[llvm-dev] RFC: We should stop merging allocas in the inliner

[Chandler Carruth via llvm-dev]

FYI, I ran some initial measurements just for sanity on the test-suite on
x86-64:

Total stack frame size: 13362520 -> 13365864 (+0.02%)
Average stack frame size: 581.814 -> 581.96 (+0.02%)
Average compile time: 5.71178 -> 5.67256 (within noise)

Number of functions is exactly the same, so no trivially visible inlining
decision changes.

I've spot checked a few of the functions that had significant change in
stack size. One shows a pattern that isn't too surprising in retrospect:
this change allows us to hoist more code which causes live ranges to expand
and increases spills. Nothing really scary here, and in many ways validates
the core premise -- this does actually remove barriers to the mid-level
optimizer.

I've also found one case where stack coloring appears to just fail for no
reason: "NArchive::N7z::GetStringForSizeValue(unsigned int)" in
7zHandler.cpp has exactly the same sequence of instructions after the
change but 3x larger stack. But this appears to be rare and mostly relating
to functions with already small stack sizes. I've filed PR28821 to track
this.

Anyways, while clearly some stuff will need cleaning up, I don't see
anything really scary jumping out here.

-Chandler

On Tue, Aug 2, 2016 at 1:02 AM Chandler Carruth <chandlerc at gmail.com> wrote:

> Seeing no big concerns raised with this direction...
>
> On Sun, Jul 31, 2016 at 9:47 PM Chandler Carruth <chandlerc at gmail.com>
> wrote:
>
>> 1) Add a flag to turn this off.
>>
>
> https://reviews.llvm.org/D23052
>
> Will proceed from there.
>
> 2) Run benchmarks with flag to make sure things are actually working.
>> 3) Change default for the flag, make sure chaos doesn't ensue.
>> 4) Delete the (quite substantial) complexity associated with this and the
>> flag.
>> 5) Profit!
>>
>> -Chandler
>>
>
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