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Re: A question about loop ivopt


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


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