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Reapply r88830 with a bugfix: this transform only applies to icmp eq/ne. This 

fixes part of PR5438.

llvm-svn: 89639
This commit is contained in:
Nick Lewycky 2009-11-23 03:17:33 +00:00
parent 6b51f28e82
commit 922d4ab574
2 changed files with 30 additions and 1 deletions
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30
llvm/lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -8539,6 +8539,36 @@ Instruction *InstCombiner::transformZExtICmp(ICmpInst *ICI, Instruction &CI,
}
}
// icmp ne A, B is equal to xor A, B when A and B only really have one bit.
// It is also profitable to transform icmp eq into not(xor(A, B)) because that
// may lead to additional simplifications.
if (ICI->isEquality() && CI.getType() == ICI->getOperand(0)->getType()) {
if (const IntegerType *ITy = dyn_cast<IntegerType>(CI.getType())) {
uint32_t BitWidth = ITy->getBitWidth();
if (BitWidth > 1) {
Value *LHS = ICI->getOperand(0);
Value *RHS = ICI->getOperand(1);
APInt KnownZeroLHS(BitWidth, 0), KnownOneLHS(BitWidth, 0);
APInt KnownZeroRHS(BitWidth, 0), KnownOneRHS(BitWidth, 0);
APInt TypeMask(APInt::getHighBitsSet(BitWidth, BitWidth-1));
ComputeMaskedBits(LHS, TypeMask, KnownZeroLHS, KnownOneLHS);
ComputeMaskedBits(RHS, TypeMask, KnownZeroRHS, KnownOneRHS);
if (KnownZeroLHS.countLeadingOnes() == BitWidth-1 &&
KnownZeroRHS.countLeadingOnes() == BitWidth-1) {
if (!DoXform) return ICI;
Value *Xor = Builder->CreateXor(LHS, RHS);
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
Xor = Builder->CreateXor(Xor, ConstantInt::get(ITy, 1));
Xor->takeName(ICI);
return ReplaceInstUsesWith(CI, Xor);
}
}
}
}
return 0;
}

1
llvm/test/Transforms/InstCombine/compare-signs.ll
View File

@ -1,5 +1,4 @@
; RUN: opt %s -instcombine -S | FileCheck %s
; XFAIL: *
; PR5438
; TODO: This should also optimize down.