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llvm / llvm-project / 77ff6f7df8691a735a2dc979cdb44835dd2d41af / . / llvm / lib / Transforms / InstCombine / InstCombineShifts.cpp
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//===- InstCombineShifts.cpp ----------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements the visitShl, visitLShr, and visitAShr functions.
//
//===----------------------------------------------------------------------===//
#include "InstCombineInternal.h"
#include "llvm/Analysis/ConstantFolding.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Transforms/InstCombine/InstCombiner.h"
using namespace llvm;
using namespace PatternMatch;
#define DEBUG_TYPE "instcombine"
bool canTryToConstantAddTwoShiftAmounts(Value *Sh0, Value *ShAmt0, Value *Sh1,
Value *ShAmt1) {
// We have two shift amounts from two different shifts. The types of those
// shift amounts may not match. If that's the case let's bailout now..
if (ShAmt0->getType() != ShAmt1->getType())
return false;
// As input, we have the following pattern:
// Sh0 (Sh1 X, Q), K
// We want to rewrite that as:
// Sh x, (Q+K) iff (Q+K) u< bitwidth(x)
// While we know that originally (Q+K) would not overflow
// (because 2 * (N-1) u<= iN -1), we have looked past extensions of
// shift amounts. so it may now overflow in smaller bitwidth.
// To ensure that does not happen, we need to ensure that the total maximal
// shift amount is still representable in that smaller bit width.
unsigned MaximalPossibleTotalShiftAmount =
(Sh0->getType()->getScalarSizeInBits() - 1) +
(Sh1->getType()->getScalarSizeInBits() - 1);
APInt MaximalRepresentableShiftAmount =
APInt::getAllOnes(ShAmt0->getType()->getScalarSizeInBits());
return MaximalRepresentableShiftAmount.uge(MaximalPossibleTotalShiftAmount);
}
// Given pattern:
// (x shiftopcode Q) shiftopcode K
// we should rewrite it as
// x shiftopcode (Q+K) iff (Q+K) u< bitwidth(x) and
//
// This is valid for any shift, but they must be identical, and we must be
// careful in case we have (zext(Q)+zext(K)) and look past extensions,
// (Q+K) must not overflow or else (Q+K) u< bitwidth(x) is bogus.
//
// AnalyzeForSignBitExtraction indicates that we will only analyze whether this
// pattern has any 2 right-shifts that sum to 1 less than original bit width.
Value *InstCombinerImpl::reassociateShiftAmtsOfTwoSameDirectionShifts(
BinaryOperator *Sh0, const SimplifyQuery &SQ,
bool AnalyzeForSignBitExtraction) {
// Look for a shift of some instruction, ignore zext of shift amount if any.
Instruction *Sh0Op0;
Value *ShAmt0;
if (!match(Sh0,
m_Shift(m_Instruction(Sh0Op0), m_ZExtOrSelf(m_Value(ShAmt0)))))
return nullptr;
// If there is a truncation between the two shifts, we must make note of it
// and look through it. The truncation imposes additional constraints on the
// transform.
Instruction *Sh1;
Value *Trunc = nullptr;
match(Sh0Op0,
m_CombineOr(m_CombineAnd(m_Trunc(m_Instruction(Sh1)), m_Value(Trunc)),
m_Instruction(Sh1)));
// Inner shift: (x shiftopcode ShAmt1)
// Like with other shift, ignore zext of shift amount if any.
Value *X, *ShAmt1;
if (!match(Sh1, m_Shift(m_Value(X), m_ZExtOrSelf(m_Value(ShAmt1)))))
return nullptr;
// Verify that it would be safe to try to add those two shift amounts.
if (!canTryToConstantAddTwoShiftAmounts(Sh0, ShAmt0, Sh1, ShAmt1))
return nullptr;
// We are only looking for signbit extraction if we have two right shifts.
bool HadTwoRightShifts = match(Sh0, m_Shr(m_Value(), m_Value())) &&
match(Sh1, m_Shr(m_Value(), m_Value()));
// ... and if it's not two right-shifts, we know the answer already.
if (AnalyzeForSignBitExtraction && !HadTwoRightShifts)
return nullptr;
// The shift opcodes must be identical, unless we are just checking whether
// this pattern can be interpreted as a sign-bit-extraction.
Instruction::BinaryOps ShiftOpcode = Sh0->getOpcode();
bool IdenticalShOpcodes = Sh0->getOpcode() == Sh1->getOpcode();
if (!IdenticalShOpcodes && !AnalyzeForSignBitExtraction)
return nullptr;
// If we saw truncation, we'll need to produce extra instruction,
// and for that one of the operands of the shift must be one-use,
// unless of course we don't actually plan to produce any instructions here.
if (Trunc && !AnalyzeForSignBitExtraction &&
!match(Sh0, m_c_BinOp(m_OneUse(m_Value()), m_Value())))
return nullptr;
// Can we fold (ShAmt0+ShAmt1) ?
auto *NewShAmt = dyn_cast_or_null<Constant>(
SimplifyAddInst(ShAmt0, ShAmt1, /*isNSW=*/false, /*isNUW=*/false,
SQ.getWithInstruction(Sh0)));
if (!NewShAmt)
return nullptr; // Did not simplify.
unsigned NewShAmtBitWidth = NewShAmt->getType()->getScalarSizeInBits();
unsigned XBitWidth = X->getType()->getScalarSizeInBits();
// Is the new shift amount smaller than the bit width of inner/new shift?
if (!match(NewShAmt, m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_ULT,
APInt(NewShAmtBitWidth, XBitWidth))))
return nullptr; // FIXME: could perform constant-folding.
// If there was a truncation, and we have a right-shift, we can only fold if
// we are left with the original sign bit. Likewise, if we were just checking
// that this is a sighbit extraction, this is the place to check it.
// FIXME: zero shift amount is also legal here, but we can't *easily* check
// more than one predicate so it's not really worth it.
if (HadTwoRightShifts && (Trunc || AnalyzeForSignBitExtraction)) {
// If it's not a sign bit extraction, then we're done.
if (!match(NewShAmt,
m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_EQ,
APInt(NewShAmtBitWidth, XBitWidth - 1))))
return nullptr;
// If it is, and that was the question, return the base value.
if (AnalyzeForSignBitExtraction)
return X;
}
assert(IdenticalShOpcodes && "Should not get here with different shifts.");
// All good, we can do this fold.
NewShAmt = ConstantExpr::getZExtOrBitCast(NewShAmt, X->getType());
BinaryOperator *NewShift = BinaryOperator::Create(ShiftOpcode, X, NewShAmt);
// The flags can only be propagated if there wasn't a trunc.
if (!Trunc) {
// If the pattern did not involve trunc, and both of the original shifts
// had the same flag set, preserve the flag.
if (ShiftOpcode == Instruction::BinaryOps::Shl) {
NewShift->setHasNoUnsignedWrap(Sh0->hasNoUnsignedWrap() &&
Sh1->hasNoUnsignedWrap());
NewShift->setHasNoSignedWrap(Sh0->hasNoSignedWrap() &&
Sh1->hasNoSignedWrap());
} else {
NewShift->setIsExact(Sh0->isExact() && Sh1->isExact());
}
}
Instruction *Ret = NewShift;
if (Trunc) {
Builder.Insert(NewShift);
Ret = CastInst::Create(Instruction::Trunc, NewShift, Sh0->getType());
}
return Ret;
}
// If we have some pattern that leaves only some low bits set, and then performs
// left-shift of those bits, if none of the bits that are left after the final
// shift are modified by the mask, we can omit the mask.
