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

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Date:   Wed, 10 Oct 2018 02:53:54 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Richard Biener <rguenther@...e.de>
Cc:     Michael Matz <matz@...e.de>, Borislav Petkov <bp@...en8.de>,
        gcc@....gnu.org, Nadav Amit <namit@...are.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        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,
        Peter Zijlstra <peterz@...radead.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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
Subject: Re: PROPOSAL: Extend inline asm syntax with size spec

On Wed, Oct 10, 2018 at 09:12:48AM +0200, Richard Biener wrote:
> On Tue, 9 Oct 2018, Segher Boessenkool 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?
> 
> Looks good.  A few nits - you need to document this in extend.texi, the

Yup.

> tree flag use needs documenting in tree-core.h,

Ah yes.

> and we need a testcase
> (I'd suggest one that shows we inline a function with "large" asm inline 
> () even at -Os).

I have one.  Oh, and I probably should do a comment at the one line of
code that isn't just bookkeeping ;-)

> Oh, and I don't think we want C and C++ to diverge - so you need to
> cook up C++ support as well.

Right, that's why I said "C only so far".

> Can kernel folks give this a second and third thought please so we
> don't implement sth that in the end won't satisfy you guys?

Or actually try it out and see if it has the desired effect!  Nothing
beats field trials.

I'll do the C++ thing today hopefully, and send things to gcc-patches@.


Segher