Author: David Rasmussen
Date: 07:46:27 01/13/02
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On January 13, 2002 at 10:22:52, Robert Hyatt wrote: > > >I've always sais that the people that say that C is just as fast as assembly >simply don't know "how to do it right". Cray has the _best_ optimizing >FORTRAN compiler on the planet, _bar_ _none_. Yet the assembly version of >Cray Blitz was roughly 5x faster when we started and even at the end of the >project it was still 3x faster. > >There are several reasons: > >1. You know more about the program than the compiler. No bounds-checking >on "switch" type statements is necessary. > >2. you know whether a value can be positive, negative, or both. The >compiler can't. > >3. you know whether a value can exceed (say) 127 or not. The compiler can't. > >4. You know how many registers the CPU has and can design code around that, >while C doesn't give you such control. > >5. you know exactly which registers procedure "x" will destroy, so you don't >have to save everything before calling it. If you are careful, you don't have >to save _anything_. > This last point is not valid I think. The compiler also knows which registers procedure "x" destroys. Or it can, anyway. I've always thought that features should be added to C/C++ so that the programmer is able to tell the compiler more about the types, values and functions, and that this information could and should be used for optimizing and debugging purposes. For example, I could tell the compiler that my Square type (which is an int), is always 0-63. If it can use this information for optimizing, good. And in debug builds, it could check whether it is satisfied. I know assert can do this last thing, but the compiler doesnt use the information that one calls assert with. There should be keywords "invariant" and "precondition" and "postcondition", I think. That would be really cool. MSVC++ have such an extension, sort of. But I don't know if it works well or not. I haven t tried using it. /David
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