[Mesa-dev] [PATCH 092/133] nir: Add a pass to lower local variables to registers

[Jason Ekstrand]

On Fri, Jan 9, 2015 at 4:38 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:

> >>>> +      case nir_intrinsic_copy_var:
> >>>> +         unreachable("There should be no copies whatsoever at this
> >>>> point");
> >>>> +         break;
> >>>
> >>>
> >>> Are you sure about this? My impression is that lower_variables will
> lower
> >>> copies involving things that aren't indirectly referenced, but if you
> have
> >>> something like:
> >>>
> >>> foo[i] = ...
> >>> bar[*] = foo[*];
> >>> ... = bar[i];
> >>>
> >>> then the copy in the middle won't get lowered, unless there's something
> >>> else I'm missing that will lower it.
> >>
> >>
> >> Yeah, there may be something missing there.  I have a pass lying around
> >> somewhere that lowers all copies.  Unfortunately, I've never actually
> seen
> >> this happen in the wild so It's untested.  I'll try and cook something
> up
> >> that I think is reliable.
> >
> >
> > Ok, more info.  Right now, GLSL IR is lowering all truely indirect
> accesses
> > to if-ladders right now so we can never hit this.  Once we can handle
> > indirects in the backends or generate the if-ladders in NIR, we will need
> > this.  Until then, let's leave it as-is to reduce the ammount of untested
> > code.
> > --Jason
>
> Huh, that's weird... it lowers input/output indirect accesses to
> if-ladders, but not temporary indirect references, right? So wouldn't
> something like:
>

It appears to lower locals as well.


>
> uniform int i;
> uniform bool f; /* false */
> in vec4 in_array[20];
> out vec4 color;
>
> void main()
> {
>    vec4 foo[20], bar[20], temp[20];
>    foo = in_array;
>    foo[i] = 0.0f;
>    while (f) {
>       temp = foo;
>       temp[i] = 1.0f;
>       foo = bar;
>       bar = temp;
>    }
>    bar[i] = 2.0f;
>    color = bar[0];
> }
>
> where GLSL IR can't eliminate the copies and doesn't turn the indirect
> references into if-ladders, and we can't lower them to SSA values,
> trigger the bug? If GLSL did turn everything into if-ladders, then
> this entire pass as well as half of lower_variables would be dead
> code...
>

I think we have a lot of dead code...
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