[Mesa-dev] [PATCH 16/78] i965/nir/vec4: Implement store_output intrinsic

Jason Ekstrand jason at jlekstrand.net
Wed Jul 22 20:20:18 PDT 2015

On Wed, Jul 22, 2015 at 4:37 AM, Eduardo Lima Mitev <elima at igalia.com> wrote:
> On 07/13/2015 01:57 PM, Jason Ekstrand wrote:
>> On Wed, Jul 8, 2015 at 11:54 PM, Eduardo Lima Mitev <elima at igalia.com> wrote:
>>> On 06/30/2015 06:51 PM, Jason Ekstrand wrote:
>>>> On Fri, Jun 26, 2015 at 1:06 AM, Eduardo Lima Mitev <elima at igalia.com> wrote:
>>>>> The index into the output_reg array where to store the destination register is
>>>>> fetched from the nir_outputs map built during nir_setup_outputs stage.
>>>>> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89580
>>>>> ---
>>>>>  src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 17 +++++++++++++++--
>>>>>  1 file changed, 15 insertions(+), 2 deletions(-)
>>>>> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
>>>>> index 8a2d335..55d4490 100644
>>>>> --- a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
>>>>> +++ b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
>>>>> @@ -520,10 +520,23 @@ vec4_visitor::nir_emit_intrinsic(nir_intrinsic_instr *instr)
>>>>>     }
>>>>>     case nir_intrinsic_store_output_indirect:
>>>>> +      has_indirect = true;
>>>>>        /* fallthrough */
>>>>> -   case nir_intrinsic_store_output:
>>>>> -      /* @TODO: Not yet implemented */
>>>>> +   case nir_intrinsic_store_output: {
>>>>> +      int offset = instr->const_index[0];
>>>>> +      int output = nir_outputs[offset];
>>>>> +
>>>>> +      src = get_nir_src(instr->src[0], nir_output_types[offset]);
>>>>> +      dest = dst_reg(src);
>>>>> +
>>>>> +      dest.writemask = brw_writemask_for_size(instr->num_components);
>>>>> +
>>>>> +      if (has_indirect)
>>>>> +         dest.reladdr = new(mem_ctx) src_reg(get_nir_src(instr->src[1]));
>>>>> +
>>>>> +      output_reg[output] = dest;
>>>> I'm very confused about the amount of indirection going on here.  It
>>>> seems to me that we should be setting these outputs up in
>>>> setup_outputs() rather than storring off a map from ints to other ints
>>>> and setting it up here.  I didn't make this comment on the patch for
>>>> setup_outputs() because I wanted to wait to see it used before I
>>>> commented on it.
>>>> I'm guessing you did it this way because the nir_assign_var_locations
>>>> is giving you bogus values.  If so, then it might be better to just
>>>> assign variable locations in setup_outputs() rather than having a
>>>> remap table.  The whole point of nir_lower_io is to make IO easy for
>>>> the back-end.  If you need a re-map table, then it's no longer making
>>>> it easy and we need to think more about what's going on.
>>>> --Jason
>>> That double indirection felt bad since the beginning, but it was needed
>>> to store the original variable's location (var->data.location). Let me
>>> explain:
>>> We are (re)using the plumbering in vec4_visitor to setup URB, so the
>>> only thing we need to do is to store the out register in "output_reg"
>>> map at the correct location. And that location is given by the original
>>> location in the shader (var->data.location).
>>> So, in this case, "nir_assign_var_locations" pass, which constructs
>>> var->data.driver_location, is not useful to us, except to give us
>>> consecutive indexes to construct the other map we have, the type map,
>>> which is needed to carry the correct type from the original variable to
>>> the output register.
>> If nir_assign_var_locations isn't doing anything for you, don't call
>> it.  You'll need to do something with var->data.driver_location.  If
>> what you really want is var->data.location, then just copy that to
>> var->data.driver_location when you do nir_setup_outputs.  Or
>> (depending on how the URB setup works, I don't actually know), put the
>> actual URB location in var->data.driver_location when you walk the
>> outputs.
>> From there, you have two options.  One would be to setup output_reg at
>> the same time with the correct types right away and emit a MOV when
>> you get a store_output.  (Copy propagation should clean up the MOV.)
>> For what it's worth, I don't think the type matters; a URB write just
>> writes data to something so as long as you don't have a type mismatch
>> in a MOV, the hardware won't care.
>> The other option, would be to directly emit the URB write in
>> store_output.  At the moment, it may be better to take the first
>> option since that better matches what the FS does right now.  But both
>> should work fine.
> Thanks for these hints, they were very useful.
> I rewrote the implementation of store_output intrinsic to avoid the
> setup phase completely. The type, as you suggested, was not important as
> long as they match while MOVing the contents of output_reg. To guarantee
> that, I had to patch the emit_urb_slot() to guarantee the types always
> match. This code is shared with vec4_visitor, so it makes sense to move
> the safeguards there instead of having both backends provide the correct
> register type in output_reg entries.
> For reference, this is the patch that implements it:
> https://github.com/Igalia/mesa/commit/8c703937f285c0b3a1e7bf6681c7ed7fe09815aa

Seems reasonable.

> I also put var->data.location in const_index[1] of the intrinsic op, and
> disabled nir_assign_var_locations() for output variables, since I don't
> need var->data.driver_location. I could have used const_index[0], but I
> prefer to leave driver_location there, and use const_index[1], to avoid
> breaking any driver that rely on current layout of const_index (like
> FS-nir). I think it is a safer approach.

You're not going to break anything by going through the output
variables and setting driver_location equal to location.  The whole
point of driver_location is to store some backend-specific index for
the variable.  In other words, to do exactly what you're doing.  The
assign_var_locations calls are simply convenience functions for
setting the driver_location field.  In other words, using
driver_location and const_index[0] is *exactly* what you should do.


> All in all, the store_output implementation got much simpler.
>>> So, before knowing that I could modify nir_lower_io, my best shot at
>>> transferring the original variable location was to create this
>>> nir_outputs map. Now, what I have done is to put that value in
>>> const_index[1] of the intrinsic instruction, which was previously
>>> unused. What do you think?
>>> That removes the offset to offset map, but we still need the type map.
>>> About your comment on initializing the register during setup stage, I'm
>>> a bit confused: the register that we need to store is not available
>>> during setup stage, because we still don't have local registers allocated.
>> What do you mean?  Because you don't have the destination of the
>> output_write intrinsic allocated?  Even if the register has a file of
>> BAD_FILE, you could still store the type there.  Also, as I said
>> above, the hardware shouldn't care about the types of data.  As long
>> as the URB write code doesn't accidentally do a float -> int
>> conversion or something, we should be fine.
>> --Jason
>>>>>        break;
>>>>> +   }
>>>>>     case nir_intrinsic_load_vertex_id:
>>>>>        unreachable("should be lowered by lower_vertex_id()");
>>>>> --

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