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

[Thomas Helland]

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?

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)

>> 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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