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

[Ian Romanick]

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]);
> 
> 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)),
>