[cygnus.project.sid] branch probability hinting

[Jeff Sturm]

On Tue, 6 Mar 2001, Boehm, Hans wrote:
> I know very little about the current gcc back end.  I'll believe that a
> 50-50 hint currently doesn't help.  But it would be nice to design this
> facility so that the right information could be supplied.

I agree.  It appears that the current backend does support a range of
probabilities, among which __builtin_expect can only select
PROB_VERY_LIKELY or PROB_VERY_UNLIKELY.  Perhaps this should be mentioned
on the GCC developers list.  I don't think many backend hackers read the
java list.

There is also a comment that the register allocator ought to consider
basic block execution counts.  Since a new experimental allocator is now
in development, there is a chance that branch probabilities can soon
become more useful.

Jeff