[PATCH] combine: Convert subreg-of-lshiftrt to zero_extract properly (PR78390)

[Michael Matz]

Hi,

On Wed, 30 Nov 2016, Segher Boessenkool wrote:

> > I don't think mode-changing _extracts are valid in this context.  From the 
> > docu:
> > 
> >   `(sign_extract:M LOC SIZE POS)'
> >   ...
> >      The mode M is the same as the mode that would be used for LOC if
> >      it were a register.
> > 
> > Probably it could be made to work just fine, but I'm not sure it'd be 
> > worth much, as then the targets would need to care for mode-changes 
> > occuring not just through subregs as usual, but also through extracts.
> 
> The patch https://gcc.gnu.org/ml/gcc-patches/2016-11/msg02987.html I
> submitted yesterday deals with this same issue, FWIW -- some ports
> apparently already do mode-changing extracts.

Yeah, saw that a bit later.  So, hmmm.  I'm not sure what to make of it, 
if the targets choose to use mode-changing extracts I guess that's fine, 
as they presumably will have written patterns that recognize them.  But I 
don't think we should willy-nilly generate such patterns as we can't know 
if the target deals with them or not.  We could of course always generate 
both variants: (subreg:M1 (extract:M2 (object:M2)) and
(extract:M1 (object:M2)) and see if either matches, but that seems a bit 
too much work.


Ciao,
Michael.