RFR: 8269598: Regressions up to 5% on aarch64 seems due to JDK-8268858

[Joshua Zhu]

On Tue, 13 Jul 2021 21:51:08 GMT, Dean Long <dlong at openjdk.org> wrote:

> I don't understand why aarcht64 needs these adjustments but x86 does not. Both are doing the same tricks with PreserveFramePointer, right?

Yes. If make the same adjustment on x86, the same wide LRG in
Compressor::compress() will also be split and be assigned to register
just like aarch64.

But this adjustment is a trade-off in my opinion. Lowering pressure
threshold would also bring unnecessary spillings in some scenarios,
result in performance degradation especially on x86 platform that has
a small quantity of GPRs. On aarch64 that has more GPRs, the benefits
of lowering pressure threshold by 1 or 2 outweigh the disadvantages.

I also performed the same SPECjvm2018 "compress" test on x86_64.
Compared to 5% performance improvement on aarch64, the same adjustment
degraded performance by 1.28% nevertheless.

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PR: https://git.openjdk.java.net/jdk/pull/4766