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 /// 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) {
96 const SCEV *FoldedExpr = 0;
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(BitWidth, 1).shl(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.push_back(IVOperand);
151 /// eliminateIVComparison - 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 // If the condition is always true or always false, replace it with
174 if (SE->isKnownPredicate(Pred, S, X))
175 ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
176 else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X))
177 ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
181 DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
184 DeadInsts.push_back(ICmp);
187 /// eliminateIVRemainder - SimplifyIVUsers helper for eliminating useless
188 /// remainder operations operating on an induction variable.
189 void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
192 // We're only interested in the case where we know something about
194 if (IVOperand != Rem->getOperand(0))
197 // Get the SCEVs for the ICmp operands.
198 const SCEV *S = SE->getSCEV(Rem->getOperand(0));
199 const SCEV *X = SE->getSCEV(Rem->getOperand(1));
201 // Simplify unnecessary loops away.
202 const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
203 S = SE->getSCEVAtScope(S, ICmpLoop);
204 X = SE->getSCEVAtScope(X, ICmpLoop);
206 // i % n --> i if i is in [0,n).
207 if ((!IsSigned || SE->isKnownNonNegative(S)) &&
208 SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
210 Rem->replaceAllUsesWith(Rem->getOperand(0));
212 // (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n).
213 const SCEV *LessOne =
214 SE->getMinusSCEV(S, SE->getConstant(S->getType(), 1));
215 if (IsSigned && !SE->isKnownNonNegative(LessOne))
218 if (!SE->isKnownPredicate(IsSigned ?
219 ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
223 ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ,
224 Rem->getOperand(0), Rem->getOperand(1));
226 SelectInst::Create(ICmp,
227 ConstantInt::get(Rem->getType(), 0),
228 Rem->getOperand(0), "tmp", Rem);
229 Rem->replaceAllUsesWith(Sel);
232 // Inform IVUsers about the new users.
234 if (Instruction *I = dyn_cast<Instruction>(Rem->getOperand(0)))
235 IU->AddUsersIfInteresting(I);
237 DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
240 DeadInsts.push_back(Rem);
243 /// eliminateIVUser - Eliminate an operation that consumes a simple IV and has
244 /// no observable side-effect given the range of IV values.
245 /// IVOperand is guaranteed SCEVable, but UseInst may not be.
246 bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
247 Instruction *IVOperand) {
248 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
249 eliminateIVComparison(ICmp, IVOperand);
252 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
253 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
254 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
255 eliminateIVRemainder(Rem, IVOperand, IsSigned);
260 // Eliminate any operation that SCEV can prove is an identity function.
261 if (!SE->isSCEVable(UseInst->getType()) ||
262 (UseInst->getType() != IVOperand->getType()) ||
263 (SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
266 DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
268 UseInst->replaceAllUsesWith(IVOperand);
271 DeadInsts.push_back(UseInst);
275 /// pushIVUsers - Add all uses of Def to the current IV's worklist.
277 static void pushIVUsers(
279 SmallPtrSet<Instruction*,16> &Simplified,
280 SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
282 for (Value::use_iterator UI = Def->use_begin(), E = Def->use_end();
284 Instruction *User = cast<Instruction>(*UI);
286 // Avoid infinite or exponential worklist processing.
287 // Also ensure unique worklist users.
288 // If Def is a LoopPhi, it may not be in the Simplified set, so check for
290 if (User != Def && Simplified.insert(User))
291 SimpleIVUsers.push_back(std::make_pair(User, Def));
295 /// isSimpleIVUser - Return true if this instruction generates a simple SCEV
296 /// expression in terms of that IV.
298 /// This is similar to IVUsers' isInteresting() but processes each instruction
299 /// non-recursively when the operand is already known to be a simpleIVUser.
301 static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
302 if (!SE->isSCEVable(I->getType()))
305 // Get the symbolic expression for this instruction.