//
// There are many variants to this pattern:
// a) (x & ((1 << MaskShAmt) - 1)) << ShiftShAmt
// b) (x & (~(-1 << MaskShAmt))) << ShiftShAmt
// c) (x & (-1 l>> MaskShAmt)) << ShiftShAmt
// d) (x & ((-1 << MaskShAmt) l>> MaskShAmt)) << ShiftShAmt
// e) ((x << MaskShAmt) l>> MaskShAmt) << ShiftShAmt
// f) ((x << MaskShAmt) a>> MaskShAmt) << ShiftShAmt
// All these patterns can be simplified to just:
// x << ShiftShAmt
// iff:
// a,b) (MaskShAmt+ShiftShAmt) u>= bitwidth(x)
// c,d,e,f) (ShiftShAmt-MaskShAmt) s>= 0 (i.e. ShiftShAmt u>= MaskShAmt)
static Instruction *
dropRedundantMaskingOfLeftShiftInput(BinaryOperator *OuterShift,
const SimplifyQuery &Q,
InstCombiner::BuilderTy &Builder) {
assert(OuterShift->getOpcode() == Instruction::BinaryOps::Shl &&
"The input must be 'shl'!");
Value *Masked, *ShiftShAmt;
match(OuterShift,
m_Shift(m_Value(Masked), m_ZExtOrSelf(m_Value(ShiftShAmt))));
// *If* there is a truncation between an outer shift and a possibly-mask,
// then said truncation *must* be one-use, else we can't perform the fold.
Value *Trunc;
if (match(Masked, m_CombineAnd(m_Trunc(m_Value(Masked)), m_Value(Trunc))) &&
!Trunc->hasOneUse())
return nullptr;
Type *NarrowestTy = OuterShift->getType();
Type *WidestTy = Masked->getType();
bool HadTrunc = WidestTy != NarrowestTy;
// The mask must be computed in a type twice as wide to ensure
// that no bits are lost if the sum-of-shifts is wider than the base type.
Type *ExtendedTy = WidestTy->getExtendedType();
Value *MaskShAmt;
// ((1 << MaskShAmt) - 1)
auto MaskA = m_Add(m_Shl(m_One(), m_Value(MaskShAmt)), m_AllOnes());
// (~(-1 << maskNbits))
auto MaskB = m_Xor(m_Shl(m_AllOnes(), m_Value(MaskShAmt)), m_AllOnes());
// (-1 l>> MaskShAmt)
auto MaskC = m_LShr(m_AllOnes(), m_Value(MaskShAmt));
// ((-1 << MaskShAmt) l>> MaskShAmt)
auto MaskD =
m_LShr(m_Shl(m_AllOnes(), m_Value(MaskShAmt)), m_Deferred(MaskShAmt));
Value *X;
Constant *NewMask;
if (match(Masked, m_c_And(m_CombineOr(MaskA, MaskB), m_Value(X)))) {
// Peek through an optional zext of the shift amount.
match(MaskShAmt, m_ZExtOrSelf(m_Value(MaskShAmt)));
// Verify that it would be safe to try to add those two shift amounts.
if (!canTryToConstantAddTwoShiftAmounts(OuterShift, ShiftShAmt, Masked,
MaskShAmt))
return nullptr;
// Can we simplify (MaskShAmt+ShiftShAmt) ?
auto *SumOfShAmts = dyn_cast_or_null<Constant>(SimplifyAddInst(
MaskShAmt, ShiftShAmt, /*IsNSW=*/false, /*IsNUW=*/false, Q));
if (!SumOfShAmts)
return nullptr; // Did not simplify.
// In this pattern SumOfShAmts correlates with the number of low bits
// that shall remain in the root value (OuterShift).
// An extend of an undef value becomes zero because the high bits are never
// completely unknown. Replace the `undef` shift amounts with final
// shift bitwidth to ensure that the value remains undef when creating the
// subsequent shift op.
SumOfShAmts = Constant::replaceUndefsWith(
SumOfShAmts, ConstantInt::get(SumOfShAmts->getType()->getScalarType(),
ExtendedTy->getScalarSizeInBits()));
auto *ExtendedSumOfShAmts = ConstantExpr::getZExt(SumOfShAmts, ExtendedTy);
// And compute the mask as usual: ~(-1 << (SumOfShAmts))
auto *ExtendedAllOnes = ConstantExpr::getAllOnesValue(ExtendedTy);
auto *ExtendedInvertedMask =
ConstantExpr::getShl(ExtendedAllOnes, ExtendedSumOfShAmts);
NewMask = ConstantExpr::getNot(ExtendedInvertedMask);
} else if (match(Masked, m_c_And(m_CombineOr(MaskC, MaskD), m_Value(X))) ||
match(Masked, m_Shr(m_Shl(m_Value(X), m_Value(MaskShAmt)),
m_Deferred(MaskShAmt)))) {
// Peek through an optional zext of the shift amount.
match(MaskShAmt, m_ZExtOrSelf(m_Value(MaskShAmt)));
// Verify that it would be safe to try to add those two shift amounts.
if (!canTryToConstantAddTwoShiftAmounts(OuterShift, ShiftShAmt, Masked,
MaskShAmt))
return nullptr;
// Can we simplify (ShiftShAmt-MaskShAmt) ?
auto *ShAmtsDiff = dyn_cast_or_null<Constant>(SimplifySubInst(
ShiftShAmt, MaskShAmt, /*IsNSW=*/false, /*IsNUW=*/false, Q));
if (!ShAmtsDiff)
return nullptr; // Did not simplify.
// In this pattern ShAmtsDiff correlates with the number of high bits that
// shall be unset in the root value (OuterShift).
// An extend of an undef value becomes zero because the high bits are never
// completely unknown. Replace the `undef` shift amounts with negated
// bitwidth of innermost shift to ensure that the value remains undef when
// creating the subsequent shift op.
unsigned WidestTyBitWidth = WidestTy->getScalarSizeInBits();
ShAmtsDiff = Constant::replaceUndefsWith(
ShAmtsDiff, ConstantInt::get(ShAmtsDiff->getType()->getScalarType(),
-WidestTyBitWidth));
auto *ExtendedNumHighBitsToClear = ConstantExpr::getZExt(
ConstantExpr::getSub(ConstantInt::get(ShAmtsDiff->getType(),
WidestTyBitWidth,
/*isSigned=*/false),
ShAmtsDiff),
ExtendedTy);
// And compute the mask as usual: (-1 l>> (NumHighBitsToClear))
auto *ExtendedAllOnes = ConstantExpr::getAllOnesValue(ExtendedTy);
NewMask =
ConstantExpr::getLShr(ExtendedAllOnes, ExtendedNumHighBitsToClear);
} else
return nullptr; // Don't know anything about this pattern.
NewMask = ConstantExpr::getTrunc(NewMask, NarrowestTy);
// Does this mask has any unset bits? If not then we can just not apply it.
bool NeedMask = !match(NewMask, m_AllOnes());
// If we need to apply a mask, there are several more restrictions we have.
if (NeedMask) {
// The old masking instruction must go away.
if (!Masked->hasOneUse())
return nullptr;
// The original "masking" instruction must not have been`ashr`.
if (match(Masked, m_AShr(m_Value(), m_Value())))
return nullptr;
}
// If we need to apply truncation, let's do it first, since we can.
// We have already ensured that the old truncation will go away.
if (HadTrunc)
X = Builder.CreateTrunc(X, NarrowestTy);
// No 'NUW'/'NSW'! We no longer know that we won't shift-out non-0 bits.
// We didn't change the Type of this outermost shift, so we can just do it.
auto *NewShift = BinaryOperator::Create(OuterShift->getOpcode(), X,
OuterShift->getOperand(1));
if (!NeedMask)
return NewShift;
Builder.Insert(NewShift);
return BinaryOperator::Create(Instruction::And, NewShift, NewMask);
}
/// If we have a shift-by-constant of a bitwise logic op that itself has a
/// shift-by-constant operand with identical opcode, we may be able to convert
/// that into 2 independent shifts followed by the logic op. This eliminates a
/// a use of an intermediate value (reduces dependency chain).
static Instruction *foldShiftOfShiftedLogic(BinaryOperator &I,
InstCombiner::BuilderTy &Builder) {
assert(I.isShift() && "Expected a shift as input");
auto *LogicInst = dyn_cast<BinaryOperator>(I.getOperand(0));
if (!LogicInst || !LogicInst->isBitwiseLogicOp() || !LogicInst->hasOneUse())
return nullptr;
Constant *C0, *C1;
if (!match(I.getOperand(1), m_Constant(C1)))
return nullptr;
Instruction::BinaryOps ShiftOpcode = I.getOpcode();
Type *Ty = I.getType();
// Find a matching one-use shift by constant. The fold is not valid if the sum
// of the shift values equals or exceeds bitwidth.
