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 #define DEBUG_TYPE "indvars"
18 #include "llvm/Instructions.h"
19 #include "llvm/Analysis/Dominators.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/Support/CommandLine.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/Support/raw_ostream.h"
27 #include "llvm/Transforms/Utils/SimplifyIndVar.h"
28 #include "llvm/Target/TargetData.h"
29 #include "llvm/ADT/SmallVector.h"
30 #include "llvm/ADT/Statistic.h"
34 STATISTIC(NumElimIdentity, "Number of IV identities eliminated");
35 STATISTIC(NumElimOperand, "Number of IV operands folded into a use");
36 STATISTIC(NumElimRem , "Number of IV remainder operations eliminated");
37 STATISTIC(NumElimCmp , "Number of IV comparisons eliminated");
40 /// SimplifyIndvar - This is a utility for simplifying induction variables
41 /// based on ScalarEvolution. It is the primary instrument of the
42 /// IndvarSimplify pass, but it may also be directly invoked to cleanup after
43 /// other loop passes that preserve SCEV.
44 class SimplifyIndvar {
49 IVUsers *IU; // NULL for DisableIVRewrite
50 const TargetData *TD; // May be NULL
52 SmallVectorImpl<WeakVH> &DeadInsts;
57 SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, LPPassManager *LPM,
58 SmallVectorImpl<WeakVH> &Dead, IVUsers *IVU = NULL) :
60 LI(LPM->getAnalysisIfAvailable<LoopInfo>()),
63 TD(LPM->getAnalysisIfAvailable<TargetData>()),
66 assert(LI && "IV simplification requires LoopInfo");
69 bool hasChanged() const { return Changed; }
71 /// Iteratively perform simplification on a worklist of users of the
72 /// specified induction variable. This is the top-level driver that applies
73 /// all simplicitions to users of an IV.
74 void simplifyUsers(PHINode *CurrIV, IVVisitor *V = NULL);
76 Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
78 bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
79 void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
80 void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
85 /// foldIVUser - 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 /// 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) {
94 const SCEV *FoldedExpr = 0;
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 (IVOperand->getNumOperands() != 2 ||
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(BitWidth, 1).shl(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 /// eliminateIVComparison - 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 /// eliminateIVRemainder - 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),
225 SelectInst::Create(ICmp,
226 ConstantInt::get(Rem->getType(), 0),
227 Rem->getOperand(0), "tmp", Rem);
228 Rem->replaceAllUsesWith(Sel);
231 // Inform IVUsers about the new users.
233 if (Instruction *I = dyn_cast<Instruction>(Rem->getOperand(0)))
234 IU->AddUsersIfInteresting(I);
236 DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
239 DeadInsts.push_back(Rem);
242 /// eliminateIVUser - Eliminate an operation that consumes a simple IV and has
243 /// no observable side-effect given the range of IV values.
244 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
245 Instruction *IVOperand) {
246 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
247 eliminateIVComparison(ICmp, IVOperand);
250 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
251 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
252 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
253 eliminateIVRemainder(Rem, IVOperand, IsSigned);
258 // Eliminate any operation that SCEV can prove is an identity function.
259 if (!SE->isSCEVable(UseInst->getType()) ||
260 (UseInst->getType() != IVOperand->getType()) ||
261 (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
264 DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
266 UseInst->replaceAllUsesWith(IVOperand);
269 DeadInsts.push_back(UseInst);
273 /// pushIVUsers - Add all uses of Def to the current IV's worklist.
275 static void pushIVUsers(
277 SmallPtrSet<Instruction*,16> &Simplified,
278 SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
280 for (Value::use_iterator UI = Def->use_begin(), E = Def->use_end();
282 Instruction *User = cast<Instruction>(*UI);
284 // Avoid infinite or exponential worklist processing.
285 // Also ensure unique worklist users.
286 // If Def is a LoopPhi, it may not be in the Simplified set, so check for
288 if (User != Def && Simplified.insert(User))
289 SimpleIVUsers.push_back(std::make_pair(User, Def));
293 /// isSimpleIVUser - Return true if this instruction generates a simple SCEV
294 /// expression in terms of that IV.
