it might worth extend __builtin_expect to take more parameters: 1) argument to specify actual probability: __builtin_expect (x, 10, 0.6) 2) a more general way of doing this is to allow specifying multiple values, and the probability is determined by # of occurances: __builtin_expect (x, 10, 10, 20) --> tells compiler x is expected to be 10 66% of the time, and 33% of time with value twenty. 3) a special value can be reserved to indicate if the branch is predictable or not.
David On Thu, Sep 26, 2013 at 3:27 PM, Jan Hubicka <hubi...@ucw.cz> wrote: >> Hi, >> >> Current default probably for builtin_expect is 0.9996. >> This makes the freq of unlikely bb very low (4), which >> suppresses the inlining of any calls within those bb. >> >> We used FDO data to measure the branch probably for >> the branch annotated with builtin_expert. >> For google >> internal benchmarks, the weight average >> (the profile count value as the weight) is 0.9081. >> >> Linux kernel is another program that is heavily annotated >> with builtin-expert. We measured its weight average as 0.8717, >> using google search as >> the workload. > > This is interesting. I was always unsure if programmers use builtin_expect > more often to mark an impossible paths (as those leading to crash) or just > unlikely paths. Your data seems to suggest the second. > > We probably also ought to get analyze_brprob working again. It was always > useful to get such a data. >> >> >> This patch sets the alternate hirate probability for >> builtin_expert >> to 90%. With the alternate hirate, we measured performance >> improvement for google >> benchmarks and Linux kernel. >> >> >> -Rong >> 2013-09-26 Rong Xu <x...@google.com> >> >> * params.def (DEFPARAM): New. >> * params.def: New. >> * predict.c (tree_predict_by_opcode): Alternate >> probablity hirate for builtin_expect. > > This also seems resonable for mainline. Please add a comment > to the parameter explaining how the value was determined. > Please also add invoke.texi documentation. > > For patches that seems resonable for mainline in FDO/IPA area, > i would apprechiate if you just added me to CC, so I do not lose > track of them. > Honza
