[ Hi Justin, enjoying winter in pgh? 8-] At Sat, 2 Feb 2002 20:04:10 +0000 (UTC), "H . J . Lu" wrote: > Does everyone agree with this? If yes, I can make a patch not to use > branch likely. But on the other hand, "gcc -mips2" will generate code > using branch likely. If branch likely doesn't buy you anything, > shouldn't we change gcc not to generate branch likely instructions? Branch-likely instructions probably _do_ buy you something (at least, slightly less code size) on some CPUs, probably even some CPUs which are still being produced. FYI, somebody, i think it was Mike Stump, submitted a patch to add a flag to disable branch-likely instructions N months ago. It was discussed a little bit, not AFAIK nothing was ever done with it. To quote the MIPS32 instruction set document from the MIPS web site: > Programming Notes: > > [ ... ] > > Software is strongly encouraged to avoid the use of the Branch > Likely instructions, as they will be removed from a future revision > of the MIPS Architecture. > > Some implementations always predict the branch will be taken, so > there is a significant penalty if the branch is not taken. Software > should only use this instruction when there is a very high > probability (98% or more) that the branch will be taken. If the > branch is not likely to be taken or if the probability of a taken > branch is unknown, software is encouraged to use the BGEZAL > instruction instead. If you have a sufficiently high static likelyhood of branch-taken, it may well be worthwhile to use branch-likely even on MIPS32/MIPS64 if it'll save you code space, or if you can determine that not clogging up your branch history / predictor buffers makes your code run faster. However, it should definitely not be the default. Anyway, from where I sit, this should be resolved like: * -mips1 - -mips5 should generate them by default (for strongly-predicted branches), -mips32 & -mips64 should not. (modify that for your favorite flag names, -march or whatever. 8-) * There should be flags to override the defaults either way, so you could say -mips2 -mno-branch-likely, or -mips32 -mbranch-likely. cgd
