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Re: PROPOSAL: Extend inline asm syntax with size spec
- From: Borislav Petkov <bp at alien8 dot de>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- Cc: gcc at gcc dot gnu dot org, Richard Biener <rguenther at suse dot de>, Michael Matz <matz at suse dot de>, Nadav Amit <namit at vmware dot com>, Ingo Molnar <mingo at redhat dot com>, linux-kernel at vger dot kernel dot org, x86 at kernel dot org, Masahiro Yamada <yamada dot masahiro at socionext dot com>, Sam Ravnborg <sam at ravnborg dot org>, Alok Kataria <akataria at vmware dot com>, Christopher Li <sparse at chrisli dot org>, Greg Kroah-Hartman <gregkh at linuxfoundation dot org>, "H. Peter Anvin" <hpa at zytor dot com>, Jan Beulich <JBeulich at suse dot com>, Josh Poimboeuf <jpoimboe at redhat dot com>, Juergen Gross <jgross at suse dot com>, Kate Stewart <kstewart at linuxfoundation dot org>, Kees Cook <keescook at chromium dot org>, linux-sparse at vger dot kernel dot org, Peter Zijlstra <peterz at infradead dot org>, Philippe Ombredanne <pombredanne at nexb dot com>, Thomas Gleixner <tglx at linutronix dot de>, virtualization at lists dot linux-foundation dot org, Linus Torvalds <torvalds at linux-foundation dot org>, Chris Zankel <chris at zankel dot net>, Max Filippov <jcmvbkbc at gmail dot com>, linux-xtensa at linux-xtensa dot org
- Date: Sun, 7 Oct 2018 16:13:49 +0200
- Subject: Re: PROPOSAL: Extend inline asm syntax with size spec
- References: <20181003213100.189959-1-namit@vmware.com> <20181007091805.GA30687@zn.tnic> <20181007132228.GJ29268@gate.crashing.org>
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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