[Mesa-dev] [PATCH 5/9] i965/nir: Emit MUL + ADD for MAD on GEN <= 5

[Connor Abbott]

On Wed, Mar 18, 2015 at 11:24 PM, Kenneth Graunke <kenneth at whitecape.org> wrote:
> On Tuesday, March 17, 2015 07:17:19 PM Jason Ekstrand wrote:
>> ---
>>  src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> index 5da8423..41f9ae2 100644
>> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
>> @@ -1234,7 +1234,13 @@ fs_visitor::nir_emit_alu(nir_alu_instr *instr)
>>        break;
>>
>>     case nir_op_ffma:
>> -      inst = emit(MAD(result, op[2], op[1], op[0]));
>> +      if (brw->gen >= 6) {
>> +         inst = emit(MAD(result, op[2], op[1], op[0]));
>> +      } else {
>> +         fs_reg temp = vgrf(glsl_type::float_type);
>> +         emit(MUL(temp, op[0], op[1]));
>> +         inst = emit(ADD(result, temp, op[2]));
>> +      }
>>        inst->saturate = instr->dest.saturate;
>>        break;
>>
>>
>
> I think this is a reasonable plan for now - it's simple and works.
> Connor is probably right that we can do better with nir_opt_algebraic,
> but I think doing that as a follow-on patch justified by statistics
> would be wise.

Actually, it seems like we already have a lower_ffma option, so just
setting that to true on gen < 6 should be even less code than this...

>
> Patches 3-6 are:
> Reviewed-by: Kenneth Graunke <kenneth at whitecape.org>
>
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