forked from OSchip/llvm-project
576 lines
20 KiB
C++
576 lines
20 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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#include "llvm/Transforms/Utils/SimplifyIndVar.h"
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#include "llvm/ADT/STLExtras.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/Dominators.h"
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#include "llvm/IR/IRBuilder.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/IntrinsicInst.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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#define DEBUG_TYPE "indvars"
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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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/// 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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SmallVectorImpl<WeakVH> &DeadInsts;
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bool Changed;
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public:
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SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, LoopInfo *LI,
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SmallVectorImpl<WeakVH> &Dead, IVUsers *IVU = nullptr)
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: L(Loop), LI(LI), SE(SE), DeadInsts(Dead), 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 = nullptr);
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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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bool strengthenOverflowingOperation(BinaryOperator *OBO, Value *IVOperand);
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Instruction *splitOverflowIntrinsic(Instruction *IVUser,
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const DominatorTree *DT);
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};
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}
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/// 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 = nullptr;
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unsigned OperIdx = 0;
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const SCEV *FoldedExpr = nullptr;
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switch (UseInst->getOpcode()) {
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default:
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return nullptr;
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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 nullptr;
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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 nullptr;
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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 nullptr;
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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 nullptr;
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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 nullptr;
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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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/// 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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/// 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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/// 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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/// Annotate BO with nsw / nuw if it provably does not signed-overflow /
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/// unsigned-overflow. Returns true if anything changed, false otherwise.
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bool SimplifyIndvar::strengthenOverflowingOperation(BinaryOperator *BO,
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Value *IVOperand) {
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// Currently we only handle instructions of the form "add <indvar> <value>"
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unsigned Op = BO->getOpcode();
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if (Op != Instruction::Add)
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return false;
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// If BO is already both nuw and nsw then there is nothing left to do
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if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
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return false;
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IntegerType *IT = cast<IntegerType>(IVOperand->getType());
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Value *OtherOperand = nullptr;
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if (BO->getOperand(0) == IVOperand) {
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OtherOperand = BO->getOperand(1);
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} else {
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assert(BO->getOperand(1) == IVOperand && "only other use!");
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OtherOperand = BO->getOperand(0);
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}
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bool Changed = false;
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const SCEV *OtherOpSCEV = SE->getSCEV(OtherOperand);
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if (OtherOpSCEV == SE->getCouldNotCompute())
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return false;
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const SCEV *IVOpSCEV = SE->getSCEV(IVOperand);
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const SCEV *ZeroSCEV = SE->getConstant(IVOpSCEV->getType(), 0);
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if (!BO->hasNoSignedWrap()) {
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// Upgrade the add to an "add nsw" if we can prove that it will never
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// sign-overflow or sign-underflow.
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const SCEV *SignedMax =
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SE->getConstant(APInt::getSignedMaxValue(IT->getBitWidth()));
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const SCEV *SignedMin =
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SE->getConstant(APInt::getSignedMinValue(IT->getBitWidth()));
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// The addition "IVOperand + OtherOp" does not sign-overflow if the result
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// is sign-representable in 2's complement in the given bit-width.
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//
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// If OtherOp is SLT 0, then for an IVOperand in [SignedMin - OtherOp,
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// SignedMax], "IVOperand + OtherOp" is in [SignedMin, SignedMax + OtherOp].
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// Everything in [SignedMin, SignedMax + OtherOp] is representable since
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// SignedMax + OtherOp is at least -1.
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//
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// If OtherOp is SGE 0, then for an IVOperand in [SignedMin, SignedMax -
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// OtherOp], "IVOperand + OtherOp" is in [SignedMin + OtherOp, SignedMax].
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// Everything in [SignedMin + OtherOp, SignedMax] is representable since
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// SignedMin + OtherOp is at most -1.
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//
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// It follows that for all values of IVOperand in [SignedMin - smin(0,
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// OtherOp), SignedMax - smax(0, OtherOp)] the result of the add is
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// representable (i.e. there is no sign-overflow).
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const SCEV *UpperDelta = SE->getSMaxExpr(ZeroSCEV, OtherOpSCEV);
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const SCEV *UpperLimit = SE->getMinusSCEV(SignedMax, UpperDelta);
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bool NeverSignedOverflows =
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SE->isKnownPredicate(ICmpInst::ICMP_SLE, IVOpSCEV, UpperLimit);
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if (NeverSignedOverflows) {
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const SCEV *LowerDelta = SE->getSMinExpr(ZeroSCEV, OtherOpSCEV);
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const SCEV *LowerLimit = SE->getMinusSCEV(SignedMin, LowerDelta);
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bool NeverSignedUnderflows =
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SE->isKnownPredicate(ICmpInst::ICMP_SGE, IVOpSCEV, LowerLimit);
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if (NeverSignedUnderflows) {
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BO->setHasNoSignedWrap(true);
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Changed = true;
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}
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}
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}
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if (!BO->hasNoUnsignedWrap()) {
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// Upgrade the add computing "IVOperand + OtherOp" to an "add nuw" if we can
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// prove that it will never unsigned-overflow (i.e. the result will always
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// be representable in the given bit-width).
