[cygnus.project.sid] branch probability hinting

Tue Mar 6 03:19:00 GMT 2001

Tom Tromey writes:
 > >> Also please everyone note that the compiler already knows that
 > >> if (p == NULL)
 > >> is unlikely
 > 
 > Hans> Presumably it will take a hint that it's likely, e.g. when
 > Hans> scanning a sparse chained hash table?
 > 
 > I don't know.
I think it's worth doing the experiment. I will if I have a moment to
spare.
 > Hans> How do you tell gcc that the branch probability is around 50-50
 > Hans> if it already believes otherwise? (E.g. half the child pointers
 > Hans> in a binary tree are NULL.)
I don't think this makes any sense, given the way gcc does this. It
can either prefer one or the other; AFAIK there is no way to be
ambivalent!
 > I don't think there is a way. Maybe this should be reported as a gcc
 > bug. Maybe you ought to be able to tell __builtin_expect the
 > probability that the branch is taken.
Cool. It might be hard to make use of this.
Andrew.