[Mesa-dev] [PATCH] i965/fs_nir: Put the immediate in src1 for commutative ALU ops

[Jason Ekstrand]

Shader-db results for fragment shaders on Broadwell:

   total instructions in shared programs: 4310987 -> 4310663 (-0.01%)
   instructions in affected programs:     43242 -> 42918 (-0.75%)
   helped:                                142
   HURT:                                  0

Shader-db results for vertex shaders on Broadwell:

   total instructions in shared programs: 2889581 -> 2844309 (-1.57%)
   instructions in affected programs:     1418720 -> 1373448 (-3.19%)
   helped:                                6139
   HURT:                                  0
---
 src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
index 555987d..161a262 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
@@ -21,6 +21,8 @@
  * IN THE SOFTWARE.
  */
 
+#include <algorithm>
+
 #include "glsl/ir.h"
 #include "glsl/ir_optimization.h"
 #include "glsl/nir/glsl_to_nir.h"
@@ -662,6 +664,16 @@ fs_visitor::nir_emit_alu(nir_alu_instr *instr)
       op[i] = offset(op[i], instr->src[i].swizzle[channel]);
    }
 
+   /* Immediates can only be used as the second source of two-source
+    * instructions.  We have code in opt_algebraic to flip them as needed
+    * for most instructions.  However, it doesn't hurt anything to just do
+    * the right thing if we can detect it at the NIR level.
+    */
+   if ((nir_op_infos[instr->op].algebraic_properties & NIR_OP_IS_COMMUTATIVE) &&
+       nir_src_as_const_value(instr->src[0].src)) {
+      std::swap(op[0], op[1]);
+   }
+
    switch (instr->op) {
    case nir_op_i2f:
    case nir_op_u2f:
-- 
2.4.0