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

Fri Aug 12 01:36:07 UTC 2016

On Aug 11, 2016 6:16 PM, "Ian Romanick" <idr at freedesktop.org> wrote:
>
> 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.

I believe so too.  "<< c" as an operator is equivalent to "* (1 << c)" so
association and distributivity apply.

> > The change looks good and is:
> >
> > Reviewed-by: Thomas Helland <thomashelland90 at gmail.com>

Reviewed-by: Jason Ekstrand <jason at jlekstrand.net>

> > (Oh, and yes, I'm back to hobby mesa development after finally
> > securing a new job and a new apartment)
>
> Welcome back!

Woohoo!

> >>> 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)),
> >>>
> >>
> >> _______________________________________________
> >> mesa-dev mailing list
> >> mesa-dev at lists.freedesktop.org
> >> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
> >
>
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