[llvm] r258690 - [DemandedBits] Fix computation of demanded bits for ICmps

[James Molloy via llvm-commits]

Merged in r258805.

Thanks hans!
> On 25 Jan 2016, at 19:45, Hans Wennborg <hans at chromium.org> wrote:
>
> Go ahead and merge with utils/release/merge.sh.
>
> On Mon, Jan 25, 2016 at 6:56 AM, James Molloy <james at jamesmolloy.co.uk> wrote:
>> Hi Hans,
>>
>> This fixes a latent bug which will be present in 3.8, and potentially
>> affects a case in Spec CPU 2006. It is low risk and an obvious bugfix.
>>
>> Could I please nominate this for inclusion in 3.8?
>>
>> James
>>
>>
>> On Mon, 25 Jan 2016 at 14:53 James Molloy via llvm-commits
>> <llvm-commits at lists.llvm.org> wrote:
>>>
>>> Author: jamesm
>>> Date: Mon Jan 25 08:49:36 2016
>>> New Revision: 258690
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=258690&view=rev
>>> Log:
>>> [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.
>>>
>>> Modified:
>>>    llvm/trunk/lib/Analysis/DemandedBits.cpp
>>>    llvm/trunk/test/Analysis/DemandedBits/basic.ll
>>>
>>> Modified: llvm/trunk/lib/Analysis/DemandedBits.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/DemandedBits.cpp?rev=258690&r1=258689&r2=258690&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/Analysis/DemandedBits.cpp (original)
>>> +++ llvm/trunk/lib/Analysis/DemandedBits.cpp Mon Jan 25 08:49:36 2016
>>> @@ -244,7 +244,7 @@ void DemandedBits::determineLiveOperandB
>>>     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);
>>>
>>> Modified: llvm/trunk/test/Analysis/DemandedBits/basic.ll
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/DemandedBits/basic.ll?rev=258690&r1=258689&r2=258690&view=diff
>>>
>>> ==============================================================================
>>> --- llvm/trunk/test/Analysis/DemandedBits/basic.ll (original)
>>> +++ llvm/trunk/test/Analysis/DemandedBits/basic.ll Mon Jan 25 08:49:36
>>> 2016
>>> @@ -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
>>> +}
>>>
>>>
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>

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