Enum processor_features adding on i386 backend

Enum processor_features adding on i386 backend

[Jiang, Haochen]

[-- Attachment #1: Type: text/plain, Size: 785 bytes --]

Hi all,

As you all know that we are continuously working on new ISA implementation for i386 backend.

There is one thing that I am really curious about when I read the code.

In gcc/config/i386/i386-cpuinfo.h, we have such comment:

/* ISA Features supported. New features have to be inserted at the end.  */

Why new features have to be inserted at the end? I did a quick investigation and found that it was
added at this mailing thread originally at libgcc/config/i386/cpuinfo.c:

https://gcc.gnu.org/pipermail/gcc-patches/2015-September/428915.html

In the thread, it seems that it might cause unwanted ABI change. Could anyone kindly tell me something
more about that? Should this rule still be kept for now after about eight years since then?

Thx,
Haochen

Re: Enum processor_features adding on i386 backend

[Andrew Pinski]

On Tue, Jul 18, 2023 at 7:50 PM Jiang, Haochen via Gcc <gcc@gcc.gnu.org> wrote:
>
> Hi all,
>
> As you all know that we are continuously working on new ISA implementation for i386 backend.
>
> There is one thing that I am really curious about when I read the code.
>
> In gcc/config/i386/i386-cpuinfo.h, we have such comment:
>
> /* ISA Features supported. New features have to be inserted at the end.  */
>
> Why new features have to be inserted at the end? I did a quick investigation and found that it was
> added at this mailing thread originally at libgcc/config/i386/cpuinfo.c:
>
> https://gcc.gnu.org/pipermail/gcc-patches/2015-September/428915.html
>
> In the thread, it seems that it might cause unwanted ABI change. Could anyone kindly tell me something
> more about that? Should this rule still be kept for now after about eight years since then?

YES we should keep it.
___builtin_cpu_supports uses those #s so the ABI is exposed to the
object level even if those #s are not exposed to the user directly
those are still exposed as an ABI that needs to be expandable that
way.

Thanks,
Andrew


>
> Thx,
> Haochen
>