306 const SCEV *S = SE->getSCEV(I);
308 // Only consider affine recurrences.
309 const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
310 if (AR && AR->getLoop() == L)
316 /// simplifyUsers - Iteratively perform simplification on a worklist of users
317 /// of the specified induction variable. Each successive simplification may push
318 /// more users which may themselves be candidates for simplification.
320 /// This algorithm does not require IVUsers analysis. Instead, it simplifies
321 /// instructions in-place during analysis. Rather than rewriting induction
322 /// variables bottom-up from their users, it transforms a chain of IVUsers
323 /// top-down, updating the IR only when it encouters a clear optimization
326 /// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
328 void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
329 if (!SE->isSCEVable(CurrIV->getType()))
332 // Instructions processed by SimplifyIndvar for CurrIV.
333 SmallPtrSet<Instruction*,16> Simplified;
335 // Use-def pairs if IV users waiting to be processed for CurrIV.
336 SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
338 // Push users of the current LoopPhi. In rare cases, pushIVUsers may be
339 // called multiple times for the same LoopPhi. This is the proper thing to
340 // do for loop header phis that use each other.
341 pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
343 while (!SimpleIVUsers.empty()) {
344 std::pair<Instruction*, Instruction*> UseOper =
345 SimpleIVUsers.pop_back_val();
346 // Bypass back edges to avoid extra work.
347 if (UseOper.first == CurrIV) continue;
349 Instruction *IVOperand = UseOper.second;
350 for (unsigned N = 0; IVOperand; ++N) {
351 assert(N <= Simplified.size() && "runaway iteration");
353 Value *NewOper = foldIVUser(UseOper.first, IVOperand);
355 break; // done folding
356 IVOperand = dyn_cast<Instruction>(NewOper);
361 if (eliminateIVUser(UseOper.first, IVOperand)) {
362 pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
365 CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
370 if (isSimpleIVUser(UseOper.first, L, SE)) {
371 pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
378 /// simplifyUsersOfIV - Simplify instructions that use this induction variable
379 /// by using ScalarEvolution to analyze the IV's recurrence.
380 bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, LPPassManager *LPM,
381 SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
383 LoopInfo *LI = &LPM->getAnalysis<LoopInfo>();
384 SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, LPM, Dead);
385 SIV.simplifyUsers(CurrIV, V);
386 return SIV.hasChanged();
389 /// simplifyLoopIVs - Simplify users of induction variables within this
390 /// loop. This does not actually change or add IVs.
391 bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, LPPassManager *LPM,
392 SmallVectorImpl<WeakVH> &Dead) {
393 bool Changed = false;
394 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
395 Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, LPM, Dead);
400 /// simplifyIVUsers - Perform simplification on instructions recorded by the
403 /// This is the old approach to IV simplification to be replaced by
405 bool simplifyIVUsers(IVUsers *IU, ScalarEvolution *SE, LPPassManager *LPM,
406 SmallVectorImpl<WeakVH> &Dead) {
407 SimplifyIndvar SIV(IU->getLoop(), SE, LPM, Dead);
409 // Each round of simplification involves a round of eliminating operations
410 // followed by a round of widening IVs. A single IVUsers worklist is used
411 // across all rounds. The inner loop advances the user. If widening exposes
412 // more uses, then another pass through the outer loop is triggered.
413 for (IVUsers::iterator I = IU->begin(); I != IU->end(); ++I) {
414 Instruction *UseInst = I->getUser();
415 Value *IVOperand = I->getOperandValToReplace();
417 if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
418 SIV.eliminateIVComparison(ICmp, IVOperand);
421 if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
422 bool IsSigned = Rem->getOpcode() == Instruction::SRem;
423 if (IsSigned || Rem->getOpcode() == Instruction::URem) {
424 SIV.eliminateIVRemainder(Rem, IVOperand, IsSigned);
429 return SIV.hasChanged();