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Summary: rL225282 introduced an ad-hoc way to promote some additions to nuw or nsw. Since then SCEV has become smarter in directly proving no-wrap; and using the canonical "ext(A op B) == ext(A) op ext(B)" method of proving no-wrap is just as powerful now. Rip out the existing complexity in favor of getting SCEV to do all the heaving lifting internally. This change does not add any unit tests because it is supposed to be a non-functional change. Tests added in rL225282 and rL226075 are valid tests for this change. Reviewers: atrick, majnemer Subscribers: llvm-commits Differential Revision: http://reviews.llvm.org/D7981 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231306 91177308-0d34-0410-b5e6-96231b3b80d8
538 lines
19 KiB
C++
538 lines
19 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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// Fastpath: we don't have any work to do if `BO` is `nuw` and `nsw`.
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if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
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return false;
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const SCEV *(ScalarEvolution::*GetExprForBO)(const SCEV *, const SCEV *,
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SCEV::NoWrapFlags);
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switch (BO->getOpcode()) {
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default:
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return false;
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case Instruction::Add:
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GetExprForBO = &ScalarEvolution::getAddExpr;
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break;
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case Instruction::Sub:
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GetExprForBO = &ScalarEvolution::getMinusSCEV;
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break;
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case Instruction::Mul:
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GetExprForBO = &ScalarEvolution::getMulExpr;
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break;
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}
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unsigned BitWidth = cast<IntegerType>(BO->getType())->getBitWidth();
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Type *WideTy = IntegerType::get(BO->getContext(), BitWidth * 2);
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const SCEV *LHS = SE->getSCEV(BO->getOperand(0));
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const SCEV *RHS = SE->getSCEV(BO->getOperand(1));
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bool Changed = false;
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if (!BO->hasNoUnsignedWrap()) {
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const SCEV *ExtendAfterOp = SE->getZeroExtendExpr(SE->getSCEV(BO), WideTy);
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const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
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SE->getZeroExtendExpr(LHS, WideTy), SE->getZeroExtendExpr(RHS, WideTy),
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SCEV::FlagAnyWrap);
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if (ExtendAfterOp == OpAfterExtend) {
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BO->setHasNoUnsignedWrap();
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SE->forgetValue(BO);
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Changed = true;
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}
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}
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if (!BO->hasNoSignedWrap()) {
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const SCEV *ExtendAfterOp = SE->getSignExtendExpr(SE->getSCEV(BO), WideTy);
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const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
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SE->getSignExtendExpr(LHS, WideTy), SE->getSignExtendExpr(RHS, WideTy),
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SCEV::FlagAnyWrap);
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if (ExtendAfterOp == OpAfterExtend) {
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BO->setHasNoSignedWrap();
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SE->forgetValue(BO);
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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;
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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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/// 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
|
|
// 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
|