[Mesa-dev] [PATCH 5/5] glsl: Apply the transformation "1/rsq(x) == sqrt(x)" in opt_algebraic.

[Eric Anholt]

The comment was stale, because the lowering in question wasn't happening
in lower_instructions.cpp.  Presumably if the lowering ever moves there,
we can plumb the lowering mask through to opt_algebraic.

total instructions in shared programs: 1618696 -> 1616810 (-0.12%)
instructions in affected programs:     243018 -> 241132 (-0.78%)
GAINED:                                0
LOST:                                  0
---
 src/glsl/opt_algebraic.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
index 448f27e..05a5899 100644
--- a/src/glsl/opt_algebraic.cpp
+++ b/src/glsl/opt_algebraic.cpp
@@ -420,10 +420,11 @@ 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];
 
-      /* FINISHME: We should do rcp(rsq(x)) -> sqrt(x) for some
-       * backends, except that some backends will have done sqrt ->
-       * rcp(rsq(x)) and we don't want to undo it for them.
+      /* 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.
        */
+      if (op_expr[0] && op_expr[0]->operation == ir_unop_rsq)
+         return sqrt(op_expr[0]->operands[0]);
 
       /* As far as we know, all backends are OK with rsq. */
       if (op_expr[0] && op_expr[0]->operation == ir_unop_sqrt) {
-- 
1.8.4.rc3