> > At least on x86 it should also be a good idea to know which way > > the branch is going to go, because it doesn't have explicit branch > > hints, you really want to be able to optimize the cold branch > > predictor case if converting from cmov to conditional branches. > > x86 as of Pentium 4 does have branch hint instruction prefixes, but their use > is somewhat > discouraged: > > from http://softwarecommunity.intel.com/articles/eng/3431.htm: > " > The Pentium? 4 Processor introduced new instructions for adding static hints > to branches. It is > not recommended that a programmer use these instructions, as they add > slightly to the size of the > code and are static hints only. It is best to use a conditional branch in the > manner that the > static predictor expects, rather than adding these branch hints.
GCC has support for this feature, but it has turned out to not gain anything and was disabled by default, since branch reordering stramlines code well enought to match the default predictor behaviour. Same conclusion was done by other compiler teams too, ICC is not generating the hints either. Honza