// TODO: Remove the one-use check if the other logic operand (Y) is constant.
Value *X, *Y;
auto matchFirstShift = [&](Value *V) {
APInt Threshold(Ty->getScalarSizeInBits(), Ty->getScalarSizeInBits());
return match(V, m_BinOp(ShiftOpcode, m_Value(), m_Value())) &&
match(V, m_OneUse(m_Shift(m_Value(X), m_Constant(C0)))) &&
match(ConstantExpr::getAdd(C0, C1),
m_SpecificInt_ICMP(ICmpInst::ICMP_ULT, Threshold));
};
// Logic ops are commutative, so check each operand for a match.
if (matchFirstShift(LogicInst->getOperand(0)))
Y = LogicInst->getOperand(1);
else if (matchFirstShift(LogicInst->getOperand(1)))
Y = LogicInst->getOperand(0);
else
return nullptr;
// shift (logic (shift X, C0), Y), C1 -> logic (shift X, C0+C1), (shift Y, C1)
Constant *ShiftSumC = ConstantExpr::getAdd(C0, C1);
Value *NewShift1 = Builder.CreateBinOp(ShiftOpcode, X, ShiftSumC);
Value *NewShift2 = Builder.CreateBinOp(ShiftOpcode, Y, I.getOperand(1));
return BinaryOperator::Create(LogicInst->getOpcode(), NewShift1, NewShift2);
}
Instruction *InstCombinerImpl::commonShiftTransforms(BinaryOperator &I) {
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
assert(Op0->getType() == Op1->getType());
// If the shift amount is a one-use `sext`, we can demote it to `zext`.
Value *Y;
if (match(Op1, m_OneUse(m_SExt(m_Value(Y))))) {
Value *NewExt = Builder.CreateZExt(Y, I.getType(), Op1->getName());
return BinaryOperator::Create(I.getOpcode(), Op0, NewExt);
}
// See if we can fold away this shift.
if (SimplifyDemandedInstructionBits(I))
return &I;
// Try to fold constant and into select arguments.
if (isa<Constant>(Op0))
if (SelectInst *SI = dyn_cast<SelectInst>(Op1))
if (Instruction *R = FoldOpIntoSelect(I, SI))
return R;
if (Constant *CUI = dyn_cast<Constant>(Op1))
if (Instruction *Res = FoldShiftByConstant(Op0, CUI, I))
return Res;
if (auto *NewShift = cast_or_null<Instruction>(
reassociateShiftAmtsOfTwoSameDirectionShifts(&I, SQ)))
return NewShift;
// (C1 shift (A add C2)) -> (C1 shift C2) shift A)
// iff A and C2 are both positive.
Value *A;
Constant *C;
if (match(Op0, m_Constant()) && match(Op1, m_Add(m_Value(A), m_Constant(C))))
if (isKnownNonNegative(A, DL, 0, &AC, &I, &DT) &&
isKnownNonNegative(C, DL, 0, &AC, &I, &DT))
return BinaryOperator::Create(
I.getOpcode(), Builder.CreateBinOp(I.getOpcode(), Op0, C), A);
// X shift (A srem C) -> X shift (A and (C - 1)) iff C is a power of 2.
// Because shifts by negative values (which could occur if A were negative)
// are undefined.
if (Op1->hasOneUse() && match(Op1, m_SRem(m_Value(A), m_Constant(C))) &&
match(C, m_Power2())) {
// FIXME: Should this get moved into SimplifyDemandedBits by saying we don't
// demand the sign bit (and many others) here??
Constant *Mask = ConstantExpr::getSub(C, ConstantInt::get(I.getType(), 1));
Value *Rem = Builder.CreateAnd(A, Mask, Op1->getName());
return replaceOperand(I, 1, Rem);
}
if (Instruction *Logic = foldShiftOfShiftedLogic(I, Builder))
return Logic;
return nullptr;
}
/// Return true if we can simplify two logical (either left or right) shifts
/// that have constant shift amounts: OuterShift (InnerShift X, C1), C2.
static bool canEvaluateShiftedShift(unsigned OuterShAmt, bool IsOuterShl,
Instruction *InnerShift,
InstCombinerImpl &IC, Instruction *CxtI) {
assert(InnerShift->isLogicalShift() && "Unexpected instruction type");
// We need constant scalar or constant splat shifts.
const APInt *InnerShiftConst;
if (!match(InnerShift->getOperand(1), m_APInt(InnerShiftConst)))
return false;
// Two logical shifts in the same direction:
// shl (shl X, C1), C2 --> shl X, C1 + C2
// lshr (lshr X, C1), C2 --> lshr X, C1 + C2
bool IsInnerShl = InnerShift->getOpcode() == Instruction::Shl;
if (IsInnerShl == IsOuterShl)
return true;
// Equal shift amounts in opposite directions become bitwise 'and':
// lshr (shl X, C), C --> and X, C'
// shl (lshr X, C), C --> and X, C'
if (*InnerShiftConst == OuterShAmt)
return true;
// If the 2nd shift is bigger than the 1st, we can fold:
// lshr (shl X, C1), C2 --> and (shl X, C1 - C2), C3
// shl (lshr X, C1), C2 --> and (lshr X, C1 - C2), C3
// but it isn't profitable unless we know the and'd out bits are already zero.
// Also, check that the inner shift is valid (less than the type width) or
// we'll crash trying to produce the bit mask for the 'and'.
unsigned TypeWidth = InnerShift->getType()->getScalarSizeInBits();
if (InnerShiftConst->ugt(OuterShAmt) && InnerShiftConst->ult(TypeWidth)) {
unsigned InnerShAmt = InnerShiftConst->getZExtValue();
unsigned MaskShift =
IsInnerShl ? TypeWidth - InnerShAmt : InnerShAmt - OuterShAmt;
APInt Mask = APInt::getLowBitsSet(TypeWidth, OuterShAmt) << MaskShift;
if (IC.MaskedValueIsZero(InnerShift->getOperand(0), Mask, 0, CxtI))
return true;
}
return false;
}
/// See if we can compute the specified value, but shifted logically to the left
/// or right by some number of bits. This should return true if the expression
/// can be computed for the same cost as the current expression tree. This is
/// used to eliminate extraneous shifting from things like:
/// %C = shl i128 %A, 64
/// %D = shl i128 %B, 96
/// %E = or i128 %C, %D
/// %F = lshr i128 %E, 64
/// where the client will ask if E can be computed shifted right by 64-bits. If
/// this succeeds, getShiftedValue() will be called to produce the value.
static bool canEvaluateShifted(Value *V, unsigned NumBits, bool IsLeftShift,
InstCombinerImpl &IC, Instruction *CxtI) {
// We can always evaluate constants shifted.
if (isa<Constant>(V))
return true;
Instruction *I = dyn_cast<Instruction>(V);
if (!I) return false;
// We can't mutate something that has multiple uses: doing so would
// require duplicating the instruction in general, which isn't profitable.
if (!I->hasOneUse()) return false;
switch (I->getOpcode()) {
default: return false;
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
// Bitwise operators can all arbitrarily be arbitrarily evaluated shifted.
return canEvaluateShifted(I->getOperand(0), NumBits, IsLeftShift, IC, I) &&
canEvaluateShifted(I->getOperand(1), NumBits, IsLeftShift, IC, I);
case Instruction::Shl:
case Instruction::LShr:
return canEvaluateShiftedShift(NumBits, IsLeftShift, I, IC, CxtI);
case Instruction::Select: {
SelectInst *SI = cast<SelectInst>(I);
Value *TrueVal = SI->getTrueValue();
Value *FalseVal = SI->getFalseValue();
return canEvaluateShifted(TrueVal, NumBits, IsLeftShift, IC, SI) &&
canEvaluateShifted(FalseVal, NumBits, IsLeftShift, IC, SI);
}
case Instruction::PHI: {
// We can change a phi if we can change all operands. Note that we never
// get into trouble with cyclic PHIs here because we only consider
// instructions with a single use.
