RFR: 8366777: Build fails unknown pseudo-op with old AS on linux-aarch64 [v6]

[Magnus Ihse Bursie]

On Fri, 5 Sep 2025 06:16:01 GMT, SendaoYan <syan at openjdk.org> wrote:

>> Hi all,
>> 
>> Build jdk on linux-aarch64 with older assembler version(as 2.30) will report compilation failre, shows as [JDK-8366777](https://bugs.openjdk.org/browse/JDK-8366777).
>> 
>> This PR add the expression `svfloat64_t a() {}` to makefile which will trigger the compilation during configure stage. After this PR, at configure stage will check the gcc and assember support `-march=armv8-a+sve` or not more comprehensive.
>> 
>> Testing:
>> - [x] On linux-aarch64 gcc12.3.1 as2.37, configure set SVE_CFLAGS as '-march=armv8-a+sve'
>> - [x] On linux-aarch64 gcc10.3.0 as2.30, configure set SVE_CFLAGS as empty
>> 
>> 
>> The compilation fails demo as below:
>> 
>> 
>> # cat sve.c 
>> #include <arm_sve.h>
>> svfloat64_t a() {}
>> # gcc -march=armv8-a+sve sve.c 
>> /tmp/ccVOzMzb.s: Assembler messages:
>> /tmp/ccVOzMzb.s:6: Error: unknown pseudo-op: `.variant_pcs'
>
> SendaoYan has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use UTIL_ARG_ENABLE instead of AC_ARG_ENABLE

make/autoconf/flags-cflags.m4 line 994:

> 992: ])
> 993: 
> 994: AC_DEFUN([BPERF_SETUP_SVE],

The `BPERF_` prefix says that this function belongs to build-performance.m4.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27073#discussion_r2332244788