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394 lines
14 KiB
C++
394 lines
14 KiB
C++
//===-- SimplifyIndVar.cpp - Induction variable simplification ------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements induction variable simplification. It does
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// not define any actual pass or policy, but provides a single function to
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// simplify a loop's induction variables based on ScalarEvolution.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "indvars"
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#include "llvm/Transforms/Utils/SimplifyIndVar.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/IVUsers.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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using namespace llvm;
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STATISTIC(NumElimIdentity, "Number of IV identities eliminated");
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STATISTIC(NumElimOperand, "Number of IV operands folded into a use");
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STATISTIC(NumElimRem , "Number of IV remainder operations eliminated");
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STATISTIC(NumElimCmp , "Number of IV comparisons eliminated");
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namespace {
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/// SimplifyIndvar - This is a utility for simplifying induction variables
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/// based on ScalarEvolution. It is the primary instrument of the
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/// IndvarSimplify pass, but it may also be directly invoked to cleanup after
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/// other loop passes that preserve SCEV.
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class SimplifyIndvar {
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Loop *L;
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LoopInfo *LI;
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ScalarEvolution *SE;
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const DataLayout *TD; // May be NULL
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SmallVectorImpl<WeakVH> &DeadInsts;
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bool Changed;
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public:
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SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, LPPassManager *LPM,
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SmallVectorImpl<WeakVH> &Dead, IVUsers *IVU = NULL) :
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L(Loop),
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LI(LPM->getAnalysisIfAvailable<LoopInfo>()),
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SE(SE),
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TD(LPM->getAnalysisIfAvailable<DataLayout>()),
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DeadInsts(Dead),
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Changed(false) {
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assert(LI && "IV simplification requires LoopInfo");
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}
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bool hasChanged() const { return Changed; }
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/// Iteratively perform simplification on a worklist of users of the
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/// specified induction variable. This is the top-level driver that applies
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/// all simplicitions to users of an IV.
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void simplifyUsers(PHINode *CurrIV, IVVisitor *V = NULL);
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Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
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bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
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void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
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void eliminateIVRemainder(BinaryOperator *Rem, Value *IVOperand,
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bool IsSigned);
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};
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}
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/// foldIVUser - Fold an IV operand into its use. This removes increments of an
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/// aligned IV when used by a instruction that ignores the low bits.
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///
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/// IVOperand is guaranteed SCEVable, but UseInst may not be.
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///
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/// Return the operand of IVOperand for this induction variable if IVOperand can
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/// be folded (in case more folding opportunities have been exposed).
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/// Otherwise return null.
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Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) {
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Value *IVSrc = 0;
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unsigned OperIdx = 0;
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const SCEV *FoldedExpr = 0;
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switch (UseInst->getOpcode()) {
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default:
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return 0;
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case Instruction::UDiv:
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case Instruction::LShr:
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// We're only interested in the case where we know something about
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// the numerator and have a constant denominator.
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if (IVOperand != UseInst->getOperand(OperIdx) ||
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!isa<ConstantInt>(UseInst->getOperand(1)))
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return 0;
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// Attempt to fold a binary operator with constant operand.
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// e.g. ((I + 1) >> 2) => I >> 2
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if (!isa<BinaryOperator>(IVOperand)
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|| !isa<ConstantInt>(IVOperand->getOperand(1)))
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return 0;
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IVSrc = IVOperand->getOperand(0);
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// IVSrc must be the (SCEVable) IV, since the other operand is const.
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assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand");
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ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1));
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if (UseInst->getOpcode() == Instruction::LShr) {
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// Get a constant for the divisor. See createSCEV.
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uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth();
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if (D->getValue().uge(BitWidth))
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return 0;
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D = ConstantInt::get(UseInst->getContext(),
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APInt::getOneBitSet(BitWidth, D->getZExtValue()));
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}
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FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
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}
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// We have something that might fold it's operand. Compare SCEVs.
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if (!SE->isSCEVable(UseInst->getType()))
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return 0;
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// Bypass the operand if SCEV can prove it has no effect.
