Author: Matt Taylor
Date: 19:28:16 12/25/02
Go up one level in this thread
On December 25, 2002 at 17:08:27, Gerd Isenberg wrote: >On December 25, 2002 at 14:13:47, Matt Taylor wrote: > >>On December 25, 2002 at 04:17:34, Dave Gomboc wrote: >> >>>On December 23, 2002 at 21:33:50, Matt Taylor wrote: >>> >>>>On December 23, 2002 at 21:29:08, Arshad F. Syed wrote: >>>> >>>>>Is there anyone here who has actually converted their C code to C++ while >>>>>keeping the NPS the same or even managed to improve upon it? Sune did mention >>>>>that he did so only at the cost of 15% degradation in speed. Why do people even >>>>>bother wasting their time with C++ for a chess program when it inevitably leads >>>>>to taking a hit in NPS? Regardless, of all the advice to simply inline the >>>>>functions, I have yet to see a living example of chess program code which >>>>>performed faster in C++. >>>>> >>>>>Regards, >>>>>Arshad >>>> >>>>The biggest hit you take in C++ is virtual functions, and inevitably people use >>>>them. The virtual functions are very damaging due to the fact that they're >>>>implemented with a function pointer, and at the machine level it translates into >>>>an indirect call. The indirect calls branch mispredict because the CPU can't >>>>figure out ahead-of-time where you're calling to, and branch mispredicts are >>>>expensive. >>> >>>Please excuse my ignorance, but where is the branch? It's an indirect jump, >>>isn't it? There's nothing optional about it, it must be followed. Is it that I >>>am not familiar enough with the terminology? e.g. I am thinking that by "branch >>>prediction" you may be including any operation that moves the program counter in >>>any way other than advancing sequentially to the next instruction. >>> >>>In any case, I'm confident that if you have a serious inheritance hierarchy (as >>>opposed to a toy example) you're not going to lose any sleep over the virtual >>>function call. Many if-then-elses aren't efficient either. >>> >>>Dave >> >>A branch is anything that changes the flow of execution from the pc++ scheme. An >>unconditional jump/call is still a branch. >> >>The problem that superscalar processors have is that they're executing multiple >>things simultaneously, and often to predict the target of a branch, you need to >>have completed execution of instructions almost immediately prior. >> >>Consider the example I gave in a previous post: >>mov esi, [ecx+SomeFuncPtr] >>call esi >>call esi >> >>This is an indirect call. The machine loads the function pointer into the esi >>register, and then the contents of that register are used as the target of the >>unconditional branch. The problem is that the machine doesn't get around to >>storing into esi until a cycle or so before the call, and the pipeline is some >>20+ cycles deep (depending on processor). After it decodes the call instruction, >>it needs to know where to go next, but it has to stall for those 20 cycles until >>it knows what gets written to esi. >> >>Static prediction assumes that it's not going to branch (in this case). >>Obviously we know that it does, but rather than stall the pipeline for those 20+ >>cycles until they know WHERE it goes to, they choose simply to continue decode >>at the wrong place. (If you make a small function call, this is actually >>marginally advantageous over doing nothing since you've already got the return >>address in the cache.) > >Hi Matt, > >I think i have it now, good explanation. > >But how about: > >mov esi, [ecx+SomeFuncPtr] >// getting and pushing several parameter >call esi > >Aren't the compiler aware to load the register as early as possible to avoid >those stalls and mispredictions? I hope they are. > >Btw. is the branch prediction technique a kind of counter productive for >hyperthreading, decoding instructions that are likely to be mispredicted, rather >than giving another hyperthead a chance? > >Regards, >Gerd > >> >>In the case of an if/then/else, some implementations will work on BOTH branches >>so you don't lose as much. Either way, the if/then/else can predict right, and >>static prediction can NEVER get it right the first time. >> >>-Matt Presumably it won't even try to guess where the indirect call goes except by using the BTB. This is the impression that I got from the manuals, and the manuals are extremely vague about it. You know, it is interesting that you point out Hyperthreading. HT could solve a lot of branching issues simply by not worrying about them -- the same mentality of Sparc, but a different answer. If you hit a branch and you're not sure where it will go, you can work on something else until you can be sure. However, in the here and now, I would presume that you are correct. -Matt
This page took 0 seconds to execute
Last modified: Thu, 15 Apr 21 08:11:13 -0700
Current Computer Chess Club Forums at Talkchess. This site by Sean Mintz.