[Mesa-dev] [PATCH 1/2] nir: Add a function to determine if a source is dynamically uniform

[Matt Turner]

On Fri, Oct 2, 2015 at 2:52 AM, Francisco Jerez <currojerez at riseup.net> wrote:
> Jason Ekstrand <jason at jlekstrand.net> writes:
>
>> On Oct 1, 2015 12:18 PM, "Matt Turner" <mattst88 at gmail.com> wrote:
>>>
>>> On Fri, Aug 7, 2015 at 9:31 AM, Neil Roberts <neil at linux.intel.com> wrote:
>>> > Adds nir_src_is_dynamically_uniform which returns true if the source
>>> > is known to be dynamically uniform. This will be used in a later patch
>>> > to add a workaround for cases that only work with dynamically uniform
>>> > sources. Note that the function is not definitive, it can return false
>>> > negatives (but not false positives). Currently it only detects
>>> > constants and uniform accesses. It could easily be extended to include
>>> > more cases.
>>> > ---
>>> >  src/glsl/nir/nir.c | 29 +++++++++++++++++++++++++++++
>>> >  src/glsl/nir/nir.h |  1 +
>>> >  2 files changed, 30 insertions(+)
>>> >
>>> > diff --git a/src/glsl/nir/nir.c b/src/glsl/nir/nir.c
>>> > index 78ff886..242f0b4 100644
>>> > --- a/src/glsl/nir/nir.c
>>> > +++ b/src/glsl/nir/nir.c
>>> > @@ -1784,6 +1784,35 @@ nir_src_as_const_value(nir_src src)
>>> >     return &load->value;
>>> >  }
>>> >
>>> > +/**
>>> > + * Returns true if the source is known to be dynamically uniform.
>> Otherwise it
>>> > + * returns false which means it may or may not be dynamically uniform
>> but it
>>> > + * can't be determined.
>>> > + */
>>> > +bool
>>> > +nir_src_is_dynamically_uniform(nir_src src)
>>> > +{
>>> > +   if (!src.is_ssa)
>>> > +      return false;
>>> > +
>>> > +   /* Constants are trivially dynamically uniform */
>>> > +   if (src.ssa->parent_instr->type == nir_instr_type_load_const)
>>> > +      return true;
>>> > +
>>> > +   /* As are uniform variables */
>>> > +   if (src.ssa->parent_instr->type == nir_instr_type_intrinsic) {
>>> > +      nir_intrinsic_instr *intr =
>> nir_instr_as_intrinsic(src.ssa->parent_instr);
>>> > +
>>> > +      if (intr->intrinsic == nir_intrinsic_load_uniform)
>>> > +         return true;
>>> > +   }
>>> > +
>>> > +   /* XXX: this could have many more tests, such as when a sampler
>> function is
>>> > +    * called with dynamically uniform arguments.
>>> > +    */
>>> > +   return false;
>>> > +}
>>>
>>> This functions seems correct as-is, so it gets a
>>>
>>> Reviewed-by: Matt Turner <mattst88 at gmail.com>
>>>
>>> On top of being useful for fixing the nonconst/nonuniform piglit
>>> tests, knowing which values are uniform can allow better optimization
>>> and knowing which branches are uniform would allow us to enable Single
>>> Program Flow.
>>>
>>> Cc'ing Jason for a discussion about how we'd like to handle this kind
>>> of information in NIR. Add a bool is_uniform or something to
>>> nir_ssa_def and hook into the metadata system?
>>
>> That was more-or-less my plan. We should probably have a proper pass that
>> handles things like phi nodes and ALU operations.
>>
>
> Heh, I hadn't noticed this series until now and happen to have started
> to sketch such an analysis pass just this week, with a completely
> unrelated purpose in mind (enable a restricted form of GCM for texturing
> ops with implicit derivatives).  Any volunteers to write it (Neil?) or
> shall I go ahead with it?

Interesting. In case you haven't seen it, Connor added nir_opt_gcm()
some time ago, but it's not enabled because it hurts many things.
Presumably you can reuse its code to implement your idea.

I suspect you should go ahead and write the uniform analysis pass.

Thanks!