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Xor'ing both sides of icmp by sign-bit is equivalent to swapping signedness of

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the predicate.

Also, make this optz'n apply in more cases where it's safe to do so.

llvm-svn: 54876
This commit is contained in:
Nick Lewycky 2008-08-17 07:34:14 +00:00
parent 661710bbd1
commit 205be593b8
2 changed files with 72 additions and 22 deletions
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72
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -5241,6 +5241,20 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
return new ICmpInst(I.getPredicate(), A, B);
}
ConstantInt *CI2;
// (icmp u/s (xor A SignBit), C) -> (icmp s/u A, (xor C SignBit))
if (!I.isEquality() &&
match(Op0, m_Xor(m_Value(A), m_ConstantInt(CI2)))) {
if (CI2->getValue().isSignBit()) {
const APInt &SignBit = CI2->getValue();
ICmpInst::Predicate Pred = I.isSignedPredicate()
? I.getUnsignedPredicate()
: I.getSignedPredicate();
return new ICmpInst(Pred, A,
ConstantInt::get(CI->getValue() ^ SignBit));
}
}
// If we have a icmp le or icmp ge instruction, turn it into the appropriate
// icmp lt or icmp gt instruction. This allows us to rely on them being
// folded in the code below.
@ -5491,35 +5505,49 @@ Instruction *InstCombiner::visitICmpInst(ICmpInst &I) {
// See if it's the same type of instruction on the left and right.
if (BinaryOperator *Op0I = dyn_cast<BinaryOperator>(Op0)) {
if (BinaryOperator *Op1I = dyn_cast<BinaryOperator>(Op1)) {
if (Op0I->getOpcode() == Op1I->getOpcode() && Op0I->hasOneUse() &&
Op1I->hasOneUse() && Op0I->getOperand(1) == Op1I->getOperand(1) &&
I.isEquality()) {
switch (Op0I->getOpcode()) {
if (Op0I->getOpcode() == Op1I->getOpcode() &&
Op0I->getOperand(1) == Op1I->getOperand(1)) {
switch (Op0I->getOpcode()) {
default: break;
case Instruction::Add:
case Instruction::Sub:
case Instruction::Xor:
// a+x icmp eq/ne b+x --> a icmp b
return new ICmpInst(I.getPredicate(), Op0I->getOperand(0),
Op1I->getOperand(0));
if (I.isEquality()) {
// icmp eq/ne a+x, b+x --> icmp eq/ne a, b
return new ICmpInst(I.getPredicate(), Op0I->getOperand(0),
Op1I->getOperand(0));
} else {
// icmp u/s (a ^ signbit), (b ^ signbit) --> icmp s/u a, b
if (ConstantInt *CI = dyn_cast<ConstantInt>(Op0I->getOperand(1))) {
if (CI->getValue().isSignBit()) {
ICmpInst::Predicate Pred = I.isSignedPredicate()
? I.getUnsignedPredicate()
: I.getSignedPredicate();
return new ICmpInst(Pred, Op0I->getOperand(0),
Op1I->getOperand(0));
}
}
}
break;
case Instruction::Mul:
if (ConstantInt *CI = dyn_cast<ConstantInt>(Op0I->getOperand(1))) {
// a * Cst icmp eq/ne b * Cst --> a & Mask icmp b & Mask
// Mask = -1 >> count-trailing-zeros(Cst).
if (!CI->isZero() && !CI->isOne()) {
const APInt &AP = CI->getValue();
ConstantInt *Mask = ConstantInt::get(
APInt::getLowBitsSet(AP.getBitWidth(),
AP.getBitWidth() -
// a * Cst icmp eq/ne b * Cst --> a & Mask icmp b & Mask
// Mask = -1 >> count-trailing-zeros(Cst).
if (Op0I->hasOneUse() && Op1I->hasOneUse() && I.isEquality()) {
if (ConstantInt *CI = dyn_cast<ConstantInt>(Op0I->getOperand(1))) {
if (!CI->isZero() && !CI->isOne()) {
const APInt &AP = CI->getValue();
ConstantInt *Mask =
ConstantInt::get(APInt::getLowBitsSet(AP.getBitWidth(),
AP.getBitWidth() -
AP.countTrailingZeros()));
Instruction *And1 = BinaryOperator::CreateAnd(Op0I->getOperand(0),
Mask);
Instruction *And2 = BinaryOperator::CreateAnd(Op1I->getOperand(0),
Mask);
InsertNewInstBefore(And1, I);
InsertNewInstBefore(And2, I);
return new ICmpInst(I.getPredicate(), And1, And2);
Instruction *And1 =
BinaryOperator::CreateAnd(Op0I->getOperand(0), Mask);
Instruction *And2 =
BinaryOperator::CreateAnd(Op1I->getOperand(0), Mask);
InsertNewInstBefore(And1, I);
InsertNewInstBefore(And2, I);
return new ICmpInst(I.getPredicate(), And1, And2);
}
}
}
break;

22
test/Transforms/InstCombine/2008-08-17-ICmpXorSignbit.ll Normal file
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@ -0,0 +1,22 @@
; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep -v xor
define i1 @test1(i8 %x, i8 %y) {
%X = xor i8 %x, 128
%Y = xor i8 %y, 128
%tmp = icmp slt i8 %X, %Y
ret i1 %tmp
}
define i1 @test2(i8 %x, i8 %y) {
%X = xor i8 %x, 128
%Y = xor i8 %y, 128
%tmp = icmp ult i8 %X, %Y
ret i1 %tmp
}
define i1 @test3(i8 %x) {
%X = xor i8 %x, 128
%tmp = icmp uge i8 %X, 15
ret i1 %tmp
}