getOperand(1)));
326 // The caller expects the new add to have the same form as the intrinsic. The
327 // IV operand position must be the same.
328 assert((AddInst->getOpcode() == Instruction::Add &&
329 AddInst->getOperand(0) == II->getOperand(0)) &&
330 "Bad add instruction created from overflow intrinsic.");
332 AddVal->replaceAllUsesWith(AddInst);
333 DeadInsts.push_back(AddVal);
337 /// Add all uses of Def to the current IV's worklist.
338 static void pushIVUsers(
340 SmallPtrSet<Instruction*,16> &Simplified,
341 SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
343 for (User *U : Def->users()) {
344 Instruction *UI = cast<Instruction>(U);
346 // Avoid infinite or exponential worklist processing.
347 // Also ensure unique worklist users.
348 // If Def is a LoopPhi, it may not be in the Simplified set, so check for
350 if (UI != Def && Simplified.insert(UI).second)
351 SimpleIVUsers.push_back(std::make_pair(UI, Def));
355 /// Return true if this instruction generates a simple SCEV
356 /// expression in terms of that IV.
358 /// This is similar to IVUsers' isInteresting() but processes each instruction
359 /// non-recursively when the operand is already known to be a simpleIVUser.
361 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
362 if (!SE->isSCEVable(I->getType()))
365 // Get the symbolic expression for this instruction.
366 const SCEV *S = SE->getSCEV(I);
368 // Only consider affine recurrences.
369 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
370 if (AR && AR->getLoop() == L)
376 /// Iteratively perform simplification on a worklist of users
377 /// of the specified induction variable. Each successive simplification may push
378 /// more users which may themselves be candidates for simplification.
380 /// This algorithm does not require IVUsers analysis. Instead, it simplifies
381 /// instructions in-place during analysis. Rather than rewriting induction
382 /// variables bottom-up from their users, it transforms a chain of IVUsers
383 /// top-down, updating the IR only when it encouters a clear optimization
386 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
388 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
389 if (!SE->isSCEVable(CurrIV->getType()))
392 // Instructions processed by SimplifyIndvar for CurrIV.
393 SmallPtrSet<Instruction*,16> Simplified;
395 // Use-def pairs if IV users waiting to be processed for CurrIV.
396 SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
398 // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
399 // called multiple times for the same LoopPhi. This is the proper thing to
400 // do for loop header phis that use each other.
401 pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
403 while (!SimpleIVUsers.empty()) {
404 std::pair<Instruction*, Instruction*> UseOper =
405 SimpleIVUsers.pop_back_val();
406 Instruction *UseInst = UseOper.first;
408 // Bypass back edges to avoid extra work.
409 if (UseInst == CurrIV) continue;
411 if (V && V->shouldSplitOverflowInstrinsics()) {
412 UseInst = splitOverflowIntrinsic(UseInst, V->getDomTree());
417 Instruction *IVOperand = UseOper.second;
418 for (unsigned N = 0; IVOperand; ++N) {
419 assert(N <= Simplified.size() && "runaway iteration");
421 Value *NewOper = foldIVUser(UseOper.first, IVOperand);
423 break; // done folding
424 IVOperand = dyn_cast<Instruction>(NewOper);
429 if (eliminateIVUser(UseOper.first, IVOperand)) {
430 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
433 CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
438 if (isSimpleIVUser(UseOper.first, L, SE)) {
439 pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
446 void IVVisitor::anchor() { }
448 /// Simplify instructions that use this induction variable
449 /// by using ScalarEvolution to analyze the IV's recurrence.
450 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, LPPassManager *LPM,
451 SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
453 LoopInfo *LI = &LPM->getAnalysis<LoopInfo>();
454 SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, LPM, Dead);
455 SIV.simplifyUsers(CurrIV, V);
456 return SIV.hasChanged();
459 /// Simplify users of induction variables within this
460 /// loop. This does not actually change or add IVs.
461 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, LPPassManager *LPM,
462 SmallVectorImpl<WeakVH> &Dead) {
463 bool Changed = false;
464 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
465 Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, LPM, Dead);