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

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Date:   Wed, 4 Sep 2019 17:18:52 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Nadav Amit <namit@...are.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: Re: [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition

On Fri, Aug 30, 2019 at 4:15 PM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
>
> This adds an asm_inline macro which expands to "asm inline" [1] when gcc
> is new enough (>= 9.1), and just asm for older gccs and other
> compilers.
>
> Using asm inline("foo") instead of asm("foo") overrules gcc's
> heuristic estimate of the size of the code represented by the asm()
> statement, and makes gcc use the minimum possible size instead. That
> can in turn affect gcc's inlining decisions.
>
> I wasn't sure whether to make this a function-like macro or not - this
> way, it can be combined with volatile as
>
>   asm_inline volatile()
>
> but perhaps we'd prefer to spell that
>
>   asm_inline_volatile()
>
> anyway.
>
> [1] Technically, asm __inline, since both inline and __inline__
> are macros that attach various attributes, making gcc barf if one
> literally does "asm inline()". However, the third spelling __inline is
> available for referring to the bare keyword.
>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
>  include/linux/compiler-gcc.h   | 4 ++++
>  include/linux/compiler_types.h | 4 ++++
>  2 files changed, 8 insertions(+)
>
> diff --git a/include/linux/compiler-gcc.h b/include/linux/compiler-gcc.h
> index d7ee4c6bad48..544b87b41b58 100644
> --- a/include/linux/compiler-gcc.h
> +++ b/include/linux/compiler-gcc.h
> @@ -172,3 +172,7 @@
>  #endif
>
>  #define __no_fgcse __attribute__((optimize("-fno-gcse")))
> +
> +#if GCC_VERSION >= 90100

Is it too late to ask for a feature test macro? Maybe one already
exists?  I was not able to find documentation or a bug on `asm
inline`.  I'm quite curious how you even found or heard of this
feature.  To the source we must go...

> +#define asm_inline asm __inline
> +#endif
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index ee49be6d6088..ba8d81b716c7 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -198,6 +198,10 @@ struct ftrace_likely_data {
>  #define asm_volatile_goto(x...) asm goto(x)
>  #endif
>
> +#ifndef asm_inline
> +#define asm_inline asm
> +#endif
> +
>  #ifndef __no_fgcse
>  # define __no_fgcse
>  #endif
> --
> 2.20.1
>


-- 
Thanks,
~Nick Desaulniers