------- Comment #10 from rguenther at suse dot de 2009-01-14 20:51 ------- Subject: Re: huge performance regression on EEMBC bitmnp01
On Wed, 14 Jan 2009, amylaar at gcc dot gnu dot org wrote: > I think the disregard for conditional execution opportunities and the > assumption that phi nodes have no execution cost are two separate issues. > I'd like to address the latter first, because it causes exponential code and > execution time growth. > > A phi node joining two constants has at least the cost of a constant load. > A phi node joining two different variables which are initialized by a graph > with constant leafs costs at least a reg-reg copy on one arm, plus the cost > of its parents if these are needed solely for this phi node. > > Therefore, if an expression is only partially anticipatable, we should compare > the cost of any phi node needed to compute it early with the estimated > likelyhod that such a computatatio, once done, is actually needed, multiplied > with the cost of the replaced operation. > > Can we use edge probabilities inside tree-pre to calculate execution > probabilities? > > Can we calculate the cost of replaced expressions? You would completely underestimate the optimization opportunities PRE unleashes. Richard. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38785
