RFR: 8312425: [vectorapi] AArch64: Optimize vector math operations with SLEEF [v4]

[Patrick Zhang]

On Wed, 20 Nov 2024 07:53:45 GMT, Patrick Zhang <qpzhang at openjdk.org> wrote:

> > > > Ah, it'll only be the redirect library that's compiled with -march=armv8-a+sve Forget that.
> > > 
> > > 
> > > But that raises an interesting question. What happens if you try to load a library compiled with `-march=armv8-a+sve` on a non-SVE system? Is the ELF flagged to require SVE so it will fail to load? I'm hoping this is the case -- if so, everything will work as it should in this PR, but I honestly don't know. (After spending like 10 years working with building, I still discover things I need to learn...).
> > 
> > 
> > I think we can handle it, when a jdk built with sve support runs on a non-sve machine, the sve related code will not be executed with the protection of UseSVE vm flag which is detected at runtime startup.
> 
> I think we should not have specified any `-march=Xxx` option here in `make/autoconf/flags-cflags.m4` under `AC_DEFUN([FLAGS_SETUP_CFLAGS_CPU_DEP]`. It generates conflicts when a user builds JDK with relevant flags in `--with-extra-cflags=-march=Xxx`. Moreover, above `-march=armv8-a+sve` can accidentally overwrite things specified by `-mcpu=Xxx` or indirectly impact `-mtune=Xxx` if a user specified them in configure cmdline, `-march` is of higher priority than `-mcpu` for a toolchain deciding what isa/cost-model would be used.
> 
> @Hamlin-Li @magicus @theRealAph

Removed my previous incorrect comments with regards to the configure sequence on flags. SVE_CFLAGS is only used for sleef library building, so the impact would be limited. While the conflicting concern of `-march=armv8-a+sve` and other `-march=` or `-mcpu` from user inputs is still there.

For example, what I captured is:

Creating support/modules_libs/jdk.incubator.vector/libsleef.so from 3 file(s)
cc1: warning: switch ‘-mcpu=ampere1a’ conflicts with ‘-march=armv8-a+sve’ switch

A degradation overriding would happen according to
https://community.arm.com/arm-community-blogs/b/tools-software-ides-blog/posts/compiler-flags-across-architectures-march-mtune-and-mcpu

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PR Comment: https://git.openjdk.org/jdk/pull/18294#issuecomment-2487899668