[Mesa-dev] [PATCH 6/6] ir_to_mesa: Try to avoid emitting a MOV_SAT to saturate an expression tree.

[Eric Anholt]

Fixes a regression in codegen quality for ff_fragment_shader
conversion to GLSL -- glean texCombine produces 7.5% fewer Mesa IR
instructions.
---
 src/mesa/program/ir_to_mesa.cpp |   28 ++++++++++++++++++++++++----
 1 files changed, 24 insertions(+), 4 deletions(-)

diff --git a/src/mesa/program/ir_to_mesa.cpp b/src/mesa/program/ir_to_mesa.cpp
index 8b4a535..f82b048 100644
--- a/src/mesa/program/ir_to_mesa.cpp
+++ b/src/mesa/program/ir_to_mesa.cpp
@@ -925,13 +925,33 @@ ir_to_mesa_visitor::try_emit_sat(ir_expression *ir)
    if (!sat_src)
       return false;
 
+   ir_to_mesa_instruction *new_inst;
+
    sat_src->accept(this);
    src_reg src = this->result;
 
-   this->result = get_temp(ir->type);
-   ir_to_mesa_instruction *inst;
-   inst = emit(ir, OPCODE_MOV, dst_reg(this->result), src);
-   inst->saturate = true;
+   /* If we generated an expression instruction into a temporary in
+    * processing the saturate's operand, apply the saturate to that
+    * instruction.  Otherwise, generate a MOV to do the saturate.
+    *
+    * Note that we have to be careful to only do this optimization if
+    * the instruction in question was in generating src->result -- for
+    * example, ir_dereference_array would generate a MUL to create the
+    * reladdr and return us a src reg using that reladdr, and that MUL
+    * not the thing we're trying to saturate.
+    */
+   ir_expression *sat_src_expr = sat_src->as_expression();
+   new_inst = (ir_to_mesa_instruction *)this->instructions.get_tail();
+   if (sat_src_expr && (sat_src_expr->operation == ir_binop_mul ||
+			sat_src_expr->operation == ir_binop_add ||
+			sat_src_expr->operation == ir_binop_dot)) {
+      new_inst->saturate = true;
+   } else {
+      this->result = get_temp(ir->type);
+      ir_to_mesa_instruction *inst;
+      inst = emit(ir, OPCODE_MOV, dst_reg(this->result), src);
+      inst->saturate = true;
+   }
 
    return true;
 }
-- 
1.7.5.4