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

[Eli Friedman via llvm-commits]

On Sun, Jun 19, 2016 at 2:58 PM, David Majnemer <david.majnemer at gmail.com>
wrote:

>
>
> On Sun, Jun 19, 2016 at 12:08 AM, Eli Friedman <eli.friedman at gmail.com>
> wrote:
>
>> 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.
>>
>
> Should be done with r273126, however accumulateConstantOffset's internal
> logic also needs to check for signed overflow.
>
>

Okay, thanks.

-Eli
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