On 23/2/19 8:54 am, Jason Ekstrand wrote:
On Fri, Feb 22, 2019 at 2:47 PM Timothy Arceri <[email protected]
<mailto:[email protected]>> wrote:
On 23/2/19 6:31 am, Rob Clark wrote:
> On Fri, Feb 22, 2019 at 12:39 PM Jason Ekstrand
<[email protected] <mailto:[email protected]>> wrote:
>>
>> On Fri, Feb 22, 2019 at 9:51 AM Eric Anholt <[email protected]
<mailto:[email protected]>> wrote:
>>>
>>> Timothy Arceri <[email protected]
<mailto:[email protected]>> 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?
Maybe checking if all components of the phi are used before converting
to scalar would be useful? Not sure but I've sent a more conservative v2
of this patch where a bunch of hurt is gone.
The remaining hurt is very small and comes from shaders like this:
if (...) {
r1 = tex();
} else {
if (...) {
r1 = tex();
} else {
r1 = undefined;
}
}
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