On 02/11/2015 02:54 PM, Matt Turner wrote:
> We propagate negations to the right-most leaves of the multiplication
> expression trees:
>
> - mul(neg(x), neg(y)) -> mul(x, y)
> - mul(neg(x), y) -> neg(mul(x, y))
> - mul(x, neg(y)) -> neg(mul(x, y))
>
> total instructions in shared programs: 5943123 -> 5937229 (-0.10%)
> instructions in affected programs: 868221 -> 862327 (-0.68%)
> helped: 4518
> HURT: 356
> GAINED: 1
This seems very counter intuitive. Why does this help? It seems like
the negate modifier on the multiplication sources should be free... how
do we get cheaper than free? :)
> ---
> 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 7bc65da..aecd6e1 100644
> --- a/src/glsl/opt_algebraic.cpp
> +++ b/src/glsl/opt_algebraic.cpp
> @@ -514,6 +514,20 @@ ir_algebraic_visitor::handle_expression(ir_expression
> *ir)
> if (is_vec_negative_one(op_const[1]))
> return neg(ir->operands[0]);
>
> + if (op_expr[0] && op_expr[0]->operation == ir_unop_neg) {
> + if (op_expr[1] && op_expr[1]->operation == ir_unop_neg) {
> + /* mul(neg(x), neg(y)) -> mul(x, y) */
> + return mul(op_expr[0]->operands[0], op_expr[1]->operands[0]);
> + }
> +
> + /* mul(neg(x), y) -> neg(mul(x, y)) */
> + return neg(mul(op_expr[0]->operands[0], ir->operands[1]));
> + }
> +
> + /* mul(x, neg(y)) -> neg(mul(x, y)) */
> + if (op_expr[1] && op_expr[1]->operation == ir_unop_neg) {
> + return neg(mul(ir->operands[0], op_expr[1]->operands[0]));
> + }
>
> /* Reassociate multiplication of constants so that we can do
> * constant folding.
>
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