Hi, Florian. >>>There is a movement to replace table-driven stack unwinding with a >>>conditional branch after most function calls... I am sorry I don't understand its meaning, forgive my poor knowledge of dynamic prediction, I am reading relative materials these days, could you explain a little more for this sentence, or give me some keywords for google ?
>>> programmers tend to misuse __builtin_expect .. I believe you must have strong factual basis saying such words, I can quite accept the CPU manufacturer's ignoring branch prediction hints for some reasons, but not this, One somewhat contrived example: if most people of a country are blind, then the government forbid the car to provide a safe road, may a better choice is asking the blind people go to hospital to cure the eye. Thanks again. 2018-08-15 17:26 GMT+08:00, Florian Weimer <fwei...@redhat.com>: > On 08/14/2018 03:36 PM, 2016 quekong wrote: > >> And Intel seems don't want to talk about it any more, because the latest >> material I found within Intel Document was written about ten years ago. > > Since branch prediction is nowadays a security feature, I doubt you will > see detailed public documentation from any processor manufacturer. > >> I know static branch prediction is (far?) less important than dynamic, >> but >> in quite a few situations, CPU will be completely lost and >> programmers(with >> compiler) are usually the best guide. Of course these situations are >> usually not performance bottleneck, because once a branch is frequently >> executed, the dynamic predictor will capture it. > > There is a movement to replace table-driven stack unwinding with a > conditional branch after most function calls, and those branches should > predict really well statically. (And so would return value checks for > almost all POSIX functions.) Statically predicting those branches as > not taken may free up branch prediction resources for other uses. > > On the other hand, programmers tend to misuse __builtin_expect to say > that something is unlikely to happen, while it can happen quite > regularly during certain executions of the program. So if > __builtin_expect results in static hints from the compiler, the CPU will > be better off ignoring those hints in most cases. > > Thanks, > Florian >
