[PATCH] D90125: [CSSPGO] Infrastructure for context-sensitive Sample PGO and Inlining

[Wenlei He via Phabricator via llvm-commits]

wenlei added a comment.

In D90125#2365948 <https://reviews.llvm.org/D90125#2365948>, @wmi wrote:

> Another question. Have you ever evaluated the performance by comparing this patch "CSSPGO profile working with existing AFDO pipeline" with the default SPGO? I understand CSSPGO's benefit has not been used without the change of CGSCC inliner. The intention of the comparison is to understand how well existing AFDO pipeline work with CSSPGO profile. It may expose problem in existing SPGO profile or CSSPGO.

Yeah, I've tried that initially on SPEC. It showed some perf win, however the problem with inliner (actually the sample loader inliner, not the CGSCC inliner) is very visible on a few cases. It's mostly because today's sample loader inliner is a replay inliner, so it won't be more aggressive than CGSCC inline from previous build, hence simple hotness heuristic works. But with CSSPGO profile, it's unbounded on hot path (as long as it's hot, there's no check on inlinee's size or inline cost) and can lead to size bloat and perf regression in some cases.

I have an upcoming change to make sample loader inliner a priority based inliner with size and cost checks. And it should work for today's AFDO as well - if we treat all call sites to inline with equal priority, and set inline limit to infinite, it should be a no-op change for AFDO, but perhaps can be tuned it to benefit AFDO later too.


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