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Re: A question about loop ivopt
- From: Richard Guenther <richard dot guenther at gmail dot com>
- To: Jiangning Liu <jiangning dot liu at arm dot com>
- Cc: gcc at gcc dot gnu dot org, Zdenek Dvorak <ook at ucw dot cz>
- Date: Tue, 15 May 2012 11:11:45 +0200
- Subject: Re: A question about loop ivopt
- References: <4fb1e3c9.6aa2b60a.59b3.ffff8598SMTPIN_ADDED@mx.google.com>
On Tue, May 15, 2012 at 7:05 AM, Jiangning Liu <email@example.com> wrote:
> 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.