PHINode *PN = cast<PHINode>(I);
for (Value *IncValue : PN->incoming_values())
if (!canEvaluateShifted(IncValue, NumBits, IsLeftShift, IC, PN))
return false;
return true;
}
}
}
/// Fold OuterShift (InnerShift X, C1), C2.
/// See canEvaluateShiftedShift() for the constraints on these instructions.
static Value *foldShiftedShift(BinaryOperator *InnerShift, unsigned OuterShAmt,
bool IsOuterShl,
InstCombiner::BuilderTy &Builder) {
bool IsInnerShl = InnerShift->getOpcode() == Instruction::Shl;
Type *ShType = InnerShift->getType();
unsigned TypeWidth = ShType->getScalarSizeInBits();
// We only accept shifts-by-a-constant in canEvaluateShifted().
const APInt *C1;
match(InnerShift->getOperand(1), m_APInt(C1));
unsigned InnerShAmt = C1->getZExtValue();
// Change the shift amount and clear the appropriate IR flags.
auto NewInnerShift = [&](unsigned ShAmt) {
InnerShift->setOperand(1, ConstantInt::get(ShType, ShAmt));
if (IsInnerShl) {
InnerShift->setHasNoUnsignedWrap(false);
InnerShift->setHasNoSignedWrap(false);
} else {
InnerShift->setIsExact(false);
}
return InnerShift;
};
// Two logical shifts in the same direction:
// shl (shl X, C1), C2 --> shl X, C1 + C2
// lshr (lshr X, C1), C2 --> lshr X, C1 + C2
if (IsInnerShl == IsOuterShl) {
// If this is an oversized composite shift, then unsigned shifts get 0.
if (InnerShAmt + OuterShAmt >= TypeWidth)
return Constant::getNullValue(ShType);
return NewInnerShift(InnerShAmt + OuterShAmt);
}
// Equal shift amounts in opposite directions become bitwise 'and':
// lshr (shl X, C), C --> and X, C'
// shl (lshr X, C), C --> and X, C'
if (InnerShAmt == OuterShAmt) {
APInt Mask = IsInnerShl
? APInt::getLowBitsSet(TypeWidth, TypeWidth - OuterShAmt)
: APInt::getHighBitsSet(TypeWidth, TypeWidth - OuterShAmt);
Value *And = Builder.CreateAnd(InnerShift->getOperand(0),
ConstantInt::get(ShType, Mask));
if (auto *AndI = dyn_cast<Instruction>(And)) {
AndI->moveBefore(InnerShift);
AndI->takeName(InnerShift);
}
return And;
}
assert(InnerShAmt > OuterShAmt &&
"Unexpected opposite direction logical shift pair");
// In general, we would need an 'and' for this transform, but
// canEvaluateShiftedShift() guarantees that the masked-off bits are not used.
// lshr (shl X, C1), C2 --> shl X, C1 - C2
// shl (lshr X, C1), C2 --> lshr X, C1 - C2
return NewInnerShift(InnerShAmt - OuterShAmt);
}
/// When canEvaluateShifted() returns true for an expression, this function
/// inserts the new computation that produces the shifted value.
static Value *getShiftedValue(Value *V, unsigned NumBits, bool isLeftShift,
InstCombinerImpl &IC, const DataLayout &DL) {
// We can always evaluate constants shifted.
if (Constant *C = dyn_cast<Constant>(V)) {
if (isLeftShift)
return IC.Builder.CreateShl(C, NumBits);
else
return IC.Builder.CreateLShr(C, NumBits);
}
Instruction *I = cast<Instruction>(V);
IC.addToWorklist(I);
switch (I->getOpcode()) {
default: llvm_unreachable("Inconsistency with CanEvaluateShifted");
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
// Bitwise operators can all arbitrarily be arbitrarily evaluated shifted.
I->setOperand(
0, getShiftedValue(I->getOperand(0), NumBits, isLeftShift, IC, DL));
I->setOperand(
1, getShiftedValue(I->getOperand(1), NumBits, isLeftShift, IC, DL));
return I;
case Instruction::Shl:
case Instruction::LShr:
return foldShiftedShift(cast<BinaryOperator>(I), NumBits, isLeftShift,
IC.Builder);
case Instruction::Select:
I->setOperand(
1, getShiftedValue(I->getOperand(1), NumBits, isLeftShift, IC, DL));
I->setOperand(
2, getShiftedValue(I->getOperand(2), NumBits, isLeftShift, IC, DL));
return I;
case Instruction::PHI: {
// We can change a phi if we can change all operands. Note that we never
// get into trouble with cyclic PHIs here because we only consider
// instructions with a single use.
PHINode *PN = cast<PHINode>(I);
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
PN->setIncomingValue(i, getShiftedValue(PN->getIncomingValue(i), NumBits,
isLeftShift, IC, DL));
return PN;
}
}
}
// If this is a bitwise operator or add with a constant RHS we might be able
// to pull it through a shift.
static bool canShiftBinOpWithConstantRHS(BinaryOperator &Shift,
BinaryOperator *BO) {
switch (BO->getOpcode()) {
default:
return false; // Do not perform transform!
case Instruction::Add:
return Shift.getOpcode() == Instruction::Shl;
case Instruction::Or:
case Instruction::And:
return true;
case Instruction::Xor:
// Do not change a 'not' of logical shift because that would create a normal
// 'xor'. The 'not' is likely better for analysis, SCEV, and codegen.
return !(Shift.isLogicalShift() && match(BO, m_Not(m_Value())));
}
}
Instruction *InstCombinerImpl::FoldShiftByConstant(Value *Op0, Constant *Op1,
BinaryOperator &I) {
const APInt *Op1C;
if (!match(Op1, m_APInt(Op1C)))
return nullptr;
// See if we can propagate this shift into the input, this covers the trivial
// cast of lshr(shl(x,c1),c2) as well as other more complex cases.
bool IsLeftShift = I.getOpcode() == Instruction::Shl;
if (I.getOpcode() != Instruction::AShr &&
canEvaluateShifted(Op0, Op1C->getZExtValue(), IsLeftShift, *this, &I)) {
LLVM_DEBUG(
dbgs() << "ICE: GetShiftedValue propagating shift through expression"
" to eliminate shift:\n IN: "
<< *Op0 << "\n SH: " << I << "\n");
return replaceInstUsesWith(
I, getShiftedValue(Op0, Op1C->getZExtValue(), IsLeftShift, *this, DL));
}
// See if we can simplify any instructions used by the instruction whose sole
// purpose is to compute bits we don't care about.
Type *Ty = I.getType();
unsigned TypeBits = Ty->getScalarSizeInBits();
assert(!Op1C->uge(TypeBits) &&
"Shift over the type width should have been removed already");
(void)TypeBits;
if (Instruction *FoldedShift = foldBinOpIntoSelectOrPhi(I))
return FoldedShift;
if (!Op0->hasOneUse())
return nullptr;
if (auto *Op0BO = dyn_cast<BinaryOperator>(Op0)) {
// If the operand is a bitwise operator with a constant RHS, and the
// shift is the only use, we can pull it out of the shift.
const APInt *Op0C;
if (match(Op0BO->getOperand(1), m_APInt(Op0C))) {
if (canShiftBinOpWithConstantRHS(I, Op0BO)) {
Constant *NewRHS = ConstantExpr::get(
I.getOpcode(), cast<Constant>(Op0BO->getOperand(1)), Op1);
Value *NewShift =
Builder.CreateBinOp(I.getOpcode(), Op0BO->getOperand(0), Op1);
NewShift->takeName(Op0BO);
return BinaryOperator::Create(Op0BO->getOpcode(), NewShift, NewRHS);
}
}
}
// If we have a select that conditionally executes some binary operator,
// see if we can pull it the select and operator through the shift.