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if (SE->getSCEV(UseInst) != FoldedExpr)
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return 0;
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DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand
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<< " -> " << *UseInst << '\n');
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UseInst->setOperand(OperIdx, IVSrc);
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assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
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++NumElimOperand;
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Changed = true;
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if (IVOperand->use_empty())
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DeadInsts.push_back(IVOperand);
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return IVSrc;
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}
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/// eliminateIVComparison - SimplifyIVUsers helper for eliminating useless
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/// comparisons against an induction variable.
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void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
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unsigned IVOperIdx = 0;
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ICmpInst::Predicate Pred = ICmp->getPredicate();
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if (IVOperand != ICmp->getOperand(0)) {
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// Swapped
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assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
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IVOperIdx = 1;
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Pred = ICmpInst::getSwappedPredicate(Pred);
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}
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// Get the SCEVs for the ICmp operands.
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const SCEV *S = SE->getSCEV(ICmp->getOperand(IVOperIdx));
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const SCEV *X = SE->getSCEV(ICmp->getOperand(1 - IVOperIdx));
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// Simplify unnecessary loops away.
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const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
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S = SE->getSCEVAtScope(S, ICmpLoop);
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X = SE->getSCEVAtScope(X, ICmpLoop);
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// If the condition is always true or always false, replace it with
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// a constant value.
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if (SE->isKnownPredicate(Pred, S, X))
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ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
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else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X))
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ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
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else
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return;
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DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
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++NumElimCmp;
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Changed = true;
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DeadInsts.push_back(ICmp);
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}
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/// eliminateIVRemainder - SimplifyIVUsers helper for eliminating useless
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/// remainder operations operating on an induction variable.
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void SimplifyIndvar::eliminateIVRemainder(BinaryOperator *Rem,
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Value *IVOperand,
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bool IsSigned) {
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// We're only interested in the case where we know something about
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// the numerator.
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if (IVOperand != Rem->getOperand(0))
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return;
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// Get the SCEVs for the ICmp operands.
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const SCEV *S = SE->getSCEV(Rem->getOperand(0));
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const SCEV *X = SE->getSCEV(Rem->getOperand(1));
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// Simplify unnecessary loops away.
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const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
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S = SE->getSCEVAtScope(S, ICmpLoop);
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X = SE->getSCEVAtScope(X, ICmpLoop);
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// i % n --> i if i is in [0,n).
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if ((!IsSigned || SE->isKnownNonNegative(S)) &&
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SE->isKnownPredicate(IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
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S, X))
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Rem->replaceAllUsesWith(Rem->getOperand(0));
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else {
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// (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n).
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const SCEV *LessOne =
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SE->getMinusSCEV(S, SE->getConstant(S->getType(), 1));
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if (IsSigned && !SE->isKnownNonNegative(LessOne))
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return;
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if (!SE->isKnownPredicate(IsSigned ?
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ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
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LessOne, X))
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return;
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ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ,
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Rem->getOperand(0), Rem->getOperand(1));
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SelectInst *Sel =
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SelectInst::Create(ICmp,
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ConstantInt::get(Rem->getType(), 0),
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Rem->getOperand(0), "tmp", Rem);
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Rem->replaceAllUsesWith(Sel);
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}
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DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
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++NumElimRem;
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Changed = true;
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DeadInsts.push_back(Rem);
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}
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/// eliminateIVUser - Eliminate an operation that consumes a simple IV and has
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/// no observable side-effect given the range of IV values.
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/// IVOperand is guaranteed SCEVable, but UseInst may not be.
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bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
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Instruction *IVOperand) {
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if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
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eliminateIVComparison(ICmp, IVOperand);
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return true;
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}
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if (BinaryOperator *Rem = dyn_cast<BinaryOperator>(UseInst)) {
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bool IsSigned = Rem->getOpcode() == Instruction::SRem;
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if (IsSigned || Rem->getOpcode() == Instruction::URem) {
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eliminateIVRemainder(Rem, IVOperand, IsSigned);
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return true;
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}
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}
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// Eliminate any operation that SCEV can prove is an identity function.
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if (!SE->isSCEVable(UseInst->getType()) ||
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(UseInst->getType() != IVOperand->getType()) ||
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(SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
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return false;
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DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
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UseInst->replaceAllUsesWith(IVOperand);
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++NumElimIdentity;
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Changed = true;
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DeadInsts.push_back(UseInst);
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return true;
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}
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/// pushIVUsers - Add all uses of Def to the current IV's worklist.
