[llvm-dev] A code layout related side-effect introduced by rL318299

[Chandler Carruth via llvm-dev]

Looks like this is the loop-oriented layout really hurting us. We don't
even consider that while.body -> return is less cold than while.cond ->
while.end because we don't really do much at all in layout out loop
*exits*. =[

On Mon, Dec 18, 2017 at 5:01 PM Chandler Carruth <chandlerc at google.com>
wrote:

> What I can't figure out is why we don't get the same basic block layout
> for b.ll....
>
> Specifically, I understand that we essentially have an iteration peeled
> out of the loop by loop rotation. But the result still should get the fancy
> layout that causes the hot conditional branch (away from the unreachable)
> to fall through into the loop header...
>
> On Mon, Dec 18, 2017 at 4:21 PM Wei Mi <wmi at google.com> wrote:
>
>> On Mon, Dec 18, 2017 at 4:14 PM, Wei Mi <wmi at google.com> wrote:
>>
>>> Hi,
>>>
>>> Recently 10% performance regression on an important benchmark showed up
>>> after we integrated https://reviews.llvm.org/rL318299. The analysis
>>> showed that rL318299 triggered loop rotation on an multi exits loop, and
>>> the loop rotation introduced code layout issue. The performance regression
>>> is a side-effect of rL318299. I got two testcases a.ll and b.ll attached to
>>> illustrate the problem. a.ll was generated by rL318298 and b.ll was
>>> generated by rL318299.
>>>
>>
>> a.ll and b.ll are generated from the same function by different versions
>> of compiler. a.s and b.s generated from a.ll and b.ll respectively showed
>> where the performance difference comes from.
>>
>
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