New submission from STINNER Victor: Attached patch is an attempt to use the GCC/Clang __builtin_expect() attribute to provide the compiler with branch prediction information, it adds _Py_likely() and _Py_unlikely() macros, similar to the Linux kernel macros.
I always wanted to experiment this change. I opened this issue to collect information and benchmark results to take a decision. I doubt that the change will really make Python faster, there is a risk of modifying a lot of code for no value, or worse: make Python slower. Extract of GCC doc: "as programmers are notoriously bad at predicting how their programs actually perform." My patch marks error cases as unlikely. A profiler like Linux perf should be used to check which branches are less likely, but I tried to use my guesses to expeeriment a first change. Since there is a risk that using such macro makes the code slower, or has no effect, I tried to restrict changes to the hot code and the most common functions/macros: * Py_EnterRecursiveCall() * _PyArg_NoKeywords() * Py_DECREF()... not sure about this case, is it really more likely that refcnt != 0 than refcnt==0? * Functions to call functions: _PyObject_FastCallKeywords(), fast_function(), etc. * etc. I'm not sure about the methodology to benchmark such patch neither. Is it worth it to benchmark a Python compiled without PGO? By the way, does GCC/Clang use __builtin_expect() information when Python is compiled with PGO? If PGO ignores this information, it would be better to not use it, since I now strongly recommend to use PGO. Otherwise, Python performance is too random because of the joy of code placement. Some links on __builtin_expect(): * http://blog.man7.org/2012/10/how-much-do-builtinexpect-likely-and.html * http://stackoverflow.com/questions/109710/likely-unlikely-macros-in-the-linux-kernel-how-do-they-work-whats-their * https://news.ycombinator.com/item?id=9411227 ---------- files: likely.patch keywords: patch messages: 287112 nosy: haypo, serhiy.storchaka priority: normal severity: normal status: open title: Experiment usage of likely/unlikely in CPython core type: performance versions: Python 3.7 Added file: http://bugs.python.org/file46539/likely.patch _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue29461> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
