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.
Good mailing practices for 400: avoid top-posting and trim the reply.
