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llvm-project/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp

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//===-- SimplifyIndVar.cpp - Induction variable simplification ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements induction variable simplification. It does
// not define any actual pass or policy, but provides a single function to
// simplify a loop's induction variables based on ScalarEvolution.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Utils/SimplifyIndVar.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;
#define DEBUG_TYPE "indvars"
STATISTIC(NumElimIdentity, "Number of IV identities eliminated");
STATISTIC(NumElimOperand, "Number of IV operands folded into a use");
STATISTIC(NumFoldedUser, "Number of IV users folded into a constant");
STATISTIC(NumElimRem , "Number of IV remainder operations eliminated");
STATISTIC(
NumSimplifiedSDiv,
"Number of IV signed division operations converted to unsigned division");
STATISTIC(
NumSimplifiedSRem,
"Number of IV signed remainder operations converted to unsigned remainder");
STATISTIC(NumElimCmp , "Number of IV comparisons eliminated");
namespace {
/// This is a utility for simplifying induction variables
/// based on ScalarEvolution. It is the primary instrument of the
/// IndvarSimplify pass, but it may also be directly invoked to cleanup after
/// other loop passes that preserve SCEV.
class SimplifyIndvar {
Loop *L;
LoopInfo *LI;
ScalarEvolution *SE;
DominatorTree *DT;
SCEVExpander &Rewriter;
SmallVectorImpl<WeakTrackingVH> &DeadInsts;
bool Changed;
public:
SimplifyIndvar(Loop *Loop, ScalarEvolution *SE, DominatorTree *DT,
LoopInfo *LI, SCEVExpander &Rewriter,
SmallVectorImpl<WeakTrackingVH> &Dead)
: L(Loop), LI(LI), SE(SE), DT(DT), Rewriter(Rewriter), DeadInsts(Dead),
Changed(false) {
assert(LI && "IV simplification requires LoopInfo");
}
bool hasChanged() const { return Changed; }
/// Iteratively perform simplification on a worklist of users of the
/// specified induction variable. This is the top-level driver that applies
/// all simplifications to users of an IV.
void simplifyUsers(PHINode *CurrIV, IVVisitor *V = nullptr);
Value *foldIVUser(Instruction *UseInst, Instruction *IVOperand);
bool eliminateIdentitySCEV(Instruction *UseInst, Instruction *IVOperand);
bool replaceIVUserWithLoopInvariant(Instruction *UseInst);
bool eliminateOverflowIntrinsic(CallInst *CI);
bool eliminateTrunc(TruncInst *TI);
bool eliminateIVUser(Instruction *UseInst, Instruction *IVOperand);
bool makeIVComparisonInvariant(ICmpInst *ICmp, Value *IVOperand);
void eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand);
void simplifyIVRemainder(BinaryOperator *Rem, Value *IVOperand,
bool IsSigned);
void replaceRemWithNumerator(BinaryOperator *Rem);
void replaceRemWithNumeratorOrZero(BinaryOperator *Rem);
void replaceSRemWithURem(BinaryOperator *Rem);
bool eliminateSDiv(BinaryOperator *SDiv);
bool strengthenOverflowingOperation(BinaryOperator *OBO, Value *IVOperand);
bool strengthenRightShift(BinaryOperator *BO, Value *IVOperand);
};
}
/// Fold an IV operand into its use. This removes increments of an
/// aligned IV when used by a instruction that ignores the low bits.
///
/// IVOperand is guaranteed SCEVable, but UseInst may not be.
///
/// Return the operand of IVOperand for this induction variable if IVOperand can
/// be folded (in case more folding opportunities have been exposed).
/// Otherwise return null.
Value *SimplifyIndvar::foldIVUser(Instruction *UseInst, Instruction *IVOperand) {
Value *IVSrc = nullptr;
const unsigned OperIdx = 0;
const SCEV *FoldedExpr = nullptr;
bool MustDropExactFlag = false;
switch (UseInst->getOpcode()) {
default:
return nullptr;
case Instruction::UDiv:
case Instruction::LShr:
// We're only interested in the case where we know something about
// the numerator and have a constant denominator.
if (IVOperand != UseInst->getOperand(OperIdx) ||
!isa<ConstantInt>(UseInst->getOperand(1)))
return nullptr;
// Attempt to fold a binary operator with constant operand.
