A question about loop ivopt

[Jiangning Liu]

Hi,

Why can't we replace function force_expr_to_var_cost directly with function
computation_cost in tree-ssa-loop-ivopt.c?

Actually I think it is inaccurate for the current recursive algorithm in
force_expr_to_var_cost to estimate expr cost. Instead computation_cost can
count some back-end factors and make the estimation more accurate.

For example, using computation_cost, we may check whether back-ends can tie
some modes transformation expressed by SUBREG or not. If we use
force_expr_to_var_cost, some more computations around type
promotion/demotion would increase the cost estimated.

Looking at the algorithm in force_expr_to_var_cost, it is just to analyze
the operator in the expression and give estimation. Should it be the same as
expanding EXPR to RTX and give estimation like in computation_cost?

Any thoughts?

Thanks,
-Jiangning