[Bug target/118815] [15 Regression] Getting `Vector size conflicts between AVX10.1 and AVX512, using 512 as max vector size` warning with -Wsystem-headers

[haochen.jiang at intel dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118815

--- Comment #7 from Haochen Jiang <haochen.jiang at intel dot com> ---
(In reply to Haochen Jiang from comment #6)
> I am testing a patch with this fix:
> 
> "
> @@ -2751,10 +2752,11 @@ ix86_option_override_internal (bool main_args_p,
>      }
> 
>    /* Set EVEX512 if one of the following conditions meets:
> -     1. AVX512 is enabled while EVEX512 is not explicitly set/unset.
> +     1. AVX512 is explicitly enabled while EVEX512 is not explicitly
> set/unset.
>       2. AVX10.1-512 is enabled.  */
>    if (TARGET_AVX10_1_512_P (opts->x_ix86_isa_flags2)
>        || (TARGET_AVX512F_P (opts->x_ix86_isa_flags)
> +         && (opts->x_ix86_isa_flags_explicit & OPTION_MASK_ISA_AVX512F)
>           && !(opts->x_ix86_isa_flags2_explicit & OPTION_MASK_ISA2_EVEX512)))
>      opts->x_ix86_isa_flags2 |= OPTION_MASK_ISA2_EVEX512;

This part will cause problem when using -march=xxx, since march is an
implicit enable on ISAs.

Thus, I will not change here, only changing the condition check on warning.

However, there is side effect on the usage for '-march=xxx -mavx10.1-256',
where xxx is with AVX512. It will not report warning on vector size for now,
while it will previously.

But it is a rare usage. Could we take it?