Messages in this thread | | | Date | Sun, 7 Oct 2018 16:13:49 +0200 | From | Borislav Petkov <> | Subject | Re: PROPOSAL: Extend inline asm syntax with size spec |
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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.
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