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//
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// "IVOperand + OtherOp" is unsigned-representable in 2's complement iff it
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// does not produce a carry. "IVOperand + OtherOp" produces no carry iff
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// IVOperand ULE (UnsignedMax - OtherOp).
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const SCEV *UnsignedMax =
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SE->getConstant(APInt::getMaxValue(IT->getBitWidth()));
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const SCEV *UpperLimit = SE->getMinusSCEV(UnsignedMax, OtherOpSCEV);
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bool NeverUnsignedOverflows =
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SE->isKnownPredicate(ICmpInst::ICMP_ULE, IVOpSCEV, UpperLimit);
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if (NeverUnsignedOverflows) {
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BO->setHasNoUnsignedWrap(true);
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Changed = true;
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}
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}
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return Changed;
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}
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/// \brief Split sadd.with.overflow into add + sadd.with.overflow to allow
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/// analysis and optimization.
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///
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/// \return A new value representing the non-overflowing add if possible,
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/// otherwise return the original value.
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Instruction *SimplifyIndvar::splitOverflowIntrinsic(Instruction *IVUser,
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const DominatorTree *DT) {
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IntrinsicInst *II = dyn_cast<IntrinsicInst>(IVUser);
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if (!II || II->getIntrinsicID() != Intrinsic::sadd_with_overflow)
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return IVUser;
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// Find a branch guarded by the overflow check.
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BranchInst *Branch = nullptr;
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Instruction *AddVal = nullptr;
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for (User *U : II->users()) {
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if (ExtractValueInst *ExtractInst = dyn_cast<ExtractValueInst>(U)) {
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if (ExtractInst->getNumIndices() != 1)
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continue;
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if (ExtractInst->getIndices()[0] == 0)
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AddVal = ExtractInst;
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else if (ExtractInst->getIndices()[0] == 1 && ExtractInst->hasOneUse())
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Branch = dyn_cast<BranchInst>(ExtractInst->user_back());
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}
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}
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if (!AddVal || !Branch)
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return IVUser;
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BasicBlock *ContinueBB = Branch->getSuccessor(1);
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if (std::next(pred_begin(ContinueBB)) != pred_end(ContinueBB))
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return IVUser;
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// Check if all users of the add are provably NSW.
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bool AllNSW = true;
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for (Use &U : AddVal->uses()) {
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if (Instruction *UseInst = dyn_cast<Instruction>(U.getUser())) {
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BasicBlock *UseBB = UseInst->getParent();
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if (PHINode *PHI = dyn_cast<PHINode>(UseInst))
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UseBB = PHI->getIncomingBlock(U);
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if (!DT->dominates(ContinueBB, UseBB)) {
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AllNSW = false;
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break;
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}
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}
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}
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if (!AllNSW)
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return IVUser;
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// Go for it...
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IRBuilder<> Builder(IVUser);
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Instruction *AddInst = dyn_cast<Instruction>(
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Builder.CreateNSWAdd(II->getOperand(0), II->getOperand(1)));
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// The caller expects the new add to have the same form as the intrinsic. The
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// IV operand position must be the same.
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assert((AddInst->getOpcode() == Instruction::Add &&
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AddInst->getOperand(0) == II->getOperand(0)) &&
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"Bad add instruction created from overflow intrinsic.");
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AddVal->replaceAllUsesWith(AddInst);
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DeadInsts.push_back(AddVal);
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return AddInst;
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}
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/// Add all uses of Def to the current IV's worklist.
