[Mesa-dev] [PATCH 1/3] glsl: Propagate negates through multiplication chains.

[Matt Turner]

We propagate negations to the right-most leaves of the multiplication
expression trees:

 - mul(neg(x), neg(y)) -> mul(x, y)
 - mul(neg(x), y) -> neg(mul(x, y))
 - mul(x, neg(y)) -> neg(mul(x, y))

total instructions in shared programs: 5943123 -> 5937229 (-0.10%)
instructions in affected programs:     868221 -> 862327 (-0.68%)
helped:                                4518
HURT:                                  356
GAINED:                                1
---
 src/glsl/opt_algebraic.cpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
index 7bc65da..aecd6e1 100644
--- a/src/glsl/opt_algebraic.cpp
+++ b/src/glsl/opt_algebraic.cpp
@@ -514,6 +514,20 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
       if (is_vec_negative_one(op_const[1]))
          return neg(ir->operands[0]);
 
+      if (op_expr[0] && op_expr[0]->operation == ir_unop_neg) {
+         if (op_expr[1] && op_expr[1]->operation == ir_unop_neg) {
+            /* mul(neg(x), neg(y)) -> mul(x, y) */
+            return mul(op_expr[0]->operands[0], op_expr[1]->operands[0]);
+         }
+
+         /* mul(neg(x), y) -> neg(mul(x, y)) */
+         return neg(mul(op_expr[0]->operands[0], ir->operands[1]));
+      }
+
+      /* mul(x, neg(y)) -> neg(mul(x, y)) */
+      if (op_expr[1] && op_expr[1]->operation == ir_unop_neg) {
+         return neg(mul(ir->operands[0], op_expr[1]->operands[0]));
+      }
 
       /* Reassociate multiplication of constants so that we can do
        * constant folding.
-- 
2.0.5