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

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Message-ID: <20181008055838.GA66819@gmail.com>
Date:   Mon, 8 Oct 2018 07:58:38 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Borislav Petkov <bp@...en8.de>, gcc@....gnu.org,
        Richard Biener <rguenther@...e.de>,
        Michael Matz <matz@...e.de>, 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


* Segher Boessenkool <segher@...nel.crashing.org> wrote:

> > > More precise *size* estimates, yes.  And if the user lies he should not
> > > be surprised to get assembler errors, etc.
> > 
> > Yes.
> > 
> > Another option would be if gcc parses the inline asm directly and
> > does a more precise size estimation. Which is a lot more involved and
> > complicated solution so I guess we wanna look at the simpler ones first.
> > 
> > :-)
> 
> Which is *impossible* to do.  Inline assembler is free-form text.

"Impossible" is false: only under GCC's model and semantics of inline
asm that is, and only under the (false) assumption that the semantics
of the asm statement (which is a GCC extension to begin with) cannot
be changed like it has been changed multiple times in the past.

"Difficult", "not worth our while", perhaps.

Thanks,

	Ingo