[PATCH] D30416: [InstCombine] Redo reduceLoadOpStoreWidth in instcombine for bitfield store optimization.

[Michael Kuperstein via llvm-commits]

On Mon, Mar 6, 2017 at 4:36 PM, Friedman, Eli <efriedma at codeaurora.org>
wrote:

> On 3/6/2017 11:06 AM, Wei Mi wrote:
>
>> After looking at the benchmark motivating the test, I find it is much
>> harder for codegenprepare to catch all the shrinking opportunities compared
>> with instcombine, because of two things:
>>
>> * loadpre may merge the original load in a shrinking pattern with other
>> load in a very early position, and there maybe other stores in between. The
>> safety check for the codegenprepare now simply scan mayWriteToMemory insns
>> between load and store in the candidate shrinking pattern, and it will fail
>> because of those stores in between.
>>
>
> It would be easy to make the check a lot more powerful using MemorySSA; I
> guess you don't have access to that in the passes where you're doing the
> transform?
>
>
We can always make it more powerful later. Perhaps it'd be better to land a
conservative version first?


> * we may see the following pattern. a.f1 is a bitfield. Ideally, we should
>> do the shrinking for all the three stores. But codegenprepare works in
>> top-down order. After the first store is shrinked, it is hard for the
>> second and the third to know %bf.set is the same as the value loaded from
>> %0. If we do this in instcombine, it is much easier because before
>> load/store elimination, %bf.set will be loaded from %0 respecitively in
>> if.then9 and in if.end41.
>>
>
> I'm not following how it helps to do this in instcombine?  Normally,
> EarlyCSE runs first.  (A complete C testcase might be useful here.)
>
> -Eli
>
> --
> Employee of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
> Foundation Collaborative Project
>
>
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