296 /// This is similar to IVUsers' isInteresting() but processes each instruction
297 /// non-recursively when the operand is already known to be a simpleIVUser.
299 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
300 if (!SE->isSCEVable(I->getType()))
303 // Get the symbolic expression for this instruction.
304 const SCEV *S = SE->getSCEV(I);
306 // Only consider affine recurrences.
307 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
308 if (AR && AR->getLoop() == L)
314 /// simplifyUsers - Iteratively perform simplification on a worklist of users
315 /// of the specified induction variable. Each successive simplification may push
316 /// more users which may themselves be candidates for simplification.
318 /// This algorithm does not require IVUsers analysis. Instead, it simplifies
319 /// instructions in-place during analysis. Rather than rewriting induction
320 /// variables bottom-up from their users, it transforms a chain of IVUsers
321 /// top-down, updating the IR only when it encouters a clear optimization
324 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
326 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
327 // Instructions processed by SimplifyIndvar for CurrIV.
328 SmallPtrSet<Instruction*,16> Simplified;
330 // Use-def pairs if IV users waiting to be processed for CurrIV.
331 SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
333 // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
334 // called multiple times for the same LoopPhi. This is the proper thing to
335 // do for loop header phis that use each other.
336 pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
338 while (!SimpleIVUsers.empty()) {
339 std::pair<Instruction*, Instruction*> UseOper =
340 SimpleIVUsers.pop_back_val();
341 // Bypass back edges to avoid extra work.
342 if (UseOper.first == CurrIV) continue;
344 Instruction *IVOperand = UseOper.second;
345 for (unsigned N = 0; IVOperand; ++N) {
346 assert(N <= Simplified.size() && "runaway iteration");
348 Value *NewOper = foldIVUser(UseOper.first, IVOperand);
350 break; // done folding
351 IVOperand = dyn_cast<Instruction>(NewOper);
356 if (eliminateIVUser(UseOper.first, IVOperand)) {
357 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
360 CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
365 if (isSimpleIVUser(UseOper.first, L, SE)) {
366 pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
373 /// simplifyUsersOfIV - Simplify instructions that use this induction variable
374 /// by using ScalarEvolution to analyze the IV's recurrence.
375 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, LPPassManager *LPM,
376 SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
378 LoopInfo *LI = &LPM->getAnalysis<LoopInfo>();
379 SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, LPM, Dead);
380 SIV.simplifyUsers(CurrIV, V);
381 return SIV.hasChanged();
384 /// simplifyLoopIVs - Simplify users of induction variables within this
385 /// loop. This does not actually change or add IVs.
386 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, LPPassManager *LPM,
387 SmallVectorImpl<WeakVH> &Dead) {
388 bool Changed = false;
389 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
390 Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, LPM, Dead);
395 /// simplifyIVUsers - Perform simplification on instructions recorded by the
398 /// This is the old approach to IV simplification to be replaced by
400 bool simplifyIVUsers(IVUsers *IU, ScalarEvolution *SE, LPPassManager *LPM,
401 SmallVectorImpl<WeakVH> &Dead) {
402 SimplifyIndvar SIV(IU->getLoop(), SE, LPM, Dead);
404 // Each round of simplification involves a round of eliminating operations
405 // followed by a round of widening IVs. A single IVUsers worklist is used
406 // across all rounds. The inner loop advances the user. If widening exposes
407 // more uses, then another pass through the outer loop is triggered.
408 for (IVUsers::iterator I = IU->begin(); I != IU->end(); ++I) {
409 Instruction *UseInst = I->getUser();
410 Value *IVOperand = I->getOperandValToReplace();
412 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
413 SIV.eliminateIVComparison(ICmp, IVOperand);
416 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
417 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
418 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
419 SIV.eliminateIVRemainder(Rem, IVOperand, IsSigned);
424 return SIV.hasChanged();