[Mesa-dev] [i915g] i915_fpc_optimize_useless_mov is invalid in the general case

[Michael Karcher]

Am Montag, den 05.12.2011, 13:03 -0800 schrieb Stéphane Marchesin:
> > This translation is correct. The mentioned optimization now kicks in
> > corrrectly removing line 9, but it also reokaces line 6/7 by
> >  6: R[1].xyz = MUL R[0], T_DIFFUSE
> > This does yield the correct result in R[1].xyz, but it does (of course)
> > not update R[0].xyz, which is a problem, because R[0].xyzz is used in
> > line 16. In this special case, we could get away with renaming R[0].xyzz
> > into R[1].xyzz in line 16, but in the general case, there is no warranty
> > that R[1].xyz still contains in line 16 what it did in line 7.
> >
> > Any suggestions? Just remove this optimization? Improve the optimizer to
> > have it check that the eliminated temporary is not used in any further
> > lines?
> 
> Yeah we need to replace the temporary in all further lines. Patches welcome.
Thanks for your quick response!

How can you be sure that R[1].xyz keeps unchanged in all further lines,
so it is a valid replacement for the elided R[0].xyz? The fragment
shader language is not static single assignment, so R[1] could already
have been reused for a different purpose.

Regards,
  Michael Karcher