// e.g. ((I + 1) >> 2) => I >> 2
if (!isa<BinaryOperator>(IVOperand)
|| !isa<ConstantInt>(IVOperand->getOperand(1)))
return nullptr;
IVSrc = IVOperand->getOperand(0);
// IVSrc must be the (SCEVable) IV, since the other operand is const.
assert(SE->isSCEVable(IVSrc->getType()) && "Expect SCEVable IV operand");
ConstantInt *D = cast<ConstantInt>(UseInst->getOperand(1));
if (UseInst->getOpcode() == Instruction::LShr) {
// Get a constant for the divisor. See createSCEV.
uint32_t BitWidth = cast<IntegerType>(UseInst->getType())->getBitWidth();
if (D->getValue().uge(BitWidth))
return nullptr;
D = ConstantInt::get(UseInst->getContext(),
APInt::getOneBitSet(BitWidth, D->getZExtValue()));
}
FoldedExpr = SE->getUDivExpr(SE->getSCEV(IVSrc), SE->getSCEV(D));
// We might have 'exact' flag set at this point which will no longer be
// correct after we make the replacement.
if (UseInst->isExact() &&
SE->getSCEV(IVSrc) != SE->getMulExpr(FoldedExpr, SE->getSCEV(D)))
MustDropExactFlag = true;
}
// We have something that might fold it's operand. Compare SCEVs.
if (!SE->isSCEVable(UseInst->getType()))
return nullptr;
// Bypass the operand if SCEV can prove it has no effect.
if (SE->getSCEV(UseInst) != FoldedExpr)
return nullptr;
LLVM_DEBUG(dbgs() << "INDVARS: Eliminated IV operand: " << *IVOperand
<< " -> " << *UseInst << '\n');
UseInst->setOperand(OperIdx, IVSrc);
assert(SE->getSCEV(UseInst) == FoldedExpr && "bad SCEV with folded oper");
if (MustDropExactFlag)
UseInst->dropPoisonGeneratingFlags();
++NumElimOperand;
Changed = true;
if (IVOperand->use_empty())
DeadInsts.emplace_back(IVOperand);
return IVSrc;
}
bool SimplifyIndvar::makeIVComparisonInvariant(ICmpInst *ICmp,
Value *IVOperand) {
unsigned IVOperIdx = 0;
ICmpInst::Predicate Pred = ICmp->getPredicate();
if (IVOperand != ICmp->getOperand(0)) {
// Swapped
assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
IVOperIdx = 1;
Pred = ICmpInst::getSwappedPredicate(Pred);
}
// Get the SCEVs for the ICmp operands (in the specific context of the
// current loop)
const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
const SCEV *S = SE->getSCEVAtScope(ICmp->getOperand(IVOperIdx), ICmpLoop);
const SCEV *X = SE->getSCEVAtScope(ICmp->getOperand(1 - IVOperIdx), ICmpLoop);
ICmpInst::Predicate InvariantPredicate;
const SCEV *InvariantLHS, *InvariantRHS;
auto *PN = dyn_cast<PHINode>(IVOperand);
if (!PN)
return false;
if (!SE->isLoopInvariantPredicate(Pred, S, X, L, InvariantPredicate,
InvariantLHS, InvariantRHS))
return false;
// Rewrite the comparison to a loop invariant comparison if it can be done
// cheaply, where cheaply means "we don't need to emit any new
// instructions".
SmallDenseMap<const SCEV*, Value*> CheapExpansions;
CheapExpansions[S] = ICmp->getOperand(IVOperIdx);
CheapExpansions[X] = ICmp->getOperand(1 - IVOperIdx);
// TODO: Support multiple entry loops? (We currently bail out of these in
// the IndVarSimplify pass)
if (auto *BB = L->getLoopPredecessor()) {
const int Idx = PN->getBasicBlockIndex(BB);
if (Idx >= 0) {
Value *Incoming = PN->getIncomingValue(Idx);
const SCEV *IncomingS = SE->getSCEV(Incoming);
CheapExpansions[IncomingS] = Incoming;
}
}
Value *NewLHS = CheapExpansions[InvariantLHS];
Value *NewRHS = CheapExpansions[InvariantRHS];
if (!NewLHS)
if (auto *ConstLHS = dyn_cast<SCEVConstant>(InvariantLHS))
NewLHS = ConstLHS->getValue();
if (!NewRHS)
if (auto *ConstRHS = dyn_cast<SCEVConstant>(InvariantRHS))
NewRHS = ConstRHS->getValue();
if (!NewLHS || !NewRHS)
// We could not find an existing value to replace either LHS or RHS.
