Re: PROPOSAL: Extend inline asm syntax with size spec

Borislav Petkov
2018-10-07T14:14:07Z

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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