//
// For example, turning:
// shl (select C, (add X, C1), X), C2
// Into:
// Y = shl X, C2
// select C, (add Y, C1 << C2), Y
Value *Cond;
BinaryOperator *TBO;
Value *FalseVal;
if (match(Op0, m_Select(m_Value(Cond), m_OneUse(m_BinOp(TBO)),
m_Value(FalseVal)))) {
const APInt *C;
if (!isa<Constant>(FalseVal) && TBO->getOperand(0) == FalseVal &&
match(TBO->getOperand(1), m_APInt(C)) &&
canShiftBinOpWithConstantRHS(I, TBO)) {
Constant *NewRHS = ConstantExpr::get(
I.getOpcode(), cast<Constant>(TBO->getOperand(1)), Op1);
Value *NewShift = Builder.CreateBinOp(I.getOpcode(), FalseVal, Op1);
Value *NewOp = Builder.CreateBinOp(TBO->getOpcode(), NewShift, NewRHS);
return SelectInst::Create(Cond, NewOp, NewShift);
}
}
BinaryOperator *FBO;
Value *TrueVal;
if (match(Op0, m_Select(m_Value(Cond), m_Value(TrueVal),
m_OneUse(m_BinOp(FBO))))) {
const APInt *C;
if (!isa<Constant>(TrueVal) && FBO->getOperand(0) == TrueVal &&
match(FBO->getOperand(1), m_APInt(C)) &&
canShiftBinOpWithConstantRHS(I, FBO)) {
Constant *NewRHS = ConstantExpr::get(
I.getOpcode(), cast<Constant>(FBO->getOperand(1)), Op1);
Value *NewShift = Builder.CreateBinOp(I.getOpcode(), TrueVal, Op1);
Value *NewOp = Builder.CreateBinOp(FBO->getOpcode(), NewShift, NewRHS);
return SelectInst::Create(Cond, NewShift, NewOp);
}
}
return nullptr;
}
Instruction *InstCombinerImpl::visitShl(BinaryOperator &I) {
const SimplifyQuery Q = SQ.getWithInstruction(&I);
if (Value *V = SimplifyShlInst(I.getOperand(0), I.getOperand(1),
I.hasNoSignedWrap(), I.hasNoUnsignedWrap(), Q))
return replaceInstUsesWith(I, V);
if (Instruction *X = foldVectorBinop(I))
return X;
if (Instruction *V = commonShiftTransforms(I))
return V;
if (Instruction *V = dropRedundantMaskingOfLeftShiftInput(&I, Q, Builder))
return V;
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
Type *Ty = I.getType();
unsigned BitWidth = Ty->getScalarSizeInBits();
const APInt *C;
if (match(Op1, m_APInt(C))) {
unsigned ShAmtC = C->getZExtValue();
// shl (zext X), C --> zext (shl X, C)
// This is only valid if X would have zeros shifted out.
Value *X;
if (match(Op0, m_OneUse(m_ZExt(m_Value(X))))) {
unsigned SrcWidth = X->getType()->getScalarSizeInBits();
if (ShAmtC < SrcWidth &&
MaskedValueIsZero(X, APInt::getHighBitsSet(SrcWidth, ShAmtC), 0, &I))
return new ZExtInst(Builder.CreateShl(X, ShAmtC), Ty);
}
// (X >> C) << C --> X & (-1 << C)
if (match(Op0, m_Shr(m_Value(X), m_Specific(Op1)))) {
APInt Mask(APInt::getHighBitsSet(BitWidth, BitWidth - ShAmtC));
return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, Mask));
}
const APInt *C1;
if (match(Op0, m_Exact(m_Shr(m_Value(X), m_APInt(C1)))) &&
C1->ult(BitWidth)) {
unsigned ShrAmt = C1->getZExtValue();
if (ShrAmt < ShAmtC) {
// If C1 < C: (X >>?,exact C1) << C --> X << (C - C1)
Constant *ShiftDiff = ConstantInt::get(Ty, ShAmtC - ShrAmt);
auto *NewShl = BinaryOperator::CreateShl(X, ShiftDiff);
NewShl->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
NewShl->setHasNoSignedWrap(I.hasNoSignedWrap());
return NewShl;
}
if (ShrAmt > ShAmtC) {
// If C1 > C: (X >>?exact C1) << C --> X >>?exact (C1 - C)
Constant *ShiftDiff = ConstantInt::get(Ty, ShrAmt - ShAmtC);
auto *NewShr = BinaryOperator::Create(
cast<BinaryOperator>(Op0)->getOpcode(), X, ShiftDiff);
NewShr->setIsExact(true);
return NewShr;
}
}
if (match(Op0, m_OneUse(m_Shr(m_Value(X), m_APInt(C1)))) &&
C1->ult(BitWidth)) {
unsigned ShrAmt = C1->getZExtValue();
if (ShrAmt < ShAmtC) {
// If C1 < C: (X >>? C1) << C --> (X << (C - C1)) & (-1 << C)
Constant *ShiftDiff = ConstantInt::get(Ty, ShAmtC - ShrAmt);
auto *NewShl = BinaryOperator::CreateShl(X, ShiftDiff);
NewShl->setHasNoUnsignedWrap(I.hasNoUnsignedWrap());
NewShl->setHasNoSignedWrap(I.hasNoSignedWrap());
Builder.Insert(NewShl);
APInt Mask(APInt::getHighBitsSet(BitWidth, BitWidth - ShAmtC));
return BinaryOperator::CreateAnd(NewShl, ConstantInt::get(Ty, Mask));
}
if (ShrAmt > ShAmtC) {
// If C1 > C: (X >>? C1) << C --> (X >>? (C1 - C)) & (-1 << C)
Constant *ShiftDiff = ConstantInt::get(Ty, ShrAmt - ShAmtC);
auto *OldShr = cast<BinaryOperator>(Op0);
auto *NewShr =
BinaryOperator::Create(OldShr->getOpcode(), X, ShiftDiff);
NewShr->setIsExact(OldShr->isExact());
Builder.Insert(NewShr);
APInt Mask(APInt::getHighBitsSet(BitWidth, BitWidth - ShAmtC));
return BinaryOperator::CreateAnd(NewShr, ConstantInt::get(Ty, Mask));
}
}
// Similar to above, but look through an intermediate trunc instruction.
BinaryOperator *Shr;
if (match(Op0, m_OneUse(m_Trunc(m_OneUse(m_BinOp(Shr))))) &&
match(Shr, m_Shr(m_Value(X), m_APInt(C1)))) {
// The larger shift direction survives through the transform.
unsigned ShrAmtC = C1->getZExtValue();
unsigned ShDiff = ShrAmtC > ShAmtC ? ShrAmtC - ShAmtC : ShAmtC - ShrAmtC;
Constant *ShiftDiffC = ConstantInt::get(X->getType(), ShDiff);
auto ShiftOpc = ShrAmtC > ShAmtC ? Shr->getOpcode() : Instruction::Shl;
// If C1 > C:
// (trunc (X >> C1)) << C --> (trunc (X >> (C1 - C))) && (-1 << C)
// If C > C1:
// (trunc (X >> C1)) << C --> (trunc (X << (C - C1))) && (-1 << C)
Value *NewShift = Builder.CreateBinOp(ShiftOpc, X, ShiftDiffC, "sh.diff");
Value *Trunc = Builder.CreateTrunc(NewShift, Ty, "tr.sh.diff");
APInt Mask(APInt::getHighBitsSet(BitWidth, BitWidth - ShAmtC));
return BinaryOperator::CreateAnd(Trunc, ConstantInt::get(Ty, Mask));
}
if (match(Op0, m_Shl(m_Value(X), m_APInt(C1))) && C1->ult(BitWidth)) {
unsigned AmtSum = ShAmtC + C1->getZExtValue();
// Oversized shifts are simplified to zero in InstSimplify.
if (AmtSum < BitWidth)
// (X << C1) << C2 --> X << (C1 + C2)
return BinaryOperator::CreateShl(X, ConstantInt::get(Ty, AmtSum));
}
// If we have an opposite shift by the same amount, we may be able to
// reorder binops and shifts to eliminate math/logic.
auto isSuitableBinOpcode = [](Instruction::BinaryOps BinOpcode) {
switch (BinOpcode) {
default:
return false;
case Instruction::Add:
case Instruction::And:
case Instruction::Or:
case Instruction::Xor:
case Instruction::Sub:
// NOTE: Sub is not commutable and the tranforms below may not be valid
// when the shift-right is operand 1 (RHS) of the sub.
return true;
}
};
BinaryOperator *Op0BO;
if (match(Op0, m_OneUse(m_BinOp(Op0BO))) &&
isSuitableBinOpcode(Op0BO->getOpcode())) {
// Commute so shift-right is on LHS of the binop.
