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

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Date:   Thu, 5 Sep 2019 16:23:11 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        "gcc-patches@....gnu.org" <gcc-patches@....gnu.org>
Subject: Re: [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition

On 05/09/2019 15.45, Segher Boessenkool wrote:
> Hi Rasmus,
> 
> On Thu, Sep 05, 2019 at 01:07:11PM +0200, Rasmus Villemoes wrote:
>> On 05/09/2019 02.18, Nick Desaulniers wrote:
>>> Is it too late to ask for a feature test macro? Maybe one already
>>> exists? 
>>
>> No, not as far as I know.
> 
> [ 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.  ]
> 
>> Perhaps something like below, though that
>> won't affect the already released gcc 9.1 and 9.2, of course.
> 
> That is one reason to not want such a predefined macro.  Another reason
> is that you usually need to compile some test programs *anyway*, to see if
> some bug is present for example, or to see if the exact implementation of
> the feature is beneficial (or harmful) to your program in some way.

OK, I think I'll just use a version check for now, and then switch to a
Kconfig test if and when clang grows support.

>> gcc maintainers, WDYT? Can we add a feature test macro for asm inline()?
> 
> 
> Why would GCC want to have macros for all features it has? 

Well, gcc has implemented __has_attribute() which is similar - one could
detect support by compiling a trivial test program. Or the same could be
said for many of the predefined macros that are conditionally defined,
e.g. __HAVE_SPECULATION_SAFE_VALUE.

But I was just throwing the question into the air, I won't pursue this
further.

Thanks,
Rasmus