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Re: A question about loop ivopt


On Tue, May 15, 2012 at 7:05 AM, Jiangning Liu <jiangning.liu@arm.com> wrote:
> 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.

Richard.

> Thanks,
> -Jiangning
>
>
>


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