Hi, > Core2 follows a similar pattern, although it's not seeing any > slowdown in the "no deps, predictable, jmp" case like K8 does. > > Any comments? (please cc me) Should gcc be using conditional jumps > more often eg. in the case of __builtin_expect())?
The problem is that in general GCC's branch prediction algorithms are very poor on predicting predictability of branch: they are pretty good on guessing outcome but that is. Only cases we do so quite reliably IMO are: 1) loop branches that are not interesting for cmov conversion 2) branches leading to noreturn calls, also not interesting 3) builtin_expect mentioned. 4) when profile feedback is around to some degree (ie we know when the branch is very likely or very unlikely. We don't simulate what hardware will do on it). I guess we can implement the machinery for 3 and 4 (in fact once I played adding EDGE_PREDICTABLE_P predicate that basically tested if the esimated probability of branch is <5% or >95%) but never got really noticeable improvements out of it and gave up. It was before Core2 times, so it might be helping now. But it needs updating for backend cost interface as ifcvt is bit inflexible in this. I had BRANCH_COST and PREDICTABLE_BRANCH_COST macros. Honza