// Generating new instructions has subtler tradeoffs, so avoid doing that
// for now.
return false;
LLVM_DEBUG(dbgs() << "INDVARS: Simplified comparison: " << *ICmp << '\n');
ICmp->setPredicate(InvariantPredicate);
ICmp->setOperand(0, NewLHS);
ICmp->setOperand(1, NewRHS);
return true;
}
/// SimplifyIVUsers helper for eliminating useless
/// comparisons against an induction variable.
void SimplifyIndvar::eliminateIVComparison(ICmpInst *ICmp, Value *IVOperand) {
unsigned IVOperIdx = 0;
ICmpInst::Predicate Pred = ICmp->getPredicate();
ICmpInst::Predicate OriginalPred = Pred;
if (IVOperand != ICmp->getOperand(0)) {
// Swapped
assert(IVOperand == ICmp->getOperand(1) && "Can't find IVOperand");
IVOperIdx = 1;
Pred = ICmpInst::getSwappedPredicate(Pred);
}
// Get the SCEVs for the ICmp operands (in the specific context of the
// current loop)
const Loop *ICmpLoop = LI->getLoopFor(ICmp->getParent());
const SCEV *S = SE->getSCEVAtScope(ICmp->getOperand(IVOperIdx), ICmpLoop);
const SCEV *X = SE->getSCEVAtScope(ICmp->getOperand(1 - IVOperIdx), ICmpLoop);
// If the condition is always true or always false, replace it with
// a constant value.
if (SE->isKnownPredicate(Pred, S, X)) {
ICmp->replaceAllUsesWith(ConstantInt::getTrue(ICmp->getContext()));
DeadInsts.emplace_back(ICmp);
LLVM_DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
} else if (SE->isKnownPredicate(ICmpInst::getInversePredicate(Pred), S, X)) {
ICmp->replaceAllUsesWith(ConstantInt::getFalse(ICmp->getContext()));
DeadInsts.emplace_back(ICmp);
LLVM_DEBUG(dbgs() << "INDVARS: Eliminated comparison: " << *ICmp << '\n');
} else if (makeIVComparisonInvariant(ICmp, IVOperand)) {
// fallthrough to end of function
} else if (ICmpInst::isSigned(OriginalPred) &&
SE->isKnownNonNegative(S) && SE->isKnownNonNegative(X)) {
// If we were unable to make anything above, all we can is to canonicalize
// the comparison hoping that it will open the doors for other
// optimizations. If we find out that we compare two non-negative values,
// we turn the instruction's predicate to its unsigned version. Note that
// we cannot rely on Pred here unless we check if we have swapped it.
assert(ICmp->getPredicate() == OriginalPred && "Predicate changed?");
LLVM_DEBUG(dbgs() << "INDVARS: Turn to unsigned comparison: " << *ICmp
<< '\n');
ICmp->setPredicate(ICmpInst::getUnsignedPredicate(OriginalPred));
} else
return;
++NumElimCmp;
Changed = true;
}
bool SimplifyIndvar::eliminateSDiv(BinaryOperator *SDiv) {
// Get the SCEVs for the ICmp operands.
auto *N = SE->getSCEV(SDiv->getOperand(0));
auto *D = SE->getSCEV(SDiv->getOperand(1));
// Simplify unnecessary loops away.
const Loop *L = LI->getLoopFor(SDiv->getParent());
N = SE->getSCEVAtScope(N, L);
D = SE->getSCEVAtScope(D, L);
// Replace sdiv by udiv if both of the operands are non-negative
if (SE->isKnownNonNegative(N) && SE->isKnownNonNegative(D)) {
auto *UDiv = BinaryOperator::Create(
BinaryOperator::UDiv, SDiv->getOperand(0), SDiv->getOperand(1),
SDiv->getName() + ".udiv", SDiv);
UDiv->setIsExact(SDiv->isExact());
SDiv->replaceAllUsesWith(UDiv);
LLVM_DEBUG(dbgs() << "INDVARS: Simplified sdiv: " << *SDiv << '\n');
++NumSimplifiedSDiv;
Changed = true;
DeadInsts.push_back(SDiv);
return true;
}
return false;
}
// i %s n -> i %u n if i >= 0 and n >= 0
void SimplifyIndvar::replaceSRemWithURem(BinaryOperator *Rem) {
auto *N = Rem->getOperand(0), *D = Rem->getOperand(1);
auto *URem = BinaryOperator::Create(BinaryOperator::URem, N, D,
Rem->getName() + ".urem", Rem);
Rem->replaceAllUsesWith(URem);
LLVM_DEBUG(dbgs() << "INDVARS: Simplified srem: " << *Rem << '\n');
++NumSimplifiedSRem;
Changed = true;
DeadInsts.emplace_back(Rem);
}
// i % n --> i if i is in [0,n).
