[llvm] r289147 - [SCCP] Make sure SCCP and ConstantFolding agree on undef >> a.

[Sanjoy Das via llvm-commits]

Why do we are about this?  What goes wrong if undef is folded to 0 in
one way and -1 in the other?

On Thu, Dec 8, 2016 at 2:28 PM, Davide Italiano via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: davide
> Date: Thu Dec  8 16:28:53 2016
> New Revision: 289147
>
> URL: http://llvm.org/viewvc/llvm-project?rev=289147&view=rev
> Log:
> [SCCP] Make sure SCCP and ConstantFolding agree on undef >> a.
>
> Currently SCCP folds the value to -1, while ConstantProp folds to
> 0. This changes SCCP to do what ConstantFolding does.
>
> Modified:
>     llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
>     llvm/trunk/test/Transforms/SCCP/undef-resolve.ll
>
> Modified: llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SCCP.cpp?rev=289147&r1=289146&r2=289147&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/SCCP.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/SCCP.cpp Thu Dec  8 16:28:53 2016
> @@ -1396,8 +1396,8 @@ bool SCCPSolver::ResolvedUndefsIn(Functi
>                break;
>          }
>
> -        // undef >>a X -> all ones
> -        markForcedConstant(&I, Constant::getAllOnesValue(ITy));
> +        // undef >>a X -> 0
> +        markForcedConstant(&I, Constant::getNullValue(ITy));
>          return true;
>        case Instruction::LShr:
>        case Instruction::Shl:
>
> Modified: llvm/trunk/test/Transforms/SCCP/undef-resolve.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SCCP/undef-resolve.ll?rev=289147&r1=289146&r2=289147&view=diff
> ==============================================================================
> --- llvm/trunk/test/Transforms/SCCP/undef-resolve.ll (original)
> +++ llvm/trunk/test/Transforms/SCCP/undef-resolve.ll Thu Dec  8 16:28:53 2016
> @@ -135,7 +135,7 @@ define i32 @test6() {
>    %t = ashr i32 undef, 31
>    ret i32 %t
>  ; CHECK-LABEL: @test6(
> -; CHECK: ret i32 -1
> +; CHECK: ret i32 0
>  }
>
>  ; Make sure lshr produces a possible value
> @@ -178,5 +178,5 @@ entry:
>    %shr4 = ashr i32 undef, zext (i1 icmp eq (i32* bitcast (i32 (i1)* @test11 to i32*), i32* @GV) to i32)
>    ret i32 %shr4
>  ; CHECK-LABEL: @test11(
> -; CHECK: ret i32 -1
> +; CHECK: ret i32 0
>  }
>
>
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-- 
Sanjoy Das
http://playingwithpointers.com