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Generalize this optimization to work on equality comparisons between any two

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integers that are constant except for a single bit (the same n-th bit in each).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@90646 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nick Lewycky 2009-12-05 05:00:00 +00:00
parent 6c7e8a02d8
commit 83e8ec73e1
2 changed files with 58 additions and 17 deletions
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39
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -8585,25 +8585,36 @@ Instruction *InstCombiner::transformZExtICmp(ICmpInst *ICI, Instruction &CI,
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);
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);
APInt KnownZeroLHS(BitWidth, 0), KnownOneLHS(BitWidth, 0);
APInt KnownZeroRHS(BitWidth, 0), KnownOneRHS(BitWidth, 0);
APInt TypeMask(APInt::getAllOnesValue(BitWidth));
ComputeMaskedBits(LHS, TypeMask, KnownZeroLHS, KnownOneLHS);
ComputeMaskedBits(RHS, TypeMask, KnownZeroRHS, KnownOneRHS);
if (KnownZeroLHS.countLeadingOnes() == BitWidth-1 &&
KnownZeroRHS.countLeadingOnes() == BitWidth-1) {
if (KnownZeroLHS == KnownZeroRHS && KnownOneLHS == KnownOneRHS) {
APInt KnownBits = KnownZeroLHS | KnownOneLHS;
APInt UnknownBit = ~KnownBits;
if (UnknownBit.countPopulation() == 1) {
if (!DoXform) return ICI;
Value *Xor = Builder->CreateXor(LHS, RHS);
Value *Result = Builder->CreateXor(LHS, RHS);
// Mask off any bits that are set and won't be shifted away.
if (KnownOneLHS.uge(UnknownBit))
Result = Builder->CreateAnd(Result,
ConstantInt::get(ITy, UnknownBit));
// Shift the bit we're testing down to the lsb.
Result = Builder->CreateLShr(
Result, ConstantInt::get(ITy, UnknownBit.countTrailingZeros()));
if (ICI->getPredicate() == ICmpInst::ICMP_EQ)
Xor = Builder->CreateXor(Xor, ConstantInt::get(ITy, 1));
Xor->takeName(ICI);
return ReplaceInstUsesWith(CI, Xor);
Result = Builder->CreateXor(Result, ConstantInt::get(ITy, 1));
Result->takeName(ICI);
return ReplaceInstUsesWith(CI, Result);
}
}
}

36
test/Transforms/InstCombine/compare-signs.ll
View File

@ -2,7 +2,7 @@
; PR5438
; TODO: This should also optimize down.
;define i32 @bar(i32 %a, i32 %b) nounwind readnone {
;define i32 @test1(i32 %a, i32 %b) nounwind readnone {
;entry:
; %0 = icmp sgt i32 %a, -1 ; <i1> [#uses=1]
; %1 = icmp slt i32 %b, 0 ; <i1> [#uses=1]
@ -11,8 +11,18 @@
; ret i32 %3
;}
define i32 @qaz(i32 %a, i32 %b) nounwind readnone {
; CHECK: @qaz
; TODO: This optimizes partially but not all the way.
;define i32 @test2(i32 %a, i32 %b) nounwind readnone {
;entry:
; %0 = and i32 %a, 8 ;<i32> [#uses=1]
; %1 = and i32 %b, 8 ;<i32> [#uses=1]
; %2 = icmp eq i32 %0, %1 ;<i1> [#uses=1]
; %3 = zext i1 %2 to i32 ;<i32> [#uses=1]
; ret i32 %3
;}
define i32 @test3(i32 %a, i32 %b) nounwind readnone {
; CHECK: @test3
entry:
; CHECK: xor i32 %a, %b
; CHECK; lshr i32 %0, 31
@ -26,3 +36,23 @@ entry:
; CHECK-NOT: zext
; CHECK: ret i32 %2
}
; Variation on @test3: checking the 2nd bit in a situation where the 5th bit
; is one, not zero.
define i32 @test3i(i32 %a, i32 %b) nounwind readnone {
; CHECK: @test3i
entry:
; CHECK: xor i32 %a, %b
; CHECK; lshr i32 %0, 31
; CHECK: xor i32 %1, 1
%0 = lshr i32 %a, 29 ; <i32> [#uses=1]
%1 = lshr i32 %b, 29 ; <i32> [#uses=1]
%2 = or i32 %0, 35
%3 = or i32 %1, 35
%4 = icmp eq i32 %2, %3 ; <i1> [#uses=1]
%5 = zext i1 %4 to i32 ; <i32> [#uses=1]
ret i32 %5
; CHECK-NOT: icmp
; CHECK-NOT: zext
; CHECK: ret i32 %2
}