[Mesa-dev] [PATCH] i965/fs: Combine tex/fb_write operations (opt)

[Ben Widawsky]

On Mon, Feb 23, 2015 at 10:02:26AM -0800, Matt Turner wrote:
> On Sun, Feb 22, 2015 at 3:06 PM, Ben Widawsky <ben at bwidawsk.net> wrote:
> > On Sun, Feb 08, 2015 at 02:48:02PM -0800, Matt Turner wrote:
> >> On Sun, Feb 8, 2015 at 1:59 PM, Ben Widawsky
> >> <benjamin.widawsky at intel.com> wrote:
> >> > +   /* The LOAD_PAYLOAD helper seems like the obvious choice here. However, it
> >> > +    * requires a lot of information about the sources to appropriately figure
> >> > +    * out the number of registers needed to be used. Given this stage in our
> >> > +    * optimization, we may not have the appropriate GRFs required by
> >> > +    * LOAD_PAYLOAD at this point (copy propogation). Therefore, we need to
> >>
> >> typo: propagation
> >>
> >> I'm not sure what "w e may not have the appropriate GRFs ..." means?
> >
> > Here is the relevant part of the original IRC conversation:
> > jekstrand       [08:52:30] No, the problem is uniforms and immediates.
> > jekstrand       [08:52:58] They can't go in a LOAD_PAYLOAD directly because we don't know how many destination registers they take up.
> > jekstrand       [08:54:16] for LOAD_PAYLOAD to work, we need more information than a regular instruction.  We need to know how many destination registers a given source takes up, we need to know whether it needs to use the second-half quarter control for the MOV that gets generated, etc.
> > jekstrand       [08:54:34] Using GRF sources more-or-less gives us this.  Immediates don't.
> > bwidawks        [08:54:55] right - this is what confuses me though... the immediates seem to already be there.
> > jekstrand       [08:55:38] Right.  The immediates can get there through copy-propagation and that's fine.  However, they're not there when it's created.
> > jekstrand       [08:55:43] It's all a mess
> >
> > Do you have a preferred way to state this concisely?
> 
> Heh, I'm not sure I understand LOAD_PAYLOAD anymore. The comment's
> probably fine as-is.
> 
> >> > @@ -3609,6 +3709,7 @@ fs_visitor::optimize()
> >> >        OPT(opt_peephole_predicated_break);
> >> >        OPT(opt_cmod_propagation);
> >> >        OPT(dead_code_eliminate);
> >> > +      OPT(opt_sampler_eot);
> >>
> >> Do you think we really need to do this in the optimization loop?
> >>
> >> I don't expect this to allow other optimization passes to make
> >> additional progress, and we can obviously do it successfully only
> >> once. I suspect we can do it after the optimization loop.
> >>
> >
> > It's possible I didn't quite spot where you want me to put the optimization. I
> > think that the way the code works right now, that will not work. The
> > optimization is depending on DCE to kill off the only LOAD_PAYLOAD and it's
> > corresponding MOVs. I agree that it is an optimization that can only occur once,
> > and generally it doesn't belong in the progress loop though.
> 
> Ah, sorry. I'd probably do it between the end of the optimization loop
> and the call to lower_load_payload().

Right. I looked at this too and at least from the 5 second glance at the code,
it seems this path can skip DCE. If you are certain that this cannot happen, I
will gladly make the change.

I defer to you regarding whether or not the optimizations can do more after this
happens (ie. we definitely want to take it out of the loop).