[Mesa-dev] [PATCH 5/5] i965/nir: use vectorization for non-scalar stages

[Jason Ekstrand]

On Tue, Nov 17, 2015 at 11:41 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> On Mon, Nov 16, 2015 at 11:00 AM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>> On Sat, Nov 14, 2015 at 6:59 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>> Shader-db results on bdw with INTEL_DEBUG=vec4:
>>>
>>> total instructions in shared programs: 1634044 -> 1612936 (-1.29%)
>>> instructions in affected programs: 802502 -> 781394 (-2.63%)
>>> helped: 5036
>>> HURT: 1442
>>>
>>> total cycles in shared programs: 9397790 -> 9355382 (-0.45%)
>>> cycles in affected programs: 5078600 -> 5036192 (-0.84%)
>>> helped: 3875
>>> HURT: 2554
>>>
>>> LOST:   0
>>> GAINED: 0
>>>
>>> Most of the hurt programs seem to be because we generate extra MOV's due
>>> to vectorizing things. For example, in
>>> shaders/non-free/steam/anomaly-2/158.shader_test, this:
>>>
>>> add(8)          g116<1>.xyF     g12<4,4,1>.xyyyF g1.4<0,4,1>.xyyyF { align16 NoDDClr 1Q };
>>> add(8)          g117<1>.xyF     g12<4,4,1>.xyyyF g1.4<0,4,1>.zwwwF { align16 NoDDClr 1Q };
>>> add(8)          g116<1>.zwF     g12<4,4,1>.xxxyF -g1.4<0,4,1>.xxxyF { align16 NoDDChk 1Q };
>>> add(8)          g117<1>.zwF     g12<4,4,1>.xxxyF -g1.4<0,4,1>.zzzwF { align16 NoDDChk 1Q };
>>>
>>> Turns into this:
>>>
>>> add(8)          g13<1>F         g12<4,4,1>.xyxyF g1.4<0,4,1>F   { align16 1Q };
>>> add(8)          g14<1>F         g12<4,4,1>.xyxyF -g1.4<0,4,1>F  { align16 1Q };
>>> mov(8)          g116<1>.xyD     g13<4,4,1>.xyyyD                { align16 NoDDClr 1Q };
>>> mov(8)          g117<1>.xyD     g13<4,4,1>.zwwwD                { align16 NoDDClr 1Q };
>>> mov(8)          g116<1>.zwD     g14<4,4,1>.xxxyD                { align16 NoDDChk 1Q };
>>> mov(8)          g117<1>.zwD     g14<4,4,1>.zzzwD                { align16 NoDDChk 1Q };
>>>
>>> So we eliminated two add's, but then had to introduce four mov's to
>>> transpose the result. I don't think there's much we can do about this at
>>> the NIR level, unfortunately.
>>
>> Given the shader-db numbers above, I think we can probably eat the
>> hurt programs.  Would you mind cherry-picking back onto a time when we
>> had GLSL IR and doing a GLSL IR vs. NIR comparison with this series?
>> This is one of the places we were still hurting so it would be good to
>> know how it changes the picture.  Not that it *really* matters at this
>> point...
>
> So I rebased the series onto right before we switched to NIR by
> default. For non-NIR vs. NIR, the numbers were:
>
> total instructions in shared programs: 1848629 -> 1647133 (-10.90%)
> instructions in affected programs:     1663319 -> 1461711 (-12.12%)
> total loops in shared programs:        223 -> 222 (-0.45%)
> helped:                                14667
> HURT:                                  1416
> GAINED:                                0
> LOST:                                  0
>
> and for non-NIR vs. NIR + vectorization:
>
> total instructions in shared programs: 1848629 -> 1619388 (-12.40%)
> instructions in affected programs:     1696613 -> 1467260 (-13.52%)
> total loops in shared programs:        223 -> 222 (-0.45%)
> helped:                                15076
> HURT:                                  1378
> GAINED:                                0
> LOST:                                  0
>
> so, indeed, it does seem to get rid of some hurt programs. My
> suspicion, though, is that it's a case of "well, NIR hurt, but the
> gain from vectorizing offset the (unrelated) problem so that now it
> helps." We don't ever split up vectorized operations in NIR (except
> when lowering to scalar, ofc) so NIR shouldn't be doing anything
> harmful that this pass would help.

Mostly, I wanted to see how big the numbers were :-)  Also, we did see
some vectorization issues in some of the shaders that were hurt.
--Jason

>>
>>> Signed-off-by: Connor Abbott <cwabbott0 at gmail.com>
>>> ---
>>>  src/mesa/drivers/dri/i965/brw_nir.c | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>>
>>> diff --git a/src/mesa/drivers/dri/i965/brw_nir.c b/src/mesa/drivers/dri/i965/brw_nir.c
>>> index fe5cad4..29cafe6 100644
>>> --- a/src/mesa/drivers/dri/i965/brw_nir.c
>>> +++ b/src/mesa/drivers/dri/i965/brw_nir.c
>>> @@ -198,6 +198,14 @@ nir_optimize(nir_shader *nir, bool is_scalar)
>>>        nir_validate_shader(nir);
>>>        progress |= nir_opt_cse(nir);
>>>        nir_validate_shader(nir);
>>> +
>>> +      if (!is_scalar) {
>>> +         progress |= nir_opt_vectorize(nir);
>>> +         nir_validate_shader(nir);
>>> +         progress |= nir_copy_prop(nir);
>>> +         nir_validate_shader(nir);
>>> +      }
>>> +
>>>        progress |= nir_opt_peephole_select(nir);
>>>        nir_validate_shader(nir);
>>>        progress |= nir_opt_algebraic(nir);
>>> --
>>> 2.4.3
>>>
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