[Mesa-dev] [PATCH 1/3] nir: Simplify fne(fneg(a), a) -> fne(a, 0.0)

[Thomas Helland]

-NaN != NaN, and -Inf != Inf, so this should be safe.
Found while working on my VRP pass.

Shader-db results on my IVB:
total instructions in shared programs: 1698267 -> 1698067 (-0.01%)
instructions in affected programs:     15785 -> 15585 (-1.27%)
helped:                                36
HURT:                                  0
GAINED:                                0
LOST:                                  0

Some shaders was found to have the following pattern in NIR:
vec1 ssa_26 = fneg ssa_21
vec1 ssa_27 = fne ssa_21, ssa_26

Make that:
vec1 ssa_27 = fne ssa_21, 0.0f

This is found in Dota2 and Brutal Legend.
One shader is cut by 8%, from 323 -> 296 instructons in SIMD8

Signed-off-by: Thomas Helland <thomashelland90 at gmail.com>
---
 src/glsl/nir/nir_opt_algebraic.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/src/glsl/nir/nir_opt_algebraic.py b/src/glsl/nir/nir_opt_algebraic.py
index 3068445..471191b 100644
--- a/src/glsl/nir/nir_opt_algebraic.py
+++ b/src/glsl/nir/nir_opt_algebraic.py
@@ -113,6 +113,7 @@ optimizations = [
    (('sge', a, b), ('b2f', ('fge', a, b)), 'options->lower_scmp'),
    (('seq', a, b), ('b2f', ('feq', a, b)), 'options->lower_scmp'),
    (('sne', a, b), ('b2f', ('fne', a, b)), 'options->lower_scmp'),
+   (('fne', ('fneg', a), a), ('fne', a, 0.0)),
    # Emulating booleans
    (('imul', ('b2i', a), ('b2i', b)), ('b2i', ('iand', a, b))),
    (('fmul', ('b2f', a), ('b2f', b)), ('b2f', ('iand', a, b))),
-- 
2.5.0