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[DemandedBits] Fix computation of demanded bits for ICmps 

The computation of ICmp demanded bits is independent of the individual operand being evaluated. We simply return a mask consisting of the minimum leading zeroes of both operands.

We were incorrectly passing "I" to ComputeKnownBits - this should be "UserI->getOperand(0)". In cases where we were evaluating the 1th operand, we were taking the minimum leading zeroes of it and itself.

This should fix PR26266.

llvm-svn: 258690
This commit is contained in:
James Molloy 2016-01-25 14:49:36 +00:00
parent 1bd7f993fc
commit 121de0bcfa
2 changed files with 12 additions and 3 deletions
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2
llvm/lib/Analysis/DemandedBits.cpp
View File

@ -244,7 +244,7 @@ void DemandedBits::determineLiveOperandBits(
break;
case Instruction::ICmp:
// Count the number of leading zeroes in each operand.
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
ComputeKnownBits(BitWidth, UserI->getOperand(0), UserI->getOperand(1));
auto NumLeadingZeroes = std::min(KnownZero.countLeadingOnes(),
KnownZero2.countLeadingOnes());
AB = ~APInt::getHighBitsSet(BitWidth, NumLeadingZeroes);

13
llvm/test/Analysis/DemandedBits/basic.ll
View File

@ -24,11 +24,20 @@ define i1 @test_icmp1(i32 %a, i32 %b) {
; CHECK-LABEL: 'test_icmp2'
; CHECK-DAG: DemandedBits: 0x1 for %3 = icmp eq i32 %1, %2
; CHECK-DAG: DemandedBits: 0xFF for %1 = and i32 %a, 255
; CHECK-DAG: DemandedBits: 0xF for %2 = ashr i32 %1, 4
; CHECK-DAG: DemandedBits: 0xFFF for %1 = and i32 %a, 255
; CHECK-DAG: DemandedBits: 0xFF for %2 = ashr i32 %1, 4
define i1 @test_icmp2(i32 %a, i32 %b) {
%1 = and i32 %a, 255
%2 = ashr i32 %1, 4
%3 = icmp eq i32 %1, %2
ret i1 %3
}
; CHECK-LABEL: 'test_icmp3'
; CHECK-DAG: DemandedBits: 0xFFFFFFFF for %1 = and i32 %a, 255
; CHECK-DAG: DemandedBits: 0x1 for %2 = icmp eq i32 -1, %1
define i1 @test_icmp3(i32 %a) {
%1 = and i32 %a, 255
%2 = icmp eq i32 -1, %1
ret i1 %2
}