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nir: Do basic constant reassociation.

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Many shaders contain expression trees of the form:

    const_1 * (value * const_2)

Reorganizing these to

    (const_1 * const_2) * value

will allow constant folding to combine the constants.  Sometimes, these
constants are 2 and 0.5, so we can remove a multiply altogether.  Other
times, it can create more immediate constants, which can actually hurt.

Finding a good balance here is tricky.  While much more could be done,
this simple patch seems to have a lot of positive benefit while having
a low downside.

shader-db results on Broadwell:

total instructions in shared programs: 8963768 -> 8961369 (-0.03%)
instructions in affected programs: 438318 -> 435919 (-0.55%)
helped: 1502
HURT: 245

total cycles in shared programs: 71527354 -> 71421516 (-0.15%)
cycles in affected programs: 11541788 -> 11435950 (-0.92%)
helped: 3445
HURT: 1224

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>
This commit is contained in:
Kenneth Graunke 2016-04-06 22:34:39 -07:00
parent 1c7ba7f156
commit 5886cd79a0
1 changed files with 8 additions and 0 deletions
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8
src/compiler/nir/nir_opt_algebraic.py
View file

@ -274,6 +274,14 @@ optimizations = [
(('fmul', ('fneg', a), b), ('fneg', ('fmul', a, b))),
(('imul', ('ineg', a), b), ('ineg', ('imul', a, b))),
# Reassociate constants in add/mul chains so they can be folded together.
# For now, we only handle cases where the constants are separated by
# a single non-constant. We could do better eventually.
(('~fmul', '#a', ('fmul', b, '#c')), ('fmul', ('fmul', a, c), b)),
(('imul', '#a', ('imul', b, '#c')), ('imul', ('imul', a, c), b)),
(('~fadd', '#a', ('fadd', b, '#c')), ('fadd', ('fadd', a, c), b)),
(('iadd', '#a', ('iadd', b, '#c')), ('iadd', ('iadd', a, c), b)),
# Misc. lowering
(('fmod', a, b), ('fsub', a, ('fmul', b, ('ffloor', ('fdiv', a, b)))), 'options->lower_fmod'),
(('uadd_carry', a, b), ('b2i', ('ult', ('iadd', a, b), a)), 'options->lower_uadd_carry'),