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

[Rask Ingemann Lambertsen]

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?

> 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.

-- 
Rask Ingemann Lambertsen