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Re: gcc feature request / RFC: extra clobbered regs

On Wed, Jul 01, 2015 at 11:23:17AM -0400, Vladimir Makarov wrote:
> >>(I'm not necessarily suggesting that we do this for the syscall bodies
> >>themselves.  I want to do it for the entry and exit helpers, so we'd
> >>still lose the five cycles in the full fast-path case, but we'd do
> >>better in the slower paths, and the slower paths are becoming
> >>increasingly important in real workloads.)
> >GCC already supports -ffixed-REG, -fcall-used-REG and -fcall-saved-REG
> >options, which allow to tweak the calling conventions; but it is per
> >translation unit right now.  It isn't clear which of these options
> >you mean with the extra_clobber.
> >I assume you are looking for a possibility to change this to be
> >per-function, with caller with a different calling convention having to
> >adjust for different ABI callee.  To some extent, recent GCC versions
> >do that automatically with -fipa-ra already - if some call used registers
> >are not clobbered by some call and the caller can analyze that callee,
> >it can stick values in such registers across the call.
> >I'd say the most natural API for this would be to allow
> >f{fixed,call-{used,saved}}-REG in target attribute.
> >
> >
> One consequence of frequent changing calling convention per function or
> register usage could be GCC slowdown.  RA calculates too many data and it
> requires a lot of time to recalculate them after something in the register
> usage convention is changed.

That is true.  i?86/x86_64 is a switchable target, so at least for the case
of info computed for the callee with non-standard calling convention such
info can be computed just once when the function with such a target
attribute would be seen first.  But for the caller side, I agree not
everything can be precomputed, if we can't use e.g. regsets saved in the
callee; as a single function can call different functions with different
ABIs.  But to some extent we have that already with -fipa-ra, don't we?


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