linux-kernel - Re: PROPOSAL: Extend inline asm syntax with size spec

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Date:   Wed, 10 Oct 2018 16:31:41 +0000
From:   Nadav Amit <namit@...are.com>
To:     Segher Boessenkool <segher@...nel.crashing.org>,
        Richard Biener <rguenther@...e.de>
CC:     Michael Matz <matz@...e.de>, Borislav Petkov <bp@...en8.de>,
        "gcc@....gnu.org" <gcc@....gnu.org>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Sam Ravnborg <sam@...nborg.org>,
        Alok Kataria <akataria@...are.com>,
        Christopher Li <sparse@...isli.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H. Peter Anvin" <hpa@...or.com>, Jan Beulich <JBeulich@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Kees Cook <keescook@...omium.org>,
        "linux-sparse@...r.kernel.org" <linux-sparse@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        "linux-xtensa@...ux-xtensa.org" <linux-xtensa@...ux-xtensa.org>
Subject: Re: PROPOSAL: Extend inline asm syntax with size spec

at 7:53 AM, Segher Boessenkool <segher@...nel.crashing.org> wrote:

> On Mon, Oct 08, 2018 at 11:07:46AM +0200, Richard Biener wrote:
>> On Mon, 8 Oct 2018, Segher Boessenkool wrote:
>>> On Sun, Oct 07, 2018 at 03:53:26PM +0000, Michael Matz wrote:
>>>> On Sun, 7 Oct 2018, Segher Boessenkool wrote:
>>>>> On Sun, Oct 07, 2018 at 11:18:06AM +0200, Borislav Petkov wrote:
>>>>>> Now, Richard suggested doing something like:
>>>>>> 
>>>>>> 1) inline asm ("...")
>>>>> 
>>>>> What would the semantics of this be?
>>>> 
>>>> The size of the inline asm wouldn't be counted towards the inliner size 
>>>> limits (or be counted as "1").
>>> 
>>> That sounds like a good option.
>> 
>> Yes, I also like it for simplicity.  It also avoids the requirement
>> of translating the number (in bytes?) given by the user to
>> "number of GIMPLE instructions" as needed by the inliner.
> 
> This patch implements this, for C only so far.  And the syntax is
> "asm inline", which is more in line with other syntax.
> 
> How does this look?

It looks good to me in general. I have a couple of reservations, but I
suspect you will not want to address them:

1. It is not backward compatible, requiring a C macro to wrap it, as the 
kernel might be built with different compilers.

2. It is specific to asm. I do not have in mind another use case (excluding
the __builtin_constant_p), but it would be nicer IMHO to have a builtin
saying “ignore the cost of this statement” for the matter of optimizations.

Regards,
Nadav