8387400: Force-inline Devirtualizer methods

[shipilev]

The purpose for Devirtualizer is avoid non-inlinable virtual calls to hot GC closures. It works fairly well, but at least in Shenandoah hot loops, I am seeing that the Devirtualizer methods themselves are not inlined. This poses a problem if GC-closure-calling code forces its own inlining, the hot GC closure targets force their own inlining, but Devirtualizer that sits in the middle is not. In this case, compilers seem to use this "opportunity" to cut the aggressive inlining off in the middle. It IMO makes sense to force-inline Devirtualizer methods themselves as well.

This improves Shenandoah marking times for about +2..10% in hot benchmarks. I measured only Shenandoah, because this was a part of larger project and I had marking loop tests ready. But I suppose it will benefit things generically. I can see no ill effects on build times, the difference is a wash in the noise.

The downside is a bump in binary size. With current version, fastdebug libjvm.so size is +0.6% larger, release libjvm.so is +0.4% larger. I presume this is a fair price to pay for a palpably faster GC code.

Additional testing:

• Ad-hoc marking time stress tests with Shenandoah
• Eyeballing build times and libjvm.so sizes
• GHA (checks cross-platform sanity)

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Issue

• JDK-8387400: Force-inline Devirtualizer methods (Enhancement - P4)

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[shipilev]

GHA failure is going to get resolved by #31707