RFR: 8345265: Minor improvements for LTO across all compilers [v2]

[Matthias Baesken]

On Tue, 17 Dec 2024 14:54:03 GMT, Julian Waters <jwaters at openjdk.org> wrote:

>> This is a general cleanup and improvement of LTO, as well as a quick fix to remove a workaround in the Makefiles that disabled LTO for g1ParScanThreadState.cpp due to the old poisoning mechanism causing trouble. The -Wno-attribute-warning change here can be removed once Kim's new poisoning solution is integrated.
>> 
>> - -fno-omit-frame-pointer is added to gcc to stop the linker from emitting code without the frame pointer
>> - -flto is set to $(JOBS) instead of auto to better match what the user requested
>> - -Gy is passed to the Microsoft compiler. This does not fully fix LTO under Microsoft, but prevents warnings about -LTCG:INCREMENTAL at least
>
> Julian Waters has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains 12 additional commits since the last revision:
> 
>  - Merge branch 'openjdk:master' into patch-16
>  - -fno-omit-frame-pointer in JvmFeatures.gmk
>  - Revert compilerWarnings_gcc.hpp
>  - General LTO fixes JvmFeatures.gmk
>  - Revert DISABLE_POISONING_STOPGAP compilerWarnings_gcc.hpp
>  - Merge branch 'openjdk:master' into patch-16
>  - Revert os.cpp
>  - Fix memory leak in jvmciEnv.cpp
>  - Stopgap fix in os.cpp
>  - Declaration fix in compilerWarnings_gcc.hpp
>  - ... and 2 more: https://git.openjdk.org/jdk/compare/06ed9958...9d05cb8e

I did some builds today with gcc14.2.0  on Linux x86_64 .
They looked good with and without LTO .
Without lto / normal opt build 
du -sh images/jdk/lib/server/libjvm.so
27M	images/jdk/lib/server/libjvm.so

WITH lto
du -sh images/jdk/lib/server/libjvm.so
24M	images/jdk/lib/server/libjvm.so

So even the code size reduction is visible.
So gcc14 seems to work for me (but so far I only used it on Linux x86_64, can't tell about aarch64/ppc64le).

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PR Comment: https://git.openjdk.org/jdk/pull/22464#issuecomment-2757921798