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, > > Do you know how huge is it? Any data on this? For GCC, is this "huge" > memory consumption is critical enough, and there aren't any other else > consuming even more memory?
no, not really (I haven't worked on this for a few years now), although of course I did some measurements when ivopts were created. Feel free to experiment with that, if it turned out that the memory consumption and extra time spent by it is negligible, it would be a nice cleanup. Zdenek