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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 (User *U : Def->users()) {
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Instruction *UI = cast<Instruction>(U);
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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 (UI != Def && Simplified.insert(UI).second)
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SimpleIVUsers.push_back(std::make_pair(UI, Def));
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}
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}
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/// 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;
|
|
|
|
// Get the symbolic expression for this instruction.
|
|
const SCEV *S = SE->getSCEV(I);
|
|
|
|
// Only consider affine recurrences.
|
|
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S);
|
|
if (AR && AR->getLoop() == L)
|
|
return true;
|
|
|
|
return false;
|
|
}
|
|
|
|
/// Iteratively perform simplification on a worklist of users
|
|
/// of the specified induction variable. Each successive simplification may push
|
|
/// more users which may themselves be candidates for simplification.
|
|
///
|
|
/// This algorithm does not require IVUsers analysis. Instead, it simplifies
|
|
/// instructions in-place during analysis. Rather than rewriting induction
|
|
/// variables bottom-up from their users, it transforms a chain of IVUsers
|
|
/// top-down, updating the IR only when it encouters a clear optimization
|
|
/// opportunitiy.
|
|
///
|
|
/// Once DisableIVRewrite is default, LSR will be the only client of IVUsers.
|
|
///
|
|
void SimplifyIndvar::simplifyUsers(PHINode *CurrIV, IVVisitor *V) {
|
|
if (!SE->isSCEVable(CurrIV->getType()))
|
|
return;
|
|
|
|
// Instructions processed by SimplifyIndvar for CurrIV.
|
|
SmallPtrSet<Instruction*,16> Simplified;
|
|
|
|
// Use-def pairs if IV users waiting to be processed for CurrIV.
|
|
SmallVector<std::pair<Instruction*, Instruction*>, 8> SimpleIVUsers;
|
|
|
|
// Push users of the current LoopPhi. In rare cases, pushIVUsers may be
|
|
// called multiple times for the same LoopPhi. This is the proper thing to
|
|
// do for loop header phis that use each other.
|
|
pushIVUsers(CurrIV, Simplified, SimpleIVUsers);
|
|
|
|
while (!SimpleIVUsers.empty()) {
|
|
std::pair<Instruction*, Instruction*> UseOper =
|
|
SimpleIVUsers.pop_back_val();
|
|
Instruction *UseInst = UseOper.first;
|
|
|
|
// Bypass back edges to avoid extra work.
|
|
if (UseInst == CurrIV) continue;
|
|
|
|
if (V && V->shouldSplitOverflowInstrinsics()) {
|
|
UseInst = splitOverflowIntrinsic(UseInst, V->getDomTree());
|
|
if (!UseInst)
|
|
continue;
|
|
}
|
|
|
|
Instruction *IVOperand = UseOper.second;
|
|
for (unsigned N = 0; IVOperand; ++N) {
|
|
assert(N <= Simplified.size() && "runaway iteration");
|
|
|
|
Value *NewOper = foldIVUser(UseOper.first, IVOperand);
|
|
if (!NewOper)
|
|
break; // done folding
|
|
IVOperand = dyn_cast<Instruction>(NewOper);
|
|
}
|
|
if (!IVOperand)
|
|
continue;
|
|
|
|
if (eliminateIVUser(UseOper.first, IVOperand)) {
|
|
pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
|
|
continue;
|
|
}
|
|
|
|
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(UseOper.first)) {
|
|
if (isa<OverflowingBinaryOperator>(BO) &&
|
|
strengthenOverflowingOperation(BO, IVOperand)) {
|
|
// re-queue uses of the now modified binary operator and fall
|
|
// through to the checks that remain.
|
|
pushIVUsers(IVOperand, Simplified, SimpleIVUsers);
|
|
}
|
|
}
|
|
|
|
CastInst *Cast = dyn_cast<CastInst>(UseOper.first);
|
|
if (V && Cast) {
|
|
V->visitCast(Cast);
|
|
continue;
|
|
}
|
|
if (isSimpleIVUser(UseOper.first, L, SE)) {
|
|
pushIVUsers(UseOper.first, Simplified, SimpleIVUsers);
|
|
}
|
|
}
|
|
}
|
|
|
|
namespace llvm {
|
|
|
|
void IVVisitor::anchor() { }
|
|
|
|
/// Simplify instructions that use this induction variable
|
|
/// by using ScalarEvolution to analyze the IV's recurrence.
|
|
bool simplifyUsersOfIV(PHINode *CurrIV, ScalarEvolution *SE, LPPassManager *LPM,
|
|
SmallVectorImpl<WeakVH> &Dead, IVVisitor *V)
|
|
{
|
|
LoopInfo *LI = &LPM->getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
|
|
SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, LI, Dead);
|
|
SIV.simplifyUsers(CurrIV, V);
|
|
return SIV.hasChanged();
|
|
}
|
|
|
|
/// Simplify users of induction variables within this
|
|
/// loop. This does not actually change or add IVs.
|
|
bool simplifyLoopIVs(Loop *L, ScalarEvolution *SE, LPPassManager *LPM,
|
|
SmallVectorImpl<WeakVH> &Dead) {
|
|
bool Changed = false;
|
|
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
|
|
Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, LPM, Dead);
|
|
}
|
|
return Changed;
|
|
}
|
|
|
|
} // namespace llvm
|