X-Git-Url: https://git.libre-soc.org/?a=blobdiff_plain;f=src%2Fglsl%2Fopt_if_simplification.cpp;h=db59b131dbc219ad3c61d45fc1e8a81ed2a713bf;hb=f25d94084ce3225e803c07672c359a4e553b0e08;hp=7e88208f7c141b19dffbc0c23faa7fcecf335c88;hpb=3f349d4e18d8c114a34fc3c36e1dc55345c1cc31;p=mesa.git

diff --git a/src/glsl/opt_if_simplification.cpp b/src/glsl/opt_if_simplification.cpp
index 7e88208f7c1..db59b131dbc 100644
--- a/src/glsl/opt_if_simplification.cpp
+++ b/src/glsl/opt_if_simplification.cpp
@@ -25,11 +25,14 @@
  * \file opt_if_simplification.cpp
  *
  * Moves constant branches of if statements out to the surrounding
- * instruction stream.
+ * instruction stream, and inverts if conditionals to avoid empty
+ * "then" blocks.
  */
 
 #include "ir.h"
 
+namespace {
+
 class ir_if_simplification_visitor : public ir_hierarchical_visitor {
 public:
    ir_if_simplification_visitor()
@@ -43,6 +46,8 @@ public:
    bool made_progress;
 };
 
+} /* unnamed namespace */
+
 /* We only care about the top level "if" instructions, so don't
  * descend into expressions.
  */
@@ -97,6 +102,30 @@ ir_if_simplification_visitor::visit_leave(ir_if *ir)
       }
       ir->remove();
       this->made_progress = true;
+      return visit_continue;
+   }
+
+   /* Turn:
+    *
+    *     if (cond) {
+    *     } else {
+    *         do_work();
+    *     }
+    *
+    * into :
+    *
+    *     if (!cond)
+    *         do_work();
+    *
+    * which avoids control flow for "else" (which is usually more
+    * expensive than normal operations), and the "not" can usually be
+    * folded into the generation of "cond" anyway.
+    */
+   if (ir->then_instructions.is_empty()) {
+      ir->condition = new(ralloc_parent(ir->condition))
+	 ir_expression(ir_unop_logic_not, ir->condition);
+      ir->else_instructions.move_nodes_to(&ir->then_instructions);
+      this->made_progress = true;
    }
 
    return visit_continue;