[cygnus.project.sid] branch probability hinting

Tue Mar 6 22:30:00 GMT 2001

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