Hi Eljay and others, just a few concluding remarks. On Tuesday 17 May 2005 13:55, Eljay Love-Jensen wrote: > Hi Peter, > > >Why doesn't the compiler manage to do the same thing for the function > > version, that it apparently does between -O1 and -O3 for the struct > > version? > > It's a pretty tall order. You are saying "Ignore what I explicitly > programmed the function to do, and inline what I specified as a function > pointer parameter because it will be more efficient". > Well, first I wasn't really thinking so much about what I was _actually_ telling the function (or the compiler) to do. For some reason I had a feeling that the function version should be faster because no class is involved. With a bit of thinking and experimenting I understood the function pointer part, but wasn't sure that that was all. The last optimization step didn't seem that difficult when the pointer was already gone. That's why I asked here. > ..... > Since I come from an assembly language (6502, > 680x0) background, I think of C as a really neat macro assembler. (But I > think of C++ as an object oriented language moreso than a excellent macro > assembler; go figure.) > I am totally on the OO side. My motivation for the function was (as I said above) that somehow I thought it should be faster in a very critical place (4 nested loops). Oh, and the implementation is completely hidden from users of the library I'm working on, of course. > And that School B is the kind of optimization that I expect out of a Java > compiler *AND* a continuously optimizing/re-optimizing JIT JVM. > > You can get some of the School B benefits in School A, if you have a > feedback mechanism in your development process to feed profiling > information back into the compilation process. (Or if your C/C++ code (or > tokenized binary thereof) was interpreted live instead of compiled.) But > even then, I really wouldn't expect* the compiler to ignore what you told > it to do because some impressive wholistic optimization analysis technique. > > * Not that it's impossible; just not what I'd expect. I'd be pleasantly > surprised if it did the kind of optimization that even changes function > signatures to remove (optimize away) parameters (such as the function > pointer parameter specified in your example)! Wow! > I _am_ very impressed with the optimizations gcc4.0.x does. The purpose of this question was not to criticize what the compiler does (or does not). I just wanted to be sure never to fall into the same trap again. So I wanted to fully understand what was happening. >From what I learned I draw the conclusion that the optimization I thought of could be done but that maybe it should not be done. And it should definitely not be expected (more than is usual for optimizations). > Sincerely, > --Eljay Thanks alot for the detailed answer to my question. I'll be staying away from the function pointers now _and_ know why. Best regards, Peter
