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

[James Molloy via llvm-commits]

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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