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///
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static void pushIVUsers(
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Instruction *Def,
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SmallPtrSet<Instruction*,16> &Simplified,
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SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
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for (Value::use_iterator UI = Def->use_begin(), E = Def->use_end();
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UI != E; ++UI) {
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Instruction *User = cast<Instruction>(*UI);
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// Avoid infinite or exponential worklist processing.
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// Also ensure unique worklist users.
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// If Def is a LoopPhi, it may not be in the Simplified set, so check for
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// self edges first.
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if (User != Def && Simplified.insert(User))
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SimpleIVUsers.push_back(std::make_pair(User, Def));
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}
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}
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/// isSimpleIVUser - Return true if this instruction generates a simple SCEV
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/// expression in terms of that IV.
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///
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/// This is similar to IVUsers' isInteresting() but processes each instruction
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/// non-recursively when the operand is already known to be a simpleIVUser.
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///
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static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
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if (!SE->isSCEVable(I->getType()))
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return false;
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// Get the symbolic expression for this instruction.
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const SCEV *S = SE->getSCEV(I);
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// Only consider affine recurrences.
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const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
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if (AR && AR->getLoop() == L)
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return true;
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return false;
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}
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/// simplifyUsers - Iteratively perform simplification on a worklist of users
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/// of the specified induction variable. Each successive simplification may push
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/// more users which may themselves be candidates for simplification.
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///
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/// This algorithm does not require IVUsers analysis. Instead, it simplifies
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/// instructions in-place during analysis. Rather than rewriting induction
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/// variables bottom-up from their users, it transforms a chain of IVUsers
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/// top-down, updating the IR only when it encouters a clear optimization
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/// opportunitiy.
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///
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/// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
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///
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void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
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if (!SE->isSCEVable(CurrIV->getType()))
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return;
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// Instructions processed by SimplifyIndvar for CurrIV.
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SmallPtrSet<Instruction*,16> Simplified;
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// Use-def pairs if IV users waiting to be processed for CurrIV.
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SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
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// Push users of the current LoopPhi. In rare cases, pushIVUsers may be
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// called multiple times for the same LoopPhi. This is the proper thing to
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// do for loop header phis that use each other.
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pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
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while (!SimpleIVUsers.empty()) {
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std::pair<Instruction*, Instruction*> UseOper =
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SimpleIVUsers.pop_back_val();
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// Bypass back edges to avoid extra work.
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if (UseOper.first == CurrIV) continue;
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Instruction *IVOperand = UseOper.second;
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for (unsigned N = 0; IVOperand; ++N) {
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assert(N <= Simplified.size() && "runaway iteration");
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Value *NewOper = foldIVUser(UseOper.first, IVOperand);
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if (!NewOper)
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break; // done folding
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IVOperand = dyn_cast<Instruction>(NewOper);
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}
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if (!IVOperand)
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continue;
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if (eliminateIVUser(UseOper.first, IVOperand)) {
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pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
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continue;
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}
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CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
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if (V && Cast) {
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V->visitCast(Cast);
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continue;
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}
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if (isSimpleIVUser(UseOper.first, L, SE)) {
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pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
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}
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}
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}
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namespace llvm {
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void IVVisitor::anchor() { }
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/// simplifyUsersOfIV - Simplify instructions that use this induction variable
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/// by using ScalarEvolution to analyze the IV's recurrence.
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bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, LPPassManager *LPM,
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SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
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{
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LoopInfo *LI = &LPM->getAnalysis<LoopInfo>();
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SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, LPM, Dead);
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SIV.simplifyUsers(CurrIV, V);
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return SIV.hasChanged();
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}
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/// simplifyLoopIVs - Simplify users of induction variables within this
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/// loop. This does not actually change or add IVs.
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bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, LPPassManager *LPM,
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SmallVectorImpl<WeakVH> &Dead) {
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bool Changed = false;
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for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
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Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, LPM, Dead);
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}
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return Changed;
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}
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} // namespace llvm
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