> I thought -Os actually disabled the basic-block reordering, doesn't it? Not in current gcc head no (just verified by stepping through) > > And I thought it did that exactly because it generates bigger code and > much worse I$ patterns (ie you have a lot of "conditional branch to other > place and then unconditional branch back" instead of "conditional branch > over the non-taken code". > > Also, I think we've had about as much good luck with guessing > "likely/unlikely" as we've had with "inline" ;) That's true. But if you look at the default heuristics that gcc has (gcc/predict.def in the gcc sources) like == NULL, < 0, branch guarding etc. I would expect a lot of them to DTRT for the kernel. Honza at some point even fixed goto to be unlikely after I complained :) > Sadly, apart from some of the "never happens" error cases, the kernel > doesn't tend to have lots of nice patterns. We have almost no loops (well, > there are loops all over, but most of them we hopefully just loop over > once or twice in any good situation), and few really predictable things. That actually makes us well suited to gcc, it has a relatively poor loop optimizer compared to other compilers ;-) > Or rather, they can easily be very predictable under one particular load, > and the totally the other way around under another .. Yes that is why we got good branch predictors in CPUs I guess. -Andi -- ak@xxxxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