void SimplifyIndvar::replaceRemWithNumerator(BinaryOperator *Rem) {
Rem->replaceAllUsesWith(Rem->getOperand(0));
LLVM_DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
++NumElimRem;
Changed = true;
DeadInsts.emplace_back(Rem);
}
// (i+1) % n --> (i+1)==n?0:(i+1) if i is in [0,n).
void SimplifyIndvar::replaceRemWithNumeratorOrZero(BinaryOperator *Rem) {
auto *T = Rem->getType();
auto *N = Rem->getOperand(0), *D = Rem->getOperand(1);
ICmpInst *ICmp = new ICmpInst(Rem, ICmpInst::ICMP_EQ, N, D);
SelectInst *Sel =
SelectInst::Create(ICmp, ConstantInt::get(T, 0), N, "iv.rem", Rem);
Rem->replaceAllUsesWith(Sel);
LLVM_DEBUG(dbgs() << "INDVARS: Simplified rem: " << *Rem << '\n');
++NumElimRem;
Changed = true;
DeadInsts.emplace_back(Rem);
}
/// SimplifyIVUsers helper for eliminating useless remainder operations
/// operating on an induction variable or replacing srem by urem.
void SimplifyIndvar::simplifyIVRemainder(BinaryOperator *Rem, Value *IVOperand,
bool IsSigned) {
auto *NValue = Rem->getOperand(0);
auto *DValue = Rem->getOperand(1);
// We're only interested in the case where we know something about
// the numerator, unless it is a srem, because we want to replace srem by urem
// in general.
bool UsedAsNumerator = IVOperand == NValue;
if (!UsedAsNumerator && !IsSigned)
return;
const SCEV *N = SE->getSCEV(NValue);
// Simplify unnecessary loops away.
const Loop *ICmpLoop = LI->getLoopFor(Rem->getParent());
N = SE->getSCEVAtScope(N, ICmpLoop);
bool IsNumeratorNonNegative = !IsSigned || SE->isKnownNonNegative(N);
// Do not proceed if the Numerator may be negative
if (!IsNumeratorNonNegative)
return;
const SCEV *D = SE->getSCEV(DValue);
D = SE->getSCEVAtScope(D, ICmpLoop);
if (UsedAsNumerator) {
auto LT = IsSigned ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
if (SE->isKnownPredicate(LT, N, D)) {
replaceRemWithNumerator(Rem);
return;
}
auto *T = Rem->getType();
const auto *NLessOne = SE->getMinusSCEV(N, SE->getOne(T));
if (SE->isKnownPredicate(LT, NLessOne, D)) {
replaceRemWithNumeratorOrZero(Rem);
return;
}
}
// Try to replace SRem with URem, if both N and D are known non-negative.
// Since we had already check N, we only need to check D now
if (!IsSigned || !SE->isKnownNonNegative(D))
return;
replaceSRemWithURem(Rem);
}
bool SimplifyIndvar::eliminateOverflowIntrinsic(CallInst *CI) {
auto *F = CI->getCalledFunction();
if (!F)
return false;
typedef const SCEV *(ScalarEvolution::*OperationFunctionTy)(
const SCEV *, const SCEV *, SCEV::NoWrapFlags, unsigned);
typedef const SCEV *(ScalarEvolution::*ExtensionFunctionTy)(
const SCEV *, Type *, unsigned);
OperationFunctionTy Operation;
ExtensionFunctionTy Extension;
Instruction::BinaryOps RawOp;
// We always have exactly one of nsw or nuw. If NoSignedOverflow is false, we
// have nuw.
bool NoSignedOverflow;
switch (F->getIntrinsicID()) {
default:
return false;
case Intrinsic::sadd_with_overflow:
Operation = &ScalarEvolution::getAddExpr;
Extension = &ScalarEvolution::getSignExtendExpr;
RawOp = Instruction::Add;
NoSignedOverflow = true;
break;
case Intrinsic::uadd_with_overflow:
Operation = &ScalarEvolution::getAddExpr;
Extension = &ScalarEvolution::getZeroExtendExpr;
RawOp = Instruction::Add;
NoSignedOverflow = false;
break;
case Intrinsic::ssub_with_overflow:
Operation = &ScalarEvolution::getMinusSCEV;
Extension = &ScalarEvolution::getSignExtendExpr;
RawOp = Instruction::Sub;
NoSignedOverflow = true;
break;
case Intrinsic::usub_with_overflow:
Operation = &ScalarEvolution::getMinusSCEV;
Extension = &ScalarEvolution::getZeroExtendExpr;
RawOp = Instruction::Sub;
NoSignedOverflow = false;
break;
}
const SCEV *LHS = SE->getSCEV(CI->getArgOperand(0));
const SCEV *RHS = SE->getSCEV(CI->getArgOperand(1));
auto *NarrowTy = cast<IntegerType>(LHS->getType());
auto *WideTy =
IntegerType::get(NarrowTy->getContext(), NarrowTy->getBitWidth() * 2);
const SCEV *A =
(SE->*Extension)((SE->*Operation)(LHS, RHS, SCEV::FlagAnyWrap, 0),
WideTy, 0);
const SCEV *B =
(SE->*Operation)((SE->*Extension)(LHS, WideTy, 0),
(SE->*Extension)(RHS, WideTy, 0), SCEV::FlagAnyWrap, 0);
if (A != B)
return false;
// Proved no overflow, nuke the overflow check and, if possible, the overflow
// intrinsic as well.
