[Mesa-dev] [PATCH] nir: add nir_instr_type_tex support to nir_lower_phis_to_scalar()

[Jason Ekstrand]

On Fri, Feb 22, 2019 at 2:47 PM Timothy Arceri <tarceri at itsqueeze.com>
wrote:

>
>
> On 23/2/19 6:31 am, Rob Clark wrote:
> > On Fri, Feb 22, 2019 at 12:39 PM Jason Ekstrand <jason at jlekstrand.net>
> wrote:
> >>
> >> On Fri, Feb 22, 2019 at 9:51 AM Eric Anholt <eric at anholt.net> wrote:
> >>>
> >>> Timothy Arceri <tarceri at itsqueeze.com> writes:
> >>>
> >>>> shader-db results i965 (SKL):
> >>>>
> >>>> total instructions in shared programs: 13219105 -> 13024761 (-1.47%)
> >>>> instructions in affected programs: 1169457 -> 975113 (-16.62%)
> >>>> helped: 599
> >>>> HURT: 154
> >>>>
> >>>> total cycles in shared programs: 333968972 -> 324822073 (-2.74%)
> >>>> cycles in affected programs: 130032440 -> 120885541 (-7.03%)
> >>>> helped: 590
> >>>> HURT: 216
> >>>>
> >>>> total spills in shared programs: 57947 -> 29130 (-49.73%)
> >>>> spills in affected programs: 53364 -> 24547 (-54.00%)
> >>>> helped: 351
> >>>> HURT: 0
> >>>>
> >>>> total fills in shared programs: 51310 -> 25468 (-50.36%)
> >>>> fills in affected programs: 44882 -> 19040 (-57.58%)
> >>>> helped: 351
> >>>> HURT: 0
> >>>> ---
> >>>>   src/compiler/nir/nir_lower_phis_to_scalar.c | 1 +
> >>>>   1 file changed, 1 insertion(+)
> >>>>
> >>>> diff --git a/src/compiler/nir/nir_lower_phis_to_scalar.c
> b/src/compiler/nir/nir_lower_phis_to_scalar.c
> >>>> index 16001f73685..f6f702bca15 100644
> >>>> --- a/src/compiler/nir/nir_lower_phis_to_scalar.c
> >>>> +++ b/src/compiler/nir/nir_lower_phis_to_scalar.c
> >>>> @@ -74,6 +74,7 @@ is_phi_src_scalarizable(nir_phi_src *src,
> >>>>         /* A phi is scalarizable if we're going to lower it */
> >>>>         return should_lower_phi(nir_instr_as_phi(src_instr), state);
> >>>>
> >>>> +   case nir_instr_type_tex:
> >>>>      case nir_instr_type_load_const:
> >>>>      case nir_instr_type_ssa_undef:
> >>>>         /* These are trivially scalarizable */
> >>>
> >>> Sounds promising, but I would definitely not describe instr_type_tex as
> >>> "trivially scalarizable" -- could you explain what's going on with this
> >>> patch?
>
> Basically it just turns:
>
> if ssa0 {
>    ...
>    vec4 ssa1 = txf .....
> } else {
>     ...
>     vec4 ssa2 = ...
> }
> vec4 ss3 = phi ssa1, ssa2
>
> Into
>
> if ssa0 {
>    ...
>    vec4 ssa1 = txf .....
>    vec1 ssa2 = imov ssa1.x
>    vec1 ssa3 = imov ssa1.y
>    vec1 ssa4 = imov ssa1.z
>    vec1 ssa5 = imov ssa1.w
> } else {
>     ...
>     vec4 ssa6 = ...
>     vec1 ssa7 = imov ssa6.x
>     vec1 ssa8 = imov ssa6.y
>     vec1 ssa9 = imov ssa6.z
>     vec1 ssa10 = imov ssa6.w
> }
> vec1 ss11 = phi ssa2, ssa7
> vec1 ss12 = phi ssa3, ssa8
> vec1 ss13 = phi ssa4, ssa9
> vec1 ss14 = phi ssa5, ssa10
>
>
> This allows a whole bunch more optimisation to take place as often not
> all of the phi channels are actually used. If some cases large chunks of
> logic can be remove from the if branch that doesn't contain the texture
> access.
>
> >>
> >>
> >> I think I can for Intel though I'm not sure how this affects other
> drivers.
> >>
> >> On Intel hardware, we almost always have to combine all the texture
> sources into one big message.  Since having more than one source is very
> common, this means that we have to make a temporary copy of the sources
> anyway.  Because we're copying them, having them contiguous (a vector in
> NIR terms) doesn't actually gain us anything.  We may as well let NIR
> scalarize them and give more freedom to the register allocator and other
> NIR passes which may need to clean things up.  We don't want to make the
> same choice for destinations as they are required to be contiguous.
> >>
> >
> > hmm, but this is abut the phi src (ie. the tex dest), not the tex src,
> isn't it?
>

Well, that's a pickle...  When I wrote this pass I did so to try and
explicitly keep from breaking up things that are known to be vectors in the
back-end such as textures.  The idea was to try and not break up things
like this:

 if (...) {
    ssa_1 = tex()
} else {
    ssa_2 = tex()
}
ssa_3 = phi (ssa_1, ssa_2)

in the hopes that it would turn into

if (...) {
    r1 = tex();
} else {
    r1 = tex();
}

Clearly, that notion was mis-placed.  At this point, I really wonder what
the complexity is saving us.  Maybe it's not worth it at all?  Maybe we
need to be more agressive and require all sources to not be vectorizable or
something?


> Yeah, thats correct. Which is why I think its fine to be bunched with
> the "These are trivially scalarizable" types?
>
> >
> > BR,
> > -R
> >
> >> Feel free to copy+paste that somewhere.  I agree with Eric that they
> are not "trivially scalarizable" but they are safe to scalarize.
> >>
> >> --Jason
> >> _______________________________________________
> >> mesa-dev mailing list
> >> mesa-dev at lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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