On 04/08/2016 01:35 AM, Kenneth Graunke wrote:
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.
Makes sense.
Reviewed-by: Eduardo Lima Mitev <el...@igalia.com>
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
FWIW, these are my results on HSW:
total instructions in shared programs: 6223995 -> 6222470 (-0.02%)
instructions in affected programs: 300945 -> 299420 (-0.51%)
helped: 1231
HURT: 240
total cycles in shared programs: 56809432 -> 56615552 (-0.34%)
cycles in affected programs: 7980160 -> 7786280 (-2.43%)
helped: 2087
HURT: 634
Eduardo
Signed-off-by: Kenneth Graunke <kenn...@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'),
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