[llvm] r348181 - [InstCombine] foldICmpWithLowBitMaskedVal(): disable 2 faulty folds.

[Friedman, Eli via llvm-commits]

On 12/3/2018 12:07 PM, Roman Lebedev via llvm-commits wrote:
> Author: lebedevri
> Date: Mon Dec  3 12:07:58 2018
> New Revision: 348181
>
> URL: http://llvm.org/viewvc/llvm-project?rev=348181&view=rev
> Log:
> [InstCombine] foldICmpWithLowBitMaskedVal(): disable 2 faulty folds.
>
> These two folds are invalid for this non-constant pattern
> when the mask ends up being all-ones:
> https://rise4fun.com/Alive/9au
> https://rise4fun.com/Alive/UcQM
>
> Fixes https://bugs.llvm.org/show_bug.cgi?id=39861
>
> Modified:
>      llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
>      llvm/trunk/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sge-to-icmp-sle.ll
>      llvm/trunk/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-slt-to-icmp-sgt.ll
>
> Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp?rev=348181&r1=348180&r2=348181&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp (original)
> +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp Mon Dec  3 12:07:58 2018
> @@ -2955,12 +2955,16 @@ static Value *foldICmpWithLowBitMaskedVa
>       //  x & (-1 >> y) s>= x    ->    x s<= (-1 >> y)
>       if (X != I.getOperand(1)) // X must be on RHS of comparison!
>         return nullptr;         // Ignore the other case.
> +    if (!match(M, m_Constant())) // Can not do this fold with non-constant.
> +      return nullptr;

Is there some guarantee that the constant can't be -1?  I mean, 
obviously "and %x, -1" will eventually fold, but it's not clear it will 
fold before this code runs.

-Eli

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