Author: Robert Hyatt
Date: 07:43:26 01/14/02
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On January 13, 2002 at 10:46:27, David Rasmussen wrote: >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 You can do that with enumerated types. But the compiler _still_ has to check the bounds on such types when needed, to avoid a "bad assumption"...
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