[Mesa-dev] [PATCH 3/3] glsl: Reassociate multiplication of mat*mat*vec.

[Matt Turner]

The typical case of mat4*mat4*vec4 is 80 scalar multiplications, but
mat4*(mat4*vec4) is only 32.

On HSW (with vec4 vertex shaders):
instructions in affected programs:     4420 -> 3194 (-27.74%)

On BDW (with scalar vertex shaders):
instructions in affected programs:     12756 -> 6726 (-47.27%)

Implementing a general matrix chain ordering is harder (or at least
tedious) because of having to walk the GLSL IR to create a list of
multiplicands. I'm guessing that this patch handles 90+% of cases, but
of course to tell definitively you'd have to implement the general
thing.
---
 src/glsl/opt_algebraic.cpp | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/src/glsl/opt_algebraic.cpp b/src/glsl/opt_algebraic.cpp
index 98c852a..a940d2f 100644
--- a/src/glsl/opt_algebraic.cpp
+++ b/src/glsl/opt_algebraic.cpp
@@ -290,6 +290,20 @@ ir_algebraic_visitor::handle_expression(ir_expression *ir)
    ir_expression *op_expr[4] = {NULL, NULL, NULL, NULL};
    unsigned int i;
 
+   if (ir->operation == ir_binop_mul &&
+       ir->operands[0]->type->is_matrix() &&
+       ir->operands[1]->type->is_vector()) {
+      ir_expression *matrix_mul = ir->operands[0]->as_expression();
+
+      if (matrix_mul && matrix_mul->operation == ir_binop_mul &&
+         matrix_mul->operands[0]->type->is_matrix() &&
+         matrix_mul->operands[1]->type->is_matrix()) {
+
+         return mul(matrix_mul->operands[0],
+                    mul(matrix_mul->operands[1], ir->operands[1]));
+      }
+   }
+
    assert(ir->get_num_operands() <= 4);
    for (i = 0; i < ir->get_num_operands(); i++) {
       if (ir->operands[i]->type->is_matrix())
-- 
2.0.5