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[InstCombine] Refactor foldICmpAndShift(); NFCI 

Separate out handling for shl, lshr and ashr. The combined handling
obscured some overly pessimistic requirements for the transform.
This commit is contained in:
Nikita Popov 2020-02-08 20:41:10 +01:00
parent 98e8eb8be0
commit a05932931c
1 changed files with 36 additions and 29 deletions
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65
llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
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@ -1642,50 +1642,57 @@ Instruction *InstCombiner::foldICmpAndShift(ICmpInst &Cmp, BinaryOperator *And,
bool IsShl = ShiftOpcode == Instruction::Shl;
const APInt *C3;
if (match(Shift->getOperand(1), m_APInt(C3))) {
bool CanFold = false;
APInt NewAndCst, NewCmpCst;
bool AnyCmpCstBitsShiftedOut;
if (ShiftOpcode == Instruction::Shl) {
// For a left shift, we can fold if the comparison is not signed. We can
// also fold a signed comparison if the mask value and comparison value
// are not negative. These constraints may not be obvious, but we can
// prove that they are correct using an SMT solver.
if (!Cmp.isSigned() || (!C2.isNegative() && !C1.isNegative()))
CanFold = true;
} else {
bool IsAshr = ShiftOpcode == Instruction::AShr;
if (Cmp.isSigned() && (C2.isNegative() || C1.isNegative()))
return nullptr;
NewCmpCst = C1.lshr(*C3);
NewAndCst = C2.lshr(*C3);
AnyCmpCstBitsShiftedOut = NewCmpCst.shl(*C3) != C1;
} else if (ShiftOpcode == Instruction::LShr) {
// For a logical right shift, we can fold if the comparison is not signed.
// We can also fold a signed comparison if the shifted mask value and the
// shifted comparison value are not negative. These constraints may not be
// obvious, but we can prove that they are correct using an SMT solver.
NewCmpCst = C1.shl(*C3);
NewAndCst = C2.shl(*C3);
AnyCmpCstBitsShiftedOut = NewCmpCst.lshr(*C3) != C1;
if (Cmp.isSigned() && (NewAndCst.isNegative() || NewCmpCst.isNegative()))
return nullptr;
} else {
// For an arithmetic shift right we can do the same, if we ensure
// the And doesn't use any bits being shifted in. Normally these would
// be turned into lshr by SimplifyDemandedBits, but not if there is an
// additional user.
if (!IsAshr || (C2.shl(*C3).lshr(*C3) == C2)) {
if (!Cmp.isSigned() ||
(!C2.shl(*C3).isNegative() && !C1.shl(*C3).isNegative()))
CanFold = true;
}
assert(ShiftOpcode == Instruction::AShr && "Unknown shift opcode");
NewCmpCst = C1.shl(*C3);
NewAndCst = C2.shl(*C3);
AnyCmpCstBitsShiftedOut = NewCmpCst.lshr(*C3) != C1;
if (NewAndCst.lshr(*C3) != C2)
return nullptr;
if (Cmp.isSigned() && (NewAndCst.isNegative() || NewCmpCst.isNegative()))
return nullptr;
}
if (CanFold) {
APInt NewCst = IsShl ? C1.lshr(*C3) : C1.shl(*C3);
APInt SameAsC1 = IsShl ? NewCst.shl(*C3) : NewCst.lshr(*C3);
// Check to see if we are shifting out any of the bits being compared.
if (SameAsC1 != C1) {
// If we shifted bits out, the fold is not going to work out. As a
// special case, check to see if this means that the result is always
// true or false now.
if (Cmp.getPredicate() == ICmpInst::ICMP_EQ)
return replaceInstUsesWith(Cmp, ConstantInt::getFalse(Cmp.getType()));
if (Cmp.getPredicate() == ICmpInst::ICMP_NE)
return replaceInstUsesWith(Cmp, ConstantInt::getTrue(Cmp.getType()));
} else {
APInt NewAndCst = IsShl ? C2.lshr(*C3) : C2.shl(*C3);
Value *NewAnd = Builder.CreateAnd(
Shift->getOperand(0), ConstantInt::get(And->getType(), NewAndCst));
return new ICmpInst(Cmp.getPredicate(),
NewAnd, ConstantInt::get(And->getType(), NewCst));
}
if (AnyCmpCstBitsShiftedOut) {
// If we shifted bits out, the fold is not going to work out. As a
// special case, check to see if this means that the result is always
// true or false now.
if (Cmp.getPredicate() == ICmpInst::ICMP_EQ)
return replaceInstUsesWith(Cmp, ConstantInt::getFalse(Cmp.getType()));
if (Cmp.getPredicate() == ICmpInst::ICMP_NE)
return replaceInstUsesWith(Cmp, ConstantInt::getTrue(Cmp.getType()));
} else {
Value *NewAnd = Builder.CreateAnd(
Shift->getOperand(0), ConstantInt::get(And->getType(), NewAndCst));
return new ICmpInst(Cmp.getPredicate(),
NewAnd, ConstantInt::get(And->getType(), NewCmpCst));
}
}