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Re: [rfc] subreg lowering pass / Overcoming double-set difficulties for CC re-use

On Mon, 16 Oct 2006, Rask Ingemann Lambertsen wrote:

> On Wed, Aug 30, 2006 at 08:28:32PM -0400, Hans-Peter Nilsson wrote:
> > On Sun, 2 Jul 2006, [utf-8] Björn Haase wrote:
> > > Ian Lance Taylor wrote on Montag, 19. Juni 2006 20:29 :
> > > > Obviously the MD file would have to be modified to add the double-SET
> > > > instructions.  Maybe most of the CC0 machine descriptions already have
> > > > those operations--I haven't checked.
> >
> > No, that's part of the pattern explosion you'll see when
> > adjusting the post-cc0 port to get the cc0 performance. :(
>    I don't quite understand why this would cause an explosion in the
> patterns. We're talking about three times the number of patterns for insns
> which modify the condition codes as a byproduct, aren't we?

For a typical cc0 machine, almost all patterns do...

> > Further details TBD.  (I think I've mentioned this idea loosely
> > before, probably on IRC.)  FWIW, I'm definitely not going to
> > code up this until I've changed over the CRIS port (done, but
> > with horrible pattern explosion and code size and performance
> > regressions; the latter solvable bug fixes to general gcc
> > parts).  It seems it's surely needed.
>   I'm curious. I'd like to see a few examples.

If what I gave wasn't illuminating enough, that'll have to
wait, sorry.

brgds, H-P

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