[Mesa-dev] [PATCH 01/14] i965: Don't copy propagate sat MOVs into LOAD_PAYLOAD

[Kenneth Graunke]

On Tuesday, October 28, 2014 03:41:45 PM Matt Turner wrote:
> On Tue, Oct 28, 2014 at 3:17 PM, Kristian Høgsberg <krh at bitplanet.net> 
wrote:
> > The LOAD_PAYLOAD opcode can't saturate its sources, so skip
> > saturating MOVs.  The register coalescing after lower_load_payload()
> > will clean up the extra MOVs.
> >
> > Signed-off-by: Kristian Høgsberg <krh at bitplanet.net>
> > ---
> >  src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp 
b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
> > index e1989cb..87ea9c2 100644
> > --- a/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
> > +++ b/src/mesa/drivers/dri/i965/brw_fs_copy_propagation.cpp
> > @@ -454,8 +454,12 @@ fs_visitor::try_constant_propagate(fs_inst *inst, 
acp_entry *entry)
> >        val.effective_width = inst->src[i].effective_width;
> >
> >        switch (inst->opcode) {
> > -      case BRW_OPCODE_MOV:
> >        case SHADER_OPCODE_LOAD_PAYLOAD:
> > +         /* LOAD_PAYLOAD can't sat its sources. */
> > +         if (entry->saturate)
> > +            break;
> > +         /* Otherwise, fall through */
> > +      case BRW_OPCODE_MOV:
> 
> I'm confused. This is the *constant* propagation function not copy
> propagation, and if we're doing a mov.sat of an immediate, we can just
> figure out what that is.
> 
> If you're seeing mov.sat with immediates, you should modify
> opt_algebraic to remove them.

Making opt_algebraic handle mov.sat with immediates seems like a good plan.  
But, shouldn't we then make this code bail on saturates for both MOV and 
LOAD_PAYLOAD?  We should be robust in case one shows up between opt_algebraic 
and this pass.

--Ken
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