i965/fs: Skip emitting MACH/MOV for small integers.

The vector backend already implemented this optimization, but
surprisingly, we never bothered to implement it in the scalar backend.

In addition to saving two instructions, this eliminates a use of the
accumulator as an explicit source, which is unsupported in SIMD16 mode
on Gen7+, which could help us gain SIMD16 programs.

Cuts 19.23% of the instructions in dolphin/efb2ram.shader_test.

v2: Rebase on is_16bit_integer_constant -> is_uint16_constant rename.

Signed-off-by: Kenneth Graunke <[email protected]>
Reviewed-by: Ian Romanick <[email protected]>
Reviewed-by: Matt Turner <[email protected]>

1 files changed, 21 insertions, 12 deletions

diff --git a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
index ce6d3dad1e5..c89f4d6a05d 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_visitor.cpp
@@ -458,18 +458,27 @@ fs_visitor::visit(ir_expression *ir)
 	  * of one of the operands (src0 on gen6, src1 on gen7).  The
 	  * MACH accumulates in the contribution of the upper 16 bits
 	  * of that operand.
-	  *
-	  * FINISHME: Emit just the MUL if we know an operand is small
-	  * enough.
-	  */
-	 if (brw->gen >= 7)
-	    no16("SIMD16 explicit accumulator operands unsupported\n");
-
-	 struct brw_reg acc = retype(brw_acc_reg(), this->result.type);
-
-	 emit(MUL(acc, op[0], op[1]));
-	 emit(MACH(reg_null_d, op[0], op[1]));
-	 emit(MOV(this->result, fs_reg(acc)));
+          */
+         if (ir->operands[0]->is_uint16_constant()) {
+            if (brw->gen < 7)
+               emit(MUL(this->result, op[0], op[1]));
+            else
+               emit(MUL(this->result, op[1], op[0]));
+         } else if (ir->operands[1]->is_uint16_constant()) {
+            if (brw->gen < 7)
+               emit(MUL(this->result, op[1], op[0]));
+            else
+               emit(MUL(this->result, op[0], op[1]));
+         } else {
+            if (brw->gen >= 7)
+               no16("SIMD16 explicit accumulator operands unsupported\n");
+
+            struct brw_reg acc = retype(brw_acc_reg(), this->result.type);
+
+            emit(MUL(acc, op[0], op[1]));
+            emit(MACH(reg_null_d, op[0], op[1]));
+            emit(MOV(this->result, fs_reg(acc)));
+         }
       } else {
 	 emit(MUL(this->result, op[0], op[1]));
       }