linux-kernel - Re: [PATCH v2 4/6] compiler-gcc.h: add asm

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Date:   Thu, 5 Sep 2019 17:52:44 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "gcc-patches@....gnu.org" <gcc-patches@....gnu.org>
Subject: Re: [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition

On Thu, Sep 5, 2019 at 3:45 PM Segher Boessenkool
<segher@...nel.crashing.org> wrote:
>
> [ That's not what a feature test macro is; a feature test macro allows the
>   user to select some optional behaviour.  Things like _GNU_SOURCE.  ]

Yes and no. GNU libc defines feature test macros like you say, but
C++'s feature macros are like Rasmus/Nick are saying. I think libc's
definition is weird, I would call those "feature selection macros"
instead, because the user is selecting between some features (whether
to enable or not, for instance), rather than testing for the features.

> Why would GCC want to have macros for all features it has?  That would be
> quite a few new ones every release.

Maybe GCC wouldn't, but its users, they surely would. For anything
that 1) is a new language feature, 2) breaks backwards-compatibility
with previous (or other compilers) and 3) is expected to be used by
end users, yes, it would be very useful to have.

For the same reasons C++ is adding feature test macros all over the
place nowadays and it is considered good practice (see SD-6: SG10
Feature Test Recommendations).

Cheers,
Miguel