[Mesa-dev] [PATCH 6/7] nir: Teach nir_opt_algebraic about adding and subtracting the same thing

[Kristian Høgsberg Kristensen]

This optimizes a + b - b to just a. Modest shader-db results (BDW):

  total instructions in shared programs: 7842452 -> 7841862 (-0.01%)
  instructions in affected programs:     61938 -> 61348 (-0.95%)
  total loops in shared programs:        2131 -> 2131 (0.00%)
  helped:                                263
  HURT:                                  0
  GAINED:                                0
  LOST:                                  0

but the optimization turns

  gl_VertexID - gl_BaseVertexARB

into just a reference to SYSTEM_VALUE_VERTEX_ID_ZERO_BASE, which the
i965 hardware supports natively. That means we can avoid using the
internal vertex buffer for gl_BaseVertexARB in this case.
---
 src/glsl/nir/nir_opt_algebraic.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/glsl/nir/nir_opt_algebraic.py b/src/glsl/nir/nir_opt_algebraic.py
index cb715c0..1fdad3d 100644
--- a/src/glsl/nir/nir_opt_algebraic.py
+++ b/src/glsl/nir/nir_opt_algebraic.py
@@ -62,6 +62,10 @@ optimizations = [
    (('iadd', ('imul', a, b), ('imul', a, c)), ('imul', a, ('iadd', b, c))),
    (('fadd', ('fneg', a), a), 0.0),
    (('iadd', ('ineg', a), a), 0),
+   (('iadd', ('ineg', a), ('iadd', a, b)), b),
+   (('iadd', a, ('iadd', ('ineg', a), b)), b),
+   (('fadd', ('fneg', a), ('fadd', a, b)), b),
+   (('fadd', a, ('fadd', ('fneg', a), b)), b),
    (('fmul', a, 0.0), 0.0),
    (('imul', a, 0), 0),
    (('umul_unorm_4x8', a, 0), 0),
-- 
2.5.0