[PATCH] D99146: [CSSPGO][llvm-profgen] Context-sensitive global pre-inliner

[Wenlei He via Phabricator via llvm-commits]

wenlei added a comment.
Herald added a subscriber: ormris.

In D99146#2649599 <https://reviews.llvm.org/D99146#2649599>, @wenlei wrote:

> In D99146#2649557 <https://reviews.llvm.org/D99146#2649557>, @davidxl wrote:
>
>> ThinLTO is known to have issues related to profile update (cross module), so we were thinking something similar in ThinLink phase.
>
> This is the exact problem we are trying to mitigate. We also considered doing this in ThinLink but adjusting profiles for thin-backends and communicating inline decisions to thin-backends would add quite a bit of complexity, which could also slow down ThinLink. With CSSPGO, doing it in profile generation and use adjusted profile to convey inline estimation/suggestion is much simpler and cheaper.
>
>> One of the issues is that the pre-inlining needs to make similar decisions as the compiler. How well is the preinliner doing in this regard?
>
> Yes, this is a challenge. We don't have data yet, but I hope with some tuning we can get them to be close. One problem with doing pre-inlining is we don't have a lot of information that compiler can see from IR, though if needed some of that can be embedded into binary (some metadata in probe descriptor, etc.) for preinliner. I hope a more accurate view on machine code byte size for inline cost can offset some of the disadvantages due to lack of IR.
>
> We'll be working on tuning the preinliner to get it to be close to compiler inliner. This is similar to the effort of transferring more inlining from cgscc inliner to sample loader inliner in that we may not see immediate results, but over time, as the new component matures, we hope to reap benefits later.

@davidxl @wmi just to report back now that we've done more work along this path, and got some results. We observed global preinliner boosting performance for a large internal workload by ~1% (baseline is CSSPGO without global preinliner). Hopefully with some work we can rely more on preinliner for AutoFDO too.


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