[Mesa-dev] [PATCH 2/2] nir/algebraic: Optimize common array indexing sequence

Fri Aug 12 01:15:58 UTC 2016

On 08/11/2016 11:54 AM, Thomas Helland wrote:
> 2016-08-11 18:19 GMT+02:00 Ian Romanick <idr at freedesktop.org>:
>> ping
>>
>> On 07/19/2016 10:37 PM, Ian Romanick wrote:
>>> From: Ian Romanick <ian.d.romanick at intel.com>
>>>
>>> Some shaders include code that looks like:
>>>
>>>    uniform int i;
>>>    uniform vec4 bones[...];
>>>
>>>    foo(bones[i * 3], bones[i * 3 + 1], bones[i * 3 + 2]);
>>>
>>> CSE would do some work on this:
>>>
>>>    x = i * 3
>>>    foo(bones[x], bones[x + 1], bones[x + 2]);
>>>
>>> The compiler may then add '<< 4 + base' to the index calculations.
>>> This results in expressions like
>>>
>>>    x = i * 3
>>>    foo(bones[x << 4], bones[(x + 1) << 4], bones[(x + 2) << 4]);
>>>
> 
> This may just be me being dense, but why is the compiler adding the shift?
> It seems like this would be "un-optimizing" and so it shouldn't be added?

The shift is to convert the array offset to a byte offset.  A vec4 is 16
bytes.

> This patch is also a good argument for adding a constant reassociation pass.
> At least the (a * #b) << #c    --->    a * (#b << #c) would be caught by that.
> 
> I was confused whether the optimization was preserving signed-ness correctly
> but after a couple rounds with myself I believe it is correct.
> 
> The change looks good and is:
> 
> Reviewed-by: Thomas Helland <thomashelland90 at gmail.com>
> 
> (Oh, and yes, I'm back to hobby mesa development after finally
> securing a new job and a new apartment)

Welcome back!

>>> Just rearranging the math to produce (i * 48) + 16 saves an
>>> instruction, and it allows CSE to do more work.
>>>
>>>    x = i * 48;
>>>    foo(bones[x], bones[x + 16], bones[x + 32]);
>>>
>>> So, ~6 instructions becomes ~3.
>>>
>>> Some individual shader-db results look pretty bad.  However, I have a
>>> really, really hard time believing the change in estimated cycles in,
>>> for example, 3dmmes-taiji/51.shader_test after looking that change in
>>> the generated code.
>>>
>>> G45
>>> total instructions in shared programs: 4020840 -> 4010070 (-0.27%)
>>> instructions in affected programs: 177460 -> 166690 (-6.07%)
>>> helped: 894
>>> HURT: 0
>>>
>>> total cycles in shared programs: 98829000 -> 98784990 (-0.04%)
>>> cycles in affected programs: 3936648 -> 3892638 (-1.12%)
>>> helped: 894
>>> HURT: 0
>>>
>>> Ironlake
>>> total instructions in shared programs: 6418887 -> 6408117 (-0.17%)
>>> instructions in affected programs: 177460 -> 166690 (-6.07%)
>>> helped: 894
>>> HURT: 0
>>>
>>> total cycles in shared programs: 143504542 -> 143460532 (-0.03%)
>>> cycles in affected programs: 3936648 -> 3892638 (-1.12%)
>>> helped: 894
>>> HURT: 0
>>>
>>> Sandy Bridge
>>> total instructions in shared programs: 8357887 -> 8339251 (-0.22%)
>>> instructions in affected programs: 432715 -> 414079 (-4.31%)
>>> helped: 2795
>>> HURT: 0
>>>
>>> total cycles in shared programs: 118284184 -> 118207412 (-0.06%)
>>> cycles in affected programs: 6114626 -> 6037854 (-1.26%)
>>> helped: 2478
>>> HURT: 317
>>>
>>> Ivy Bridge
>>> total instructions in shared programs: 7669390 -> 7653822 (-0.20%)
>>> instructions in affected programs: 388234 -> 372666 (-4.01%)
>>> helped: 2795
>>> HURT: 0
>>>
>>> total cycles in shared programs: 68381982 -> 68263684 (-0.17%)
>>> cycles in affected programs: 1972658 -> 1854360 (-6.00%)
>>> helped: 2458
>>> HURT: 307
>>>
>>> Haswell
>>> total instructions in shared programs: 7082636 -> 7067068 (-0.22%)
>>> instructions in affected programs: 388234 -> 372666 (-4.01%)
>>> helped: 2795
>>> HURT: 0
>>>
>>> total cycles in shared programs: 68282020 -> 68164158 (-0.17%)
>>> cycles in affected programs: 1891820 -> 1773958 (-6.23%)
>>> helped: 2459
>>> HURT: 261
>>>
>>> Broadwell
>>> total instructions in shared programs: 9002466 -> 8985875 (-0.18%)
>>> instructions in affected programs: 658784 -> 642193 (-2.52%)
>>> helped: 2795
>>> HURT: 5
>>>
>>> total cycles in shared programs: 78503092 -> 78450404 (-0.07%)
>>> cycles in affected programs: 2873304 -> 2820616 (-1.83%)
>>> helped: 2275
>>> HURT: 415
>>>
>>> Skylake
>>> total instructions in shared programs: 9156978 -> 9140387 (-0.18%)
>>> instructions in affected programs: 682625 -> 666034 (-2.43%)
>>> helped: 2795
>>> HURT: 5
>>>
>>> total cycles in shared programs: 75591392 -> 75550574 (-0.05%)
>>> cycles in affected programs: 3192120 -> 3151302 (-1.28%)
>>> helped: 2271
>>> HURT: 425
>>>
>>> Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
>>> ---
>>>  src/compiler/nir/nir_opt_algebraic.py | 11 +++++++++++
>>>  1 file changed, 11 insertions(+)
>>>
>>> diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
>>> index 0f0896b..37cb700 100644
>>> --- a/src/compiler/nir/nir_opt_algebraic.py
>>> +++ b/src/compiler/nir/nir_opt_algebraic.py
>>> @@ -119,6 +119,17 @@ optimizations = [
>>>     (('~fadd at 64', a, ('fmul',         c , ('fadd', b, ('fneg', a)))), ('flrp', a, b, c), '!options->lower_flrp64'),
>>>     (('ffma', a, b, c), ('fadd', ('fmul', a, b), c), 'options->lower_ffma'),
>>>     (('~fadd', ('fmul', a, b), c), ('ffma', a, b, c), 'options->fuse_ffma'),
>>> +
>>> +   # (a * #b + #c) << #d
>>> +   # ((a * #b) << #d) + (#c << #d)
>>> +   # (a * (#b << #d)) + (#c << #d)
>>> +   (('ishl', ('iadd', ('imul', a, '#b'), '#c'), '#d'),
>>> +    ('iadd', ('imul', a, ('ishl', b, d)), ('ishl', c, d))),
>>> +
>>> +   # (a * #b) << #c
>>> +   # a * (#b << #c)
>>> +   (('ishl', ('imul', a, '#b'), '#c'), ('imul', a, ('ishl', b, c))),
>>> +
>>>     # Comparison simplifications
>>>     (('~inot', ('flt', a, b)), ('fge', a, b)),
>>>     (('~inot', ('fge', a, b)), ('flt', a, b)),
>>>
>>
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> 