BinaryOperator *NewResult = BinaryOperator::Create(
RawOp, CI->getArgOperand(0), CI->getArgOperand(1), "", CI);
if (NoSignedOverflow)
NewResult->setHasNoSignedWrap(true);
else
NewResult->setHasNoUnsignedWrap(true);
SmallVector<ExtractValueInst *, 4> ToDelete;
for (auto *U : CI->users()) {
if (auto *EVI = dyn_cast<ExtractValueInst>(U)) {
if (EVI->getIndices()[0] == 1)
EVI->replaceAllUsesWith(ConstantInt::getFalse(CI->getContext()));
else {
assert(EVI->getIndices()[0] == 0 && "Only two possibilities!");
EVI->replaceAllUsesWith(NewResult);
}
ToDelete.push_back(EVI);
}
}
for (auto *EVI : ToDelete)
EVI->eraseFromParent();
if (CI->use_empty())
CI->eraseFromParent();
return true;
}
bool SimplifyIndvar::eliminateTrunc(TruncInst *TI) {
// It is always legal to replace
// icmp <pred> i32 trunc(iv), n
// with
// icmp <pred> i64 sext(trunc(iv)), sext(n), if pred is signed predicate.
// Or with
// icmp <pred> i64 zext(trunc(iv)), zext(n), if pred is unsigned predicate.
// Or with either of these if pred is an equality predicate.
//
// If we can prove that iv == sext(trunc(iv)) or iv == zext(trunc(iv)) for
// every comparison which uses trunc, it means that we can replace each of
// them with comparison of iv against sext/zext(n). We no longer need trunc
// after that.
//
// TODO: Should we do this if we can widen *some* comparisons, but not all
// of them? Sometimes it is enough to enable other optimizations, but the
// trunc instruction will stay in the loop.
Value *IV = TI->getOperand(0);
Type *IVTy = IV->getType();
const SCEV *IVSCEV = SE->getSCEV(IV);
const SCEV *TISCEV = SE->getSCEV(TI);
// Check if iv == zext(trunc(iv)) and if iv == sext(trunc(iv)). If so, we can
// get rid of trunc
bool DoesSExtCollapse = false;
bool DoesZExtCollapse = false;
if (IVSCEV == SE->getSignExtendExpr(TISCEV, IVTy))
DoesSExtCollapse = true;
if (IVSCEV == SE->getZeroExtendExpr(TISCEV, IVTy))
DoesZExtCollapse = true;
// If neither sext nor zext does collapse, it is not profitable to do any
// transform. Bail.
if (!DoesSExtCollapse && !DoesZExtCollapse)
return false;
// Collect users of the trunc that look like comparisons against invariants.
// Bail if we find something different.
SmallVector<ICmpInst *, 4> ICmpUsers;
for (auto *U : TI->users()) {
// We don't care about users in unreachable blocks.
if (isa<Instruction>(U) &&
!DT->isReachableFromEntry(cast<Instruction>(U)->getParent()))
continue;
if (ICmpInst *ICI = dyn_cast<ICmpInst>(U)) {
if (ICI->getOperand(0) == TI && L->isLoopInvariant(ICI->getOperand(1))) {
assert(L->contains(ICI->getParent()) && "LCSSA form broken?");
// If we cannot get rid of trunc, bail.
if (ICI->isSigned() && !DoesSExtCollapse)
return false;
if (ICI->isUnsigned() && !DoesZExtCollapse)
return false;
// For equality, either signed or unsigned works.
