[Mesa-dev] [PATCH 1/3] i965/vs: Use the MAD instruction when possible.

[Eric Anholt]

This is different from how we do it in the FS - we are using MAD even when
some of the args are constants, because with the relatively unrestrained
ability to schedule a MOV to prepare a temporary with that data, we can
get lower latency for the sequence of instructions.

No significant performance difference on GLB2.7 trex (n=33/34), though it
doesn't have that many MADs.  I noticed MAD opportunities while reading
the code for the DOTA2 bug.
---
 src/mesa/drivers/dri/i965/brw_vec4.h               |  1 +
 .../drivers/dri/i965/brw_vec4_copy_propagation.cpp |  1 +
 src/mesa/drivers/dri/i965/brw_vec4_emit.cpp        |  4 +++
 src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp     | 37 ++++++++++++++++++++++
 4 files changed, 43 insertions(+)

diff --git a/src/mesa/drivers/dri/i965/brw_vec4.h b/src/mesa/drivers/dri/i965/brw_vec4.h
index e6e59bc..a72d694 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4.h
+++ b/src/mesa/drivers/dri/i965/brw_vec4.h
@@ -468,6 +468,7 @@ public:
 				int base_offset);
 
    bool try_emit_sat(ir_expression *ir);
+   bool try_emit_mad(ir_expression *ir, int mul_arg);
    void resolve_ud_negate(src_reg *reg);
 
    src_reg get_timestamp();
diff --git a/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp b/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
index 39eef4b..1a667eb 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_copy_propagation.cpp
@@ -216,6 +216,7 @@ vec4_visitor::try_copy_propagation(struct intel_context *intel,
       return false;
 
    bool is_3src_inst = (inst->opcode == BRW_OPCODE_LRP ||
+                        inst->opcode == BRW_OPCODE_MAD ||
                         inst->opcode == BRW_OPCODE_BFE ||
                         inst->opcode == BRW_OPCODE_BFI2);
    if (is_3src_inst && value.file == UNIFORM)
diff --git a/src/mesa/drivers/dri/i965/brw_vec4_emit.cpp b/src/mesa/drivers/dri/i965/brw_vec4_emit.cpp
index 91101f2..fbb93db 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_emit.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_emit.cpp
@@ -772,6 +772,10 @@ vec4_generator::generate_code(exec_list *instructions)
 	 brw_set_acc_write_control(p, 0);
 	 break;
 
+      case BRW_OPCODE_MAD:
+         brw_MAD(p, dst, src[0], src[1], src[2]);
+         break;
+
       case BRW_OPCODE_FRC:
 	 brw_FRC(p, dst, src[0]);
 	 break;
diff --git a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
index 33c1b24..451f7d5 100644
--- a/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
+++ b/src/mesa/drivers/dri/i965/brw_vec4_visitor.cpp
@@ -1250,6 +1250,38 @@ vec4_visitor::try_emit_sat(ir_expression *ir)
    return true;
 }
 
+bool
+vec4_visitor::try_emit_mad(ir_expression *ir, int mul_arg)
+{
+   /* 3-src instructions were introduced in gen6. */
+   if (intel->gen < 6)
+      return false;
+
+   /* MAD can only handle floating-point data. */
+   if (ir->type->base_type != GLSL_TYPE_FLOAT)
+      return false;
+
+   ir_rvalue *nonmul = ir->operands[1 - mul_arg];
+   ir_expression *mul = ir->operands[mul_arg]->as_expression();
+
+   if (!mul || mul->operation != ir_binop_mul)
+      return false;
+
+   nonmul->accept(this);
+   src_reg src0 = fix_3src_operand(this->result);
+
+   mul->operands[0]->accept(this);
+   src_reg src1 = fix_3src_operand(this->result);
+
+   mul->operands[1]->accept(this);
+   src_reg src2 = fix_3src_operand(this->result);
+
+   this->result = src_reg(this, ir->type);
+   emit(BRW_OPCODE_MAD, dst_reg(this->result), src0, src1, src2);
+
+   return true;
+}
+
 void
 vec4_visitor::emit_bool_comparison(unsigned int op,
 				 dst_reg dst, src_reg src0, src_reg src1)
@@ -1293,6 +1325,11 @@ vec4_visitor::visit(ir_expression *ir)
    if (try_emit_sat(ir))
       return;
 
+   if (ir->operation == ir_binop_add) {
+      if (try_emit_mad(ir, 0) || try_emit_mad(ir, 1))
+	 return;
+   }
+
    for (operand = 0; operand < ir->get_num_operands(); operand++) {
       this->result.file = BAD_FILE;
       ir->operands[operand]->accept(this);
-- 
1.8.3.rc0