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

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Message-ID: <20181007141349.GD30687@zn.tnic>
Date:   Sun, 7 Oct 2018 16:13:49 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     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

On Sun, Oct 07, 2018 at 08:22:28AM -0500, Segher Boessenkool wrote:
> GCC already estimates the *size* of inline asm, and this is required
> *for correctness*.

I didn't say it didn't - but the heuristic could use improving.

> So I guess the real issue is that the inline asm size estimate for x86
> isn't very good (since it has to be pessimistic, and x86 insns can be
> huge)?

Well, the size thing could be just a "parameter" or "hint" of sorts, to
tell gcc to inline the function X which is inlining the asm statement
into the function Y which is calling function X. If you look at the
patchset, it is moving everything to asm macros where gcc is apparently
able to do better inlining.

> >  3) asm ("...") __attribute__((asm_size(<size-expr>)));
> 
> Eww.

Why?

> 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.

:-)

> I don't like 2) either.  But 1) looks interesting, depends what its
> semantics would be?  "Don't count this insn's size for inlining decisions",
> maybe?

Or simply "this asm statement has a size of 1" to mean, inline it
everywhere. Which has the same caveats as above.

> Another option is to just force inlining for those few functions where
> GCC currently makes an inlining decision you don't like.  Or are there
> more than a few?

I'm afraid they're more than a few and this should work automatically,
if possible.

Thx.

-- 
Regards/Gruss,
    Boris.

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