[PATCH] [InstCombine] Fold endian-independent load sequence into a single load.

[Ahmed Bougacha]

On Thu, Oct 23, 2014 at 1:00 PM, Michael Spencer <bigcheesegs at gmail.com> wrote:
> On Thu, Oct 23, 2014 at 11:48 AM, Hal Finkel <hfinkel at anl.gov> wrote:
>> Chandler, Michael, before we go farther, could you please comment on whether this is an optimal place for this optimization? We do have a LoadCombine pass, maybe it should really go there?
>>
>> Ahmed, I think we also need to check for other uses of the loads... if they also have other uses, then we likely don't want to double-load the same data.
>>
>>  -Hal
>
> The load combiner was made to deal with this case, however it's not
> enabled right now because load slicing still needs to be implemented
> to deal with some of the performance regressions.

I didn't know about the load combiner; if you have any pointers, I can
try to look into it rather than continuing with this (less general)
patch then?

- Ahmed

> Right now it's in the proper place. The issue is that when we do
> enable load combining, this will be redundant.
>
> - Michael Spencer
>
>>
>> ----- Original Message -----
>>> From: "Ahmed Bougacha" <ahmed.bougacha at gmail.com>
>>> To: "ahmed bougacha" <ahmed.bougacha at gmail.com>, "david majnemer" <david.majnemer at gmail.com>, hfinkel at anl.gov
>>> Cc: "amara emerson" <amara.emerson at arm.com>, llvm-commits at cs.uiuc.edu
>>> Sent: Wednesday, October 22, 2014 1:58:47 PM
>>> Subject: Re: [PATCH] [InstCombine] Fold endian-independent load sequence into a single load.
>>>
>>> Keep the alignment from the first byte load instruction.
>>> Also, refactor the ByteLoads handling to make it endian-independent
>>> (from a memory standpoint).
>>>
>>> The previous patch relied on the later InstCombines to set the
>>> alignment, but we can also do it here. I didn't do that from the
>>> beginning because I couldn't think of a case where that's possible
>>> but the later InstCombine isn't. I still can't, but that's no reason
>>> not to do it here!
>>>
>>> http://reviews.llvm.org/D5898
>>>
>>> Files:
>>>   lib/Transforms/InstCombine/InstCombine.h
>>>   lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
>>>   test/Transforms/InstCombine/endian-independent-load-BE.ll
>>>   test/Transforms/InstCombine/endian-independent-load-LE-aliasing.ll
>>>   test/Transforms/InstCombine/endian-independent-load-LE.ll
>>>
>>
>> --
>> Hal Finkel
>> Assistant Computational Scientist
>> Leadership Computing Facility
>> Argonne National Laboratory