[Mesa-dev] [PATCH] glsl: Optimize 1/exp(x) into exp(-x).

[Matt Turner]

Lots of shaders divide by exp2(...) which we turn into a multiplication
by the reciprocal. We can avoid the reciprocal by simply negating exp2's
argument.

total instructions in shared programs: 5947154 -> 5946695 (-0.01%)
instructions in affected programs:     118661 -> 118202 (-0.39%)
helped:                                380
---
 src/glsl/opt_algebraic.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
index c6f4a9c..616ed37 100644
--- a/src/glsl/opt_algebraic.cpp
+++ b/src/glsl/opt_algebraic.cpp
@@ -747,6 +747,12 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
       if (op_expr[0] && op_expr[0]->operation == ir_unop_rcp)
 	 return op_expr[0]->operands[0];
 
+      if (op_expr[0] && (op_expr[0]->operation == ir_unop_exp2 ||
+                         op_expr[0]->operation == ir_unop_exp)) {
+         return new(mem_ctx) ir_expression(op_expr[0]->operation, ir->type,
+                                           neg(op_expr[0]->operands[0]));
+      }
+
       /* While ir_to_mesa.cpp will lower sqrt(x) to rcp(rsq(x)), it does so at
        * its IR level, so we can always apply this transformation.
        */
-- 
2.0.5