[Mesa-dev] [PATCH 1/4] nir: Do basic constant reassociation.

[Kenneth Graunke]

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 at whitecape.org>
---
 src/compiler/nir/nir_opt_algebraic.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/compiler/nir/nir_opt_algebraic.py b/src/compiler/nir/nir_opt_algebraic.py
index e72b4a7..420d9d9 100644
--- a/src/compiler/nir/nir_opt_algebraic.py
+++ b/src/compiler/nir/nir_opt_algebraic.py
@@ -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'),
-- 
2.8.0