RFR: 8366777: Build fails unknown pseudo-op with old AS on linux-aarch64 [v9]

[Magnus Ihse Bursie]

On Wed, 10 Sep 2025 09:38:59 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:
> 
>   Include the prefix argument when calling FLAGS_COMPILER_CHECK_ARGUMENTS

I agree, that seems much better. That way it is also trivial to fix the `auto` case, which I believe is broken now -- the point is that with auto, we should turn on the flag if it is possible, otherwise we should turn off it. Not having the functionality is not an error nor should it print a warning -- just information that the functionality is not present, so it is turned off.

This test should be done *before* UTIL_ARG_ENABLE, and the result of whether the functionality is present or not should be passed as true/false in the AVAILABLE argument to UTIL_ARG_ENABLE. 

I would recommend you to have a look at how ENABLE_FULL_DOCS is setup in jdk-options.m4. I think this test should be handled in a similar way.

If the test is simple (I don't think it is in this case), it can also be inlined as a "lambda" with CHECK_AVAILABLE. (For that, see for instance how UBSAN_ENABLED is setup).

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PR Comment: https://git.openjdk.org/jdk/pull/27073#issuecomment-3275233238