// (Y bop (X >> C)) << C -> ((X >> C) bop Y) << C
// (Y bop ((X >> C) & CC)) << C -> (((X >> C) & CC) bop Y) << C
Value *Shr = Op0BO->getOperand(0);
Value *Y = Op0BO->getOperand(1);
Value *X;
const APInt *CC;
if (Op0BO->isCommutative() && Y->hasOneUse() &&
(match(Y, m_Shr(m_Value(), m_Specific(Op1))) ||
match(Y, m_And(m_OneUse(m_Shr(m_Value(), m_Specific(Op1))),
m_APInt(CC)))))
std::swap(Shr, Y);
// ((X >> C) bop Y) << C -> (X bop (Y << C)) & (~0 << C)
if (match(Shr, m_OneUse(m_Shr(m_Value(X), m_Specific(Op1))))) {
// Y << C
Value *YS = Builder.CreateShl(Y, Op1, Op0BO->getName());
// (X bop (Y << C))
Value *B =
Builder.CreateBinOp(Op0BO->getOpcode(), X, YS, Shr->getName());
unsigned Op1Val = C->getLimitedValue(BitWidth);
APInt Bits = APInt::getHighBitsSet(BitWidth, BitWidth - Op1Val);
Constant *Mask = ConstantInt::get(Ty, Bits);
return BinaryOperator::CreateAnd(B, Mask);
}
// (((X >> C) & CC) bop Y) << C -> (X & (CC << C)) bop (Y << C)
if (match(Shr,
m_OneUse(m_And(m_OneUse(m_Shr(m_Value(X), m_Specific(Op1))),
m_APInt(CC))))) {
// Y << C
Value *YS = Builder.CreateShl(Y, Op1, Op0BO->getName());
// X & (CC << C)
Value *M = Builder.CreateAnd(X, ConstantInt::get(Ty, CC->shl(*C)),
X->getName() + ".mask");
return BinaryOperator::Create(Op0BO->getOpcode(), M, YS);
}
}
// (C1 - X) << C --> (C1 << C) - (X << C)
if (match(Op0, m_OneUse(m_Sub(m_APInt(C1), m_Value(X))))) {
Constant *NewLHS = ConstantInt::get(Ty, C1->shl(*C));
Value *NewShift = Builder.CreateShl(X, Op1);
return BinaryOperator::CreateSub(NewLHS, NewShift);
}
// If the shifted-out value is known-zero, then this is a NUW shift.
if (!I.hasNoUnsignedWrap() &&
MaskedValueIsZero(Op0, APInt::getHighBitsSet(BitWidth, ShAmtC), 0,
&I)) {
I.setHasNoUnsignedWrap();
return &I;
}
// If the shifted-out value is all signbits, then this is a NSW shift.
if (!I.hasNoSignedWrap() && ComputeNumSignBits(Op0, 0, &I) > ShAmtC) {
I.setHasNoSignedWrap();
return &I;
}
}
// Transform (x >> y) << y to x & (-1 << y)
// Valid for any type of right-shift.
Value *X;
if (match(Op0, m_OneUse(m_Shr(m_Value(X), m_Specific(Op1))))) {
Constant *AllOnes = ConstantInt::getAllOnesValue(Ty);
Value *Mask = Builder.CreateShl(AllOnes, Op1);
return BinaryOperator::CreateAnd(Mask, X);
}
Constant *C1;
if (match(Op1, m_Constant(C1))) {
Constant *C2;
Value *X;
// (C2 << X) << C1 --> (C2 << C1) << X
if (match(Op0, m_OneUse(m_Shl(m_Constant(C2), m_Value(X)))))
return BinaryOperator::CreateShl(ConstantExpr::getShl(C2, C1), X);
// (X * C2) << C1 --> X * (C2 << C1)
if (match(Op0, m_Mul(m_Value(X), m_Constant(C2))))
return BinaryOperator::CreateMul(X, ConstantExpr::getShl(C2, C1));
// shl (zext i1 X), C1 --> select (X, 1 << C1, 0)
if (match(Op0, m_ZExt(m_Value(X))) && X->getType()->isIntOrIntVectorTy(1)) {
auto *NewC = ConstantExpr::getShl(ConstantInt::get(Ty, 1), C1);
return SelectInst::Create(X, NewC, ConstantInt::getNullValue(Ty));
}
}
// (1 << (C - x)) -> ((1 << C) >> x) if C is bitwidth - 1
if (match(Op0, m_One()) &&
match(Op1, m_Sub(m_SpecificInt(BitWidth - 1), m_Value(X))))
return BinaryOperator::CreateLShr(
ConstantInt::get(Ty, APInt::getSignMask(BitWidth)), X);
return nullptr;
}
Instruction *InstCombinerImpl::visitLShr(BinaryOperator &I) {
if (Value *V = SimplifyLShrInst(I.getOperand(0), I.getOperand(1), I.isExact(),
SQ.getWithInstruction(&I)))
return replaceInstUsesWith(I, V);
if (Instruction *X = foldVectorBinop(I))
return X;
if (Instruction *R = commonShiftTransforms(I))
return R;
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
Type *Ty = I.getType();
const APInt *C;
if (match(Op1, m_APInt(C))) {
unsigned ShAmtC = C->getZExtValue();
unsigned BitWidth = Ty->getScalarSizeInBits();
auto *II = dyn_cast<IntrinsicInst>(Op0);
if (II && isPowerOf2_32(BitWidth) && Log2_32(BitWidth) == ShAmtC &&
(II->getIntrinsicID() == Intrinsic::ctlz ||
II->getIntrinsicID() == Intrinsic::cttz ||
II->getIntrinsicID() == Intrinsic::ctpop)) {
// ctlz.i32(x)>>5 --> zext(x == 0)
// cttz.i32(x)>>5 --> zext(x == 0)
// ctpop.i32(x)>>5 --> zext(x == -1)
bool IsPop = II->getIntrinsicID() == Intrinsic::ctpop;
Constant *RHS = ConstantInt::getSigned(Ty, IsPop ? -1 : 0);
Value *Cmp = Builder.CreateICmpEQ(II->getArgOperand(0), RHS);
return new ZExtInst(Cmp, Ty);
}
Value *X;
const APInt *C1;
if (match(Op0, m_Shl(m_Value(X), m_APInt(C1))) && C1->ult(BitWidth)) {
if (C1->ult(ShAmtC)) {
unsigned ShlAmtC = C1->getZExtValue();
Constant *ShiftDiff = ConstantInt::get(Ty, ShAmtC - ShlAmtC);
if (cast<BinaryOperator>(Op0)->hasNoUnsignedWrap()) {
// (X <<nuw C1) >>u C --> X >>u (C - C1)
auto *NewLShr = BinaryOperator::CreateLShr(X, ShiftDiff);
NewLShr->setIsExact(I.isExact());
return NewLShr;
}
// (X << C1) >>u C --> (X >>u (C - C1)) & (-1 >> C)
Value *NewLShr = Builder.CreateLShr(X, ShiftDiff, "", I.isExact());
APInt Mask(APInt::getLowBitsSet(BitWidth, BitWidth - ShAmtC));
return BinaryOperator::CreateAnd(NewLShr, ConstantInt::get(Ty, Mask));
}
if (C1->ugt(ShAmtC)) {
unsigned ShlAmtC = C1->getZExtValue();
Constant *ShiftDiff = ConstantInt::get(Ty, ShlAmtC - ShAmtC);
if (cast<BinaryOperator>(Op0)->hasNoUnsignedWrap()) {
// (X <<nuw C1) >>u C --> X <<nuw (C1 - C)
auto *NewShl = BinaryOperator::CreateShl(X, ShiftDiff);
NewShl->setHasNoUnsignedWrap(true);
return NewShl;
}
// (X << C1) >>u C --> X << (C1 - C) & (-1 >> C)
Value *NewShl = Builder.CreateShl(X, ShiftDiff);
APInt Mask(APInt::getLowBitsSet(BitWidth, BitWidth - ShAmtC));
return BinaryOperator::CreateAnd(NewShl, ConstantInt::get(Ty, Mask));
