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Re: Patch to Avoid Bad Prefetching


Hi,

> An estimate of 10 iterations can lead to a serious degradation in
> performance if the actual trip count turns out to be less than 10 and
> less than the ahead distance.

so just require that the number of iterations is at least 3*ahead (I
think prefetching would not be useful for such loops with just a few
iterations, anyway), or possibly disable the prefetching completely if
we do not have a reasonable estimate (i.e., if
estimate_number_of_iterations fails).

> However, the user can always override that conservative estimate by
> setting the prefetch-unknown-trip-count parameter that I am proposing.
> It seems to me that setting this parameter provides the programmer with
> an easy way to give a hint about the trip count if he wants. 

I think programmers are even less likely to play with the hundreds
of options gcc provides than to use profile feedback,

Zdenek


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