[llvm] r273105 - [LoadCombine] Combine Loads formed from GEPS with negative indexes

[Eli Friedman via llvm-commits]

On Sat, Jun 18, 2016 at 11:14 PM, David Majnemer via llvm-commits <
llvm-commits at lists.llvm.org> wrote:

> Author: majnemer
> Date: Sun Jun 19 01:14:56 2016
> New Revision: 273105
>
> URL: http://llvm.org/viewvc/llvm-project?rev=273105&view=rev
> Log:
> [LoadCombine] Combine Loads formed from GEPS with negative indexes
>
> Change the underlying offset and comparisons to use int64_t instead of
> uint64_t.
>
> Patch by River Riddle!
>
> Differential Revision: http://reviews.llvm.org/D21499
>
> Added:
>     llvm/trunk/test/Transforms/LoadCombine/load-combine-negativegep.ll
> Modified:
>     llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp?rev=273105&r1=273104&r2=273105&view=diff
>
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/LoadCombine.cpp Sun Jun 19 01:14:56
> 2016
> @@ -40,7 +40,7 @@ STATISTIC(NumLoadsCombined, "Number of l
>  namespace {
>  struct PointerOffsetPair {
>    Value *Pointer;
> -  uint64_t Offset;
> +  int64_t Offset;
>  };
>
>  struct LoadPOPPair {
> @@ -102,7 +102,7 @@ PointerOffsetPair LoadCombine::getPointe
>        unsigned BitWidth = DL.getPointerTypeSizeInBits(GEP->getType());
>        APInt Offset(BitWidth, 0);
>        if (GEP->accumulateConstantOffset(DL, Offset))
> -        POP.Offset += Offset.getZExtValue();
> +        POP.Offset += Offset.getSExtValue();
>

This line seems a bit suspcious... one, it assumes pointers are never more
than 64 bits wide, and two, the addition could have signed overflow.

-Eli
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