RFR: 8319429: Don't zero out mxcsr flag bits on ECore

[David Holmes]

On Fri, 3 Nov 2023 22:32:44 GMT, Volodymyr Paprotski <duke at openjdk.org> wrote:

> Improves vector rounding on ECore about 10x
> 
> (BEFORE) FpRoundingBenchmark.test_round_float        2048  thrpt    3  40.912 ± 0.044  ops/ms
> (AFTER ) FpRoundingBenchmark.test_round_float        2048  thrpt    3  431.682 ± 0.727  ops/ms

Changes requested by dholmes (Reviewer).

src/hotspot/cpu/x86/vm_version_x86.cpp line 864:

> 862:     (_model == 0x97 || _model == 0xAC || _model == 0xAF)) {
> 863:     FLAG_SET_DEFAULT(DoEcoreOpt, true);
> 864:   }

And what should happen if the flag is set true by the user and there is no Ecore? What affect will that have? Should it be allowed?

src/hotspot/share/runtime/globals.hpp line 574:

> 572:   product(bool, DoEcoreOpt, false, DIAGNOSTIC,                              \
> 573:           "Perform Ecore Optimization")                                     \
> 574:                                                                             \

I think this should be a CPU specific flag in ./cpu/x86/globals_x86.hpp (similar to how we have linux specific flags in ./os/linux/globals_linux.hpp). Also the description should clarify that the default is actually true for Ecore systems, and false elsewhere.

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PR Review: https://git.openjdk.org/jdk/pull/16504#pullrequestreview-1714266950
PR Review Comment: https://git.openjdk.org/jdk/pull/16504#discussion_r1382784036
PR Review Comment: https://git.openjdk.org/jdk/pull/16504#discussion_r1382783715