ICmpUsers.push_back(ICI);
} else
return false;
} else
return false;
}
auto CanUseZExt = [&](ICmpInst *ICI) {
// Unsigned comparison can be widened as unsigned.
if (ICI->isUnsigned())
return true;
// Is it profitable to do zext?
if (!DoesZExtCollapse)
return false;
// For equality, we can safely zext both parts.
if (ICI->isEquality())
return true;
// Otherwise we can only use zext when comparing two non-negative or two
// negative values. But in practice, we will never pass DoesZExtCollapse
// check for a negative value, because zext(trunc(x)) is non-negative. So
// it only make sense to check for non-negativity here.
const SCEV *SCEVOP1 = SE->getSCEV(ICI->getOperand(0));
const SCEV *SCEVOP2 = SE->getSCEV(ICI->getOperand(1));
return SE->isKnownNonNegative(SCEVOP1) && SE->isKnownNonNegative(SCEVOP2);
};
// Replace all comparisons against trunc with comparisons against IV.
for (auto *ICI : ICmpUsers) {
auto *Op1 = ICI->getOperand(1);
Instruction *Ext = nullptr;
// For signed/unsigned predicate, replace the old comparison with comparison
// of immediate IV against sext/zext of the invariant argument. If we can
// use either sext or zext (i.e. we are dealing with equality predicate),
// then prefer zext as a more canonical form.
// TODO: If we see a signed comparison which can be turned into unsigned,
// we can do it here for canonicalization purposes.
ICmpInst::Predicate Pred = ICI->getPredicate();
if (CanUseZExt(ICI)) {
assert(DoesZExtCollapse && "Unprofitable zext?");
Ext = new ZExtInst(Op1, IVTy, "zext", ICI);
Pred = ICmpInst::getUnsignedPredicate(Pred);
} else {
assert(DoesSExtCollapse && "Unprofitable sext?");
Ext = new SExtInst(Op1, IVTy, "sext", ICI);
assert(Pred == ICmpInst::getSignedPredicate(Pred) && "Must be signed!");
}
bool Changed;
L->makeLoopInvariant(Ext, Changed);
(void)Changed;
ICmpInst *NewICI = new ICmpInst(ICI, Pred, IV, Ext);
ICI->replaceAllUsesWith(NewICI);
DeadInsts.emplace_back(ICI);
}
// Trunc no longer needed.
TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
DeadInsts.emplace_back(TI);
return true;
}
/// Eliminate an operation that consumes a simple IV and has no observable
/// side-effect given the range of IV values. IVOperand is guaranteed SCEVable,
/// but UseInst may not be.
bool SimplifyIndvar::eliminateIVUser(Instruction *UseInst,
Instruction *IVOperand) {
if (ICmpInst *ICmp = dyn_cast<ICmpInst>(UseInst)) {
eliminateIVComparison(ICmp, IVOperand);
return true;
}
if (BinaryOperator *Bin = dyn_cast<BinaryOperator>(UseInst)) {
bool IsSRem = Bin->getOpcode() == Instruction::SRem;
if (IsSRem || Bin->getOpcode() == Instruction::URem) {
simplifyIVRemainder(Bin, IVOperand, IsSRem);
return true;
}
if (Bin->getOpcode() == Instruction::SDiv)
return eliminateSDiv(Bin);
}
if (auto *CI = dyn_cast<CallInst>(UseInst))
if (eliminateOverflowIntrinsic(CI))
return true;
if (auto *TI = dyn_cast<TruncInst>(UseInst))
if (eliminateTrunc(TI))
return true;
if (eliminateIdentitySCEV(UseInst, IVOperand))
return true;
return false;
}
static Instruction *GetLoopInvariantInsertPosition(Loop *L, Instruction *Hint) {
if (auto *BB = L->getLoopPreheader())
return BB->getTerminator();
return Hint;
}
/// Replace the UseInst with a constant if possible.
bool SimplifyIndvar::replaceIVUserWithLoopInvariant(Instruction *I) {
if (!SE->isSCEVable(I->getType()))
return false;
// Get the symbolic expression for this instruction.
const SCEV *S = SE->getSCEV(I);
if (!SE->isLoopInvariant(S, L))
return false;
// Do not generate something ridiculous even if S is loop invariant.
if (Rewriter.isHighCostExpansion(S, L, I))
return false;
auto *IP = GetLoopInvariantInsertPosition(L, I);
auto *Invariant = Rewriter.expandCodeFor(S, I->getType(), IP);
I->replaceAllUsesWith(Invariant);
LLVM_DEBUG(dbgs() << "INDVARS: Replace IV user: " << *I
<< " with loop invariant: " << *S << '\n');
++NumFoldedUser;
Changed = true;
DeadInsts.emplace_back(I);
return true;
}
/// Eliminate any operation that SCEV can prove is an identity function.
