RFR: 8344607: Link Time Optimization - basic support for clang [v4]

[Joachim Kern]

On Tue, 3 Dec 2024 16:46:04 GMT, Matthias Baesken <mbaesken at openjdk.org> wrote:

>> Support the clang toolchain when link time optimization is configured.
>> Please note that this is NOT intended to enable lto by default and NOT to fix all possible clang-supporting builds or test issues .
>> 
>> It works on my Linux x86_64 SUSE 15 test machine with  clang15.0.7, also on macOS with Xcode 13.1 devkit  and Xcode 15.4 .
>> 
>> To be able to build on macOS with Xcode, I had to deal with one issue.  The lto-link process runs into this error 
>> 
>> 
>> ld: <inline asm>:11:1: symbol 'SpinPause_return' is already defined
>> SpinPause_return:        
>> ^
>> for architecture arm64
>> clang: error: linker command failed with exit code 1 (use -v to see invocation)
>> 
>> 
>> Seems that when building with LTO,  the  inline assembler code in SpinPause() is not handled well. In the lto inlining the label  SpinPause_return is generated multiple times and causes an 'already defined' error. This goes away when adding NOINLINE to this function.
>> Linux/clang did not show this issue, probably the toolchain there does not have the error.
>
> Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:
> 
>   use fat lto objects on AIX/clang

Marked as reviewed by jkern (Committer).

make/hotspot/lib/JvmFeatures.gmk line 182:

> 180:       JVM_CFLAGS_FEATURES += -ffat-lto-objects
> 181:     endif
> 182:     JVM_LDFLAGS_FEATURES += $(CXX_O_FLAG_HIGHEST_JVM) -flto -fno-strict-aliasing

LGTM. ffat-lto-objects is needed on AIX, because gtest/gtestLauncher.cpp is compiled with -flto for libjvm.so, but also linked to libjvm/gtest/gtestLauncher without -flto in the link statement.

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PR Review: https://git.openjdk.org/jdk/pull/22412#pullrequestreview-2481095181
PR Review Comment: https://git.openjdk.org/jdk/pull/22412#discussion_r1871014580