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? > > I suppose Zdenek may remember.
computation_cost actually expands the expression to RTL. Since cost estimates are computed a lot in ivopts, using it directly would require a huge amount of memory, Zdenek