bool SimplifyIndvar::eliminateIdentitySCEV(Instruction *UseInst,
Instruction *IVOperand) {
if (!SE->isSCEVable(UseInst->getType()) ||
(UseInst->getType() != IVOperand->getType()) ||
(SE->getSCEV(UseInst) != SE->getSCEV(IVOperand)))
return false;
// getSCEV(X) == getSCEV(Y) does not guarantee that X and Y are related in the
// dominator tree, even if X is an operand to Y. For instance, in
//
// %iv = phi i32 {0,+,1}
// br %cond, label %left, label %merge
//
// left:
// %X = add i32 %iv, 0
// br label %merge
//
// merge:
// %M = phi (%X, %iv)
//
// getSCEV(%M) == getSCEV(%X) == {0,+,1}, but %X does not dominate %M, and
// %M.replaceAllUsesWith(%X) would be incorrect.
if (isa<PHINode>(UseInst))
// If UseInst is not a PHI node then we know that IVOperand dominates
// UseInst directly from the legality of SSA.
if (!DT || !DT->dominates(IVOperand, UseInst))
return false;
if (!LI->replacementPreservesLCSSAForm(UseInst, IVOperand))
return false;
LLVM_DEBUG(dbgs() << "INDVARS: Eliminated identity: " << *UseInst << '\n');
UseInst->replaceAllUsesWith(IVOperand);
++NumElimIdentity;
Changed = true;
DeadInsts.emplace_back(UseInst);
return true;
}
/// Annotate BO with nsw / nuw if it provably does not signed-overflow /
/// unsigned-overflow. Returns true if anything changed, false otherwise.
bool SimplifyIndvar::strengthenOverflowingOperation(BinaryOperator *BO,
Value *IVOperand) {
// Fastpath: we don't have any work to do if `BO` is `nuw` and `nsw`.
if (BO->hasNoUnsignedWrap() && BO->hasNoSignedWrap())
return false;
const SCEV *(ScalarEvolution::*GetExprForBO)(const SCEV *, const SCEV *,
SCEV::NoWrapFlags, unsigned);
switch (BO->getOpcode()) {
default:
return false;
case Instruction::Add:
GetExprForBO = &ScalarEvolution::getAddExpr;
break;
case Instruction::Sub:
GetExprForBO = &ScalarEvolution::getMinusSCEV;
break;
case Instruction::Mul:
GetExprForBO = &ScalarEvolution::getMulExpr;
break;
}
unsigned BitWidth = cast<IntegerType>(BO->getType())->getBitWidth();
Type *WideTy = IntegerType::get(BO->getContext(), BitWidth * 2);
const SCEV *LHS = SE->getSCEV(BO->getOperand(0));
const SCEV *RHS = SE->getSCEV(BO->getOperand(1));
bool Changed = false;
if (!BO->hasNoUnsignedWrap()) {
const SCEV *ExtendAfterOp = SE->getZeroExtendExpr(SE->getSCEV(BO), WideTy);
const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
SE->getZeroExtendExpr(LHS, WideTy), SE->getZeroExtendExpr(RHS, WideTy),
SCEV::FlagAnyWrap, 0u);
if (ExtendAfterOp == OpAfterExtend) {
BO->setHasNoUnsignedWrap();
SE->forgetValue(BO);
Changed = true;
}
}
if (!BO->hasNoSignedWrap()) {
const SCEV *ExtendAfterOp = SE->getSignExtendExpr(SE->getSCEV(BO), WideTy);
const SCEV *OpAfterExtend = (SE->*GetExprForBO)(
SE->getSignExtendExpr(LHS, WideTy), SE->getSignExtendExpr(RHS, WideTy),
SCEV::FlagAnyWrap, 0u);
if (ExtendAfterOp == OpAfterExtend) {
BO->setHasNoSignedWrap();
SE->forgetValue(BO);
Changed = true;
}
}
return Changed;
}
/// Annotate the Shr in (X << IVOperand) >> C as exact using the
/// information from the IV's range. Returns true if anything changed, false
/// otherwise.
bool SimplifyIndvar::strengthenRightShift(BinaryOperator *BO,
Value *IVOperand) {
using namespace llvm::PatternMatch;
if (BO->getOpcode() == Instruction::Shl) {
bool Changed = false;
ConstantRange IVRange = SE->getUnsignedRange(SE->getSCEV(IVOperand));
for (auto *U : BO->users()) {
const APInt *C;
if (match(U,
m_AShr(m_Shl(m_Value(), m_Specific(IVOperand)), m_APInt(C))) ||
match(U,
m_LShr(m_Shl(m_Value(), m_Specific(IVOperand)), m_APInt(C)))) {
BinaryOperator *Shr = cast<BinaryOperator>(U);
if (!Shr->isExact() && IVRange.getUnsignedMin().uge(*C)) {
Shr->setIsExact(true);
Changed = true;
}
}
}
return Changed;
}
return false;
}
/// Add all uses of Def to the current IV's worklist.