}
assert(*C1 == ShAmtC);
// (X << C) >>u C --> X & (-1 >>u C)
APInt Mask(APInt::getLowBitsSet(BitWidth, BitWidth - ShAmtC));
return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, Mask));
}
if (match(Op0, m_OneUse(m_ZExt(m_Value(X)))) &&
(!Ty->isIntegerTy() || shouldChangeType(Ty, X->getType()))) {
assert(ShAmtC < X->getType()->getScalarSizeInBits() &&
"Big shift not simplified to zero?");
// lshr (zext iM X to iN), C --> zext (lshr X, C) to iN
Value *NewLShr = Builder.CreateLShr(X, ShAmtC);
return new ZExtInst(NewLShr, Ty);
}
if (match(Op0, m_SExt(m_Value(X)))) {
unsigned SrcTyBitWidth = X->getType()->getScalarSizeInBits();
// lshr (sext i1 X to iN), C --> select (X, -1 >> C, 0)
if (SrcTyBitWidth == 1) {
auto *NewC = ConstantInt::get(
Ty, APInt::getLowBitsSet(BitWidth, BitWidth - ShAmtC));
return SelectInst::Create(X, NewC, ConstantInt::getNullValue(Ty));
}
if ((!Ty->isIntegerTy() || shouldChangeType(Ty, X->getType())) &&
Op0->hasOneUse()) {
// Are we moving the sign bit to the low bit and widening with high
// zeros? lshr (sext iM X to iN), N-1 --> zext (lshr X, M-1) to iN
if (ShAmtC == BitWidth - 1) {
Value *NewLShr = Builder.CreateLShr(X, SrcTyBitWidth - 1);
return new ZExtInst(NewLShr, Ty);
}
// lshr (sext iM X to iN), N-M --> zext (ashr X, min(N-M, M-1)) to iN
if (ShAmtC == BitWidth - SrcTyBitWidth) {
// The new shift amount can't be more than the narrow source type.
unsigned NewShAmt = std::min(ShAmtC, SrcTyBitWidth - 1);
Value *AShr = Builder.CreateAShr(X, NewShAmt);
return new ZExtInst(AShr, Ty);
}
}
}
Value *Y;
if (ShAmtC == BitWidth - 1) {
// lshr i32 or(X,-X), 31 --> zext (X != 0)
if (match(Op0, m_OneUse(m_c_Or(m_Neg(m_Value(X)), m_Deferred(X)))))
return new ZExtInst(Builder.CreateIsNotNull(X), Ty);
// lshr i32 (X -nsw Y), 31 --> zext (X < Y)
if (match(Op0, m_OneUse(m_NSWSub(m_Value(X), m_Value(Y)))))
return new ZExtInst(Builder.CreateICmpSLT(X, Y), Ty);
// Check if a number is negative and odd:
// lshr i32 (srem X, 2), 31 --> and (X >> 31), X
if (match(Op0, m_OneUse(m_SRem(m_Value(X), m_SpecificInt(2))))) {
Value *Signbit = Builder.CreateLShr(X, ShAmtC);
return BinaryOperator::CreateAnd(Signbit, X);
}
}
// (X >>u C1) >>u C --> X >>u (C1 + C)
if (match(Op0, m_LShr(m_Value(X), m_APInt(C1)))) {
// Oversized shifts are simplified to zero in InstSimplify.
unsigned AmtSum = ShAmtC + C1->getZExtValue();
if (AmtSum < BitWidth)
return BinaryOperator::CreateLShr(X, ConstantInt::get(Ty, AmtSum));
}
Instruction *TruncSrc;
if (match(Op0, m_OneUse(m_Trunc(m_Instruction(TruncSrc)))) &&
match(TruncSrc, m_LShr(m_Value(X), m_APInt(C1)))) {
unsigned SrcWidth = X->getType()->getScalarSizeInBits();
unsigned AmtSum = ShAmtC + C1->getZExtValue();
// If the combined shift fits in the source width:
// (trunc (X >>u C1)) >>u C --> and (trunc (X >>u (C1 + C)), MaskC
//
// If the first shift covers the number of bits truncated, then the
// mask instruction is eliminated (and so the use check is relaxed).
if (AmtSum < SrcWidth &&
(TruncSrc->hasOneUse() || C1->uge(SrcWidth - BitWidth))) {
Value *SumShift = Builder.CreateLShr(X, AmtSum, "sum.shift");
Value *Trunc = Builder.CreateTrunc(SumShift, Ty, I.getName());
// If the first shift does not cover the number of bits truncated, then
// we require a mask to get rid of high bits in the result.
APInt MaskC = APInt::getAllOnes(BitWidth).lshr(ShAmtC);
return BinaryOperator::CreateAnd(Trunc, ConstantInt::get(Ty, MaskC));
}
}
// Look for a "splat" mul pattern - it replicates bits across each half of
// a value, so a right shift is just a mask of the low bits:
// lshr i32 (mul nuw X, Pow2+1), 16 --> and X, Pow2-1
// TODO: Generalize to allow more than just half-width shifts?
const APInt *MulC;
if (match(Op0, m_NUWMul(m_Value(X), m_APInt(MulC))) &&
ShAmtC * 2 == BitWidth && (*MulC - 1).isPowerOf2() &&
MulC->logBase2() == ShAmtC)
return BinaryOperator::CreateAnd(X, ConstantInt::get(Ty, *MulC - 2));
// If the shifted-out value is known-zero, then this is an exact shift.
if (!I.isExact() &&
MaskedValueIsZero(Op0, APInt::getLowBitsSet(BitWidth, ShAmtC), 0, &I)) {
I.setIsExact();
return &I;
}
}
// Transform (x << y) >> y to x & (-1 >> y)
Value *X;
if (match(Op0, m_OneUse(m_Shl(m_Value(X), m_Specific(Op1))))) {
Constant *AllOnes = ConstantInt::getAllOnesValue(Ty);
Value *Mask = Builder.CreateLShr(AllOnes, Op1);
return BinaryOperator::CreateAnd(Mask, X);
}
return nullptr;
}
Instruction *
InstCombinerImpl::foldVariableSignZeroExtensionOfVariableHighBitExtract(
BinaryOperator &OldAShr) {
assert(OldAShr.getOpcode() == Instruction::AShr &&
"Must be called with arithmetic right-shift instruction only.");
// Check that constant C is a splat of the element-wise bitwidth of V.
auto BitWidthSplat = [](Constant *C, Value *V) {
return match(
C, m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_EQ,
APInt(C->getType()->getScalarSizeInBits(),
V->getType()->getScalarSizeInBits())));
};
// It should look like variable-length sign-extension on the outside:
// (Val << (bitwidth(Val)-Nbits)) a>> (bitwidth(Val)-Nbits)
Value *NBits;
Instruction *MaybeTrunc;
Constant *C1, *C2;
if (!match(&OldAShr,
m_AShr(m_Shl(m_Instruction(MaybeTrunc),
m_ZExtOrSelf(m_Sub(m_Constant(C1),
m_ZExtOrSelf(m_Value(NBits))))),
m_ZExtOrSelf(m_Sub(m_Constant(C2),
m_ZExtOrSelf(m_Deferred(NBits)))))) ||
!BitWidthSplat(C1, &OldAShr) || !BitWidthSplat(C2, &OldAShr))
return nullptr;
// There may or may not be a truncation after outer two shifts.
