RFR: 8272873: C2: Inlining should not depend on absolute call site counts

[Dean Long]

On Tue, 24 Aug 2021 15:44:41 GMT, Igor Veresov <iveresov at openjdk.org> wrote:

> C2 considers absolute call site counts in its inlining decisions, which seems very wrong considering the asynchronous nature of profiling and background compilation (See InlineTree::should_inline()). It causes substantial over-inlining, which in presence of a depth-first inlining can lead to an early cut off. It also is inherently unstable. C2 already uses call frequency as an additional factor and it's better to consider only that in the inlining heuristic. I did extensive benchmarking it yielded almost no losses and single-digit wins (up to 5%) on some benchmarks. I think it's safe to remove/deprecate InlineFrequencyCount and continue using InlineFrequencyRatio instead. I found that converting the frequency computation to FP and setting InlineFrequencyRatio=0.25 (inline a method that is called a least 25% of the time) works best.

It looks like we still have UseCounterDecay, which used to trigger decay in older compilation policies.  Is that flag obsolete now and replaced by something better?  It seems like that's part of the reason InlineFrequencyCount is now less useful, because the counts no longer decay.

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PR: https://git.openjdk.java.net/jdk/pull/5238