static void pushIVUsers(
Instruction *Def, Loop *L,
SmallPtrSet<Instruction*,16> &Simplified,
SmallVectorImpl< std::pair<Instruction*,Instruction*> > &SimpleIVUsers) {
for (User *U : Def->users()) {
Instruction *UI = cast<Instruction>(U);
// Avoid infinite or exponential worklist processing.
// Also ensure unique worklist users.
// If Def is a LoopPhi, it may not be in the Simplified set, so check for
// self edges first.
if (UI == Def)
continue;
// Only change the current Loop, do not change the other parts (e.g. other
// Loops).
if (!L->contains(UI))
continue;
// Do not push the same instruction more than once.
if (!Simplified.insert(UI).second)
continue;
SimpleIVUsers.push_back(std::make_pair(UI, Def));
}
}
/// Return true if this instruction generates a simple SCEV
/// expression in terms of that IV.
///
/// This is similar to IVUsers' isInteresting() but processes each instruction
/// non-recursively when the operand is already known to be a simpleIVUser.
///
static bool isSimpleIVUser(Instruction *I, const Loop *L, ScalarEvolution *SE) {
if (!SE->isSCEVable(I->getType()))
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 encounters a clear optimization
/// opportunity.
///
/// 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, L, Simplified, SimpleIVUsers);
while (!SimpleIVUsers.empty()) {
std::pair<Instruction*, Instruction*> UseOper =
SimpleIVUsers.pop_back_val();
Instruction *UseInst = UseOper.first;
// If a user of the IndVar is trivially dead, we prefer just to mark it dead
// rather than try to do some complex analysis or transformation (such as
// widening) basing on it.
// TODO: Propagate TLI and pass it here to handle more cases.
if (isInstructionTriviallyDead(UseInst, /* TLI */ nullptr)) {
DeadInsts.emplace_back(UseInst);
continue;
}
// Bypass back edges to avoid extra work.
if (UseInst == CurrIV) continue;
// Try to replace UseInst with a loop invariant before any other
// simplifications.
if (replaceIVUserWithLoopInvariant(UseInst))
continue;
Instruction *IVOperand = UseOper.second;
for (unsigned N = 0; IVOperand; ++N) {
assert(N <= Simplified.size() && "runaway iteration");
Value *NewOper = foldIVUser(UseInst, IVOperand);
if (!NewOper)
break; // done folding
IVOperand = dyn_cast<Instruction>(NewOper);
}
if (!IVOperand)
continue;
if (eliminateIVUser(UseInst, IVOperand)) {
pushIVUsers(IVOperand, L, Simplified, SimpleIVUsers);
continue;
}
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(UseInst)) {
if ((isa<OverflowingBinaryOperator>(BO) &&
strengthenOverflowingOperation(BO, IVOperand)) ||
(isa<ShlOperator>(BO) && strengthenRightShift(BO, IVOperand))) {
// re-queue uses of the now modified binary operator and fall
// through to the checks that remain.
pushIVUsers(IVOperand, L, Simplified, SimpleIVUsers);
}
}
CastInst *Cast = dyn_cast<CastInst>(UseInst);
if (V && Cast) {
V->visitCast(Cast);
continue;
}
if (isSimpleIVUser(UseInst, L, SE)) {
pushIVUsers(UseInst, L, 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, DominatorTree *DT,
LoopInfo *LI, SmallVectorImpl<WeakTrackingVH> &Dead,
SCEVExpander &Rewriter, IVVisitor *V) {
SimplifyIndvar SIV(LI->getLoopFor(CurrIV->getParent()), SE, DT, LI, Rewriter,
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, DominatorTree *DT,
LoopInfo *LI, SmallVectorImpl<WeakTrackingVH> &Dead) {
SCEVExpander Rewriter(*SE, SE->getDataLayout(), "indvars");
#ifndef NDEBUG
Rewriter.setDebugType(DEBUG_TYPE);
#endif
bool Changed = false;
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
Changed |= simplifyUsersOfIV(cast<PHINode>(I), SE, DT, LI, Dead, Rewriter);
}
return Changed;
}
} // namespace llvm
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