Instruction *HighBitExtract;
match(MaybeTrunc, m_TruncOrSelf(m_Instruction(HighBitExtract)));
bool HadTrunc = MaybeTrunc != HighBitExtract;
// And finally, the innermost part of the pattern must be a right-shift.
Value *X, *NumLowBitsToSkip;
if (!match(HighBitExtract, m_Shr(m_Value(X), m_Value(NumLowBitsToSkip))))
return nullptr;
// Said right-shift must extract high NBits bits - C0 must be it's bitwidth.
Constant *C0;
if (!match(NumLowBitsToSkip,
m_ZExtOrSelf(
m_Sub(m_Constant(C0), m_ZExtOrSelf(m_Specific(NBits))))) ||
!BitWidthSplat(C0, HighBitExtract))
return nullptr;
// Since the NBits is identical for all shifts, if the outermost and
// innermost shifts are identical, then outermost shifts are redundant.
// If we had truncation, do keep it though.
if (HighBitExtract->getOpcode() == OldAShr.getOpcode())
return replaceInstUsesWith(OldAShr, MaybeTrunc);
// Else, if there was a truncation, then we need to ensure that one
// instruction will go away.
if (HadTrunc && !match(&OldAShr, m_c_BinOp(m_OneUse(m_Value()), m_Value())))
return nullptr;
// Finally, bypass two innermost shifts, and perform the outermost shift on
// the operands of the innermost shift.
Instruction *NewAShr =
BinaryOperator::Create(OldAShr.getOpcode(), X, NumLowBitsToSkip);
NewAShr->copyIRFlags(HighBitExtract); // We can preserve 'exact'-ness.
if (!HadTrunc)
return NewAShr;
Builder.Insert(NewAShr);
return TruncInst::CreateTruncOrBitCast(NewAShr, OldAShr.getType());
}
Instruction *InstCombinerImpl::visitAShr(BinaryOperator &I) {
if (Value *V = SimplifyAShrInst(I.getOperand(0), I.getOperand(1), I.isExact(),
SQ.getWithInstruction(&I)))
return replaceInstUsesWith(I, V);
if (Instruction *X = foldVectorBinop(I))
return X;
if (Instruction *R = commonShiftTransforms(I))
return R;
Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
Type *Ty = I.getType();
unsigned BitWidth = Ty->getScalarSizeInBits();
const APInt *ShAmtAPInt;
if (match(Op1, m_APInt(ShAmtAPInt)) && ShAmtAPInt->ult(BitWidth)) {
unsigned ShAmt = ShAmtAPInt->getZExtValue();
// If the shift amount equals the difference in width of the destination
// and source scalar types:
// ashr (shl (zext X), C), C --> sext X
Value *X;
if (match(Op0, m_Shl(m_ZExt(m_Value(X)), m_Specific(Op1))) &&
ShAmt == BitWidth - X->getType()->getScalarSizeInBits())
return new SExtInst(X, Ty);
// We can't handle (X << C1) >>s C2. It shifts arbitrary bits in. However,
// we can handle (X <<nsw C1) >>s C2 since it only shifts in sign bits.
const APInt *ShOp1;
if (match(Op0, m_NSWShl(m_Value(X), m_APInt(ShOp1))) &&
ShOp1->ult(BitWidth)) {
unsigned ShlAmt = ShOp1->getZExtValue();
if (ShlAmt < ShAmt) {
// (X <<nsw C1) >>s C2 --> X >>s (C2 - C1)
Constant *ShiftDiff = ConstantInt::get(Ty, ShAmt - ShlAmt);
auto *NewAShr = BinaryOperator::CreateAShr(X, ShiftDiff);
NewAShr->setIsExact(I.isExact());
return NewAShr;
}
if (ShlAmt > ShAmt) {
// (X <<nsw C1) >>s C2 --> X <<nsw (C1 - C2)
Constant *ShiftDiff = ConstantInt::get(Ty, ShlAmt - ShAmt);
auto *NewShl = BinaryOperator::Create(Instruction::Shl, X, ShiftDiff);
NewShl->setHasNoSignedWrap(true);
return NewShl;
}
}
if (match(Op0, m_AShr(m_Value(X), m_APInt(ShOp1))) &&
ShOp1->ult(BitWidth)) {
unsigned AmtSum = ShAmt + ShOp1->getZExtValue();
// Oversized arithmetic shifts replicate the sign bit.
AmtSum = std::min(AmtSum, BitWidth - 1);
// (X >>s C1) >>s C2 --> X >>s (C1 + C2)
return BinaryOperator::CreateAShr(X, ConstantInt::get(Ty, AmtSum));
}
if (match(Op0, m_OneUse(m_SExt(m_Value(X)))) &&
(Ty->isVectorTy() || shouldChangeType(Ty, X->getType()))) {
// ashr (sext X), C --> sext (ashr X, C')
Type *SrcTy = X->getType();
ShAmt = std::min(ShAmt, SrcTy->getScalarSizeInBits() - 1);
Value *NewSh = Builder.CreateAShr(X, ConstantInt::get(SrcTy, ShAmt));
return new SExtInst(NewSh, Ty);
}
if (ShAmt == BitWidth - 1) {
// ashr i32 or(X,-X), 31 --> sext (X != 0)
if (match(Op0, m_OneUse(m_c_Or(m_Neg(m_Value(X)), m_Deferred(X)))))
return new SExtInst(Builder.CreateIsNotNull(X), Ty);
// ashr i32 (X -nsw Y), 31 --> sext (X < Y)
Value *Y;
if (match(Op0, m_OneUse(m_NSWSub(m_Value(X), m_Value(Y)))))
return new SExtInst(Builder.CreateICmpSLT(X, Y), Ty);
}
// If the shifted-out value is known-zero, then this is an exact shift.
if (!I.isExact() &&
MaskedValueIsZero(Op0, APInt::getLowBitsSet(BitWidth, ShAmt), 0, &I)) {
I.setIsExact();
return &I;
}
}
// Prefer `-(x & 1)` over `(x << (bitwidth(x)-1)) a>> (bitwidth(x)-1)`
// as the pattern to splat the lowest bit.
// FIXME: iff X is already masked, we don't need the one-use check.
Value *X;
if (match(Op1, m_SpecificIntAllowUndef(BitWidth - 1)) &&
match(Op0, m_OneUse(m_Shl(m_Value(X),
m_SpecificIntAllowUndef(BitWidth - 1))))) {
Constant *Mask = ConstantInt::get(Ty, 1);
// Retain the knowledge about the ignored lanes.
Mask = Constant::mergeUndefsWith(
Constant::mergeUndefsWith(Mask, cast<Constant>(Op1)),
cast<Constant>(cast<Instruction>(Op0)->getOperand(1)));
X = Builder.CreateAnd(X, Mask);
return BinaryOperator::CreateNeg(X);
}
if (Instruction *R = foldVariableSignZeroExtensionOfVariableHighBitExtract(I))
return R;
// See if we can turn a signed shr into an unsigned shr.
if (MaskedValueIsZero(Op0, APInt::getSignMask(BitWidth), 0, &I))
return BinaryOperator::CreateLShr(Op0, Op1);
// ashr (xor %x, -1), %y --> xor (ashr %x, %y), -1
if (match(Op0, m_OneUse(m_Not(m_Value(X))))) {
// Note that we must drop 'exact'-ness of the shift!
// Note that we can't keep undef's in -1 vector constant!
auto *NewAShr = Builder.CreateAShr(X, Op1, Op0->getName() + ".not");
return BinaryOperator::CreateNot(NewAShr);
}
return nullptr;
}