Re: [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition

From: Nadav Amit <namit@vmware.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)"
	<x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Subject: Re: [PATCH v2 4/6] compiler-gcc.h: add asm_inline definition
Date: Thu, 5 Sep 2019 05:43:05 +0000	[thread overview]
Message-ID: <C0D6E68C-AA52-4097-A626-5B8FBC67D445@vmware.com> (raw)
In-Reply-To: <CAKwvOdktYpMH8WnEQwNE2JJdKn4w0CHv3L=YHkqU2JzQ6Qwkew@mail.gmail.com>

> On Sep 4, 2019, at 5:18 PM, Nick Desaulniers <ndesaulniers@google.com> wrote:
> 
> On Fri, Aug 30, 2019 at 4:15 PM Rasmus Villemoes
> <linux@rasmusvillemoes.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@rasmusvillemoes.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...

When I had some free time I wrote a detailed blog post about this issue:
https://nadav.amit.zone/linux/2018/10/10/newline.html

Which later Borislav took to gcc people:
https://lore.kernel.org/lkml/20181007091805.GA30687@zn.tnic/