Author: Robert Hyatt
Date: 05:23:47 07/05/01
Go up one level in this thread
On July 05, 2001 at 08:07:58, Vincent Diepeveen wrote: >Hi, risking to be off topic, >here a small attempt to adress the intel c++ compiler 5.01 build 15 > >I tried several options with it for DIEP and figured out that >it is 1.5% slower for me as msvc 6.0, nevertheless that's very >good considering that 5.00 completely crashed. > >this version does *not* crash. it produces an exe which runs and >which doesn't crash. > >I measured however in >another program of mine some weird things in floating point >unit calculations. > >An AI program of mine, >which fiddles with parameters (neural based) is regrettably >no longer deterministic. When i use optimizations > -O3 and some optimizations that allow Pentiumpro instructions and > MMX and SSE and similar instructions, then > i get completely different results. > >Especially the function > > y = ax^2 + bx + c > >which i use everywhere in the network, is giving different results >when using different optimizations. This is a bad thing! > >I cannot explain it but i feel that it has to do with the >fact that all these values are 'float'. > >Then when casting them back to parameters in 1/1000 pawn, i >get differences in testruns of up to 1 millipawn for the average >calculation! Some IEEE FP hardware has the ability to round or truncate on the LSB. Perhaps Intel is doing that opposite to MSVC? Another explanation is a program bug that has an unitialized variable. High levels of optimization often cause such bugs to produce non-deterministic results. > >This is WEIRD!!!!!!!!!!!!!!!!!!!!!!!! > >I cannot explain it!!!!!!!!!!!!!1 > >So i cannot use this compiler. Note that i find it pretty bad that >a compiler which was already very good some time ago, that it still >can't beat at a P3-800, which is an intel chip, the visual c++ compiler. > >It was 0.5% slower this intel c++ some years ago as visual. Latest visual >c++ compiler is 1.5% faster, despite that it has all kind of instructions >nowadays this intel c++! why is this "pretty bad"? MS has a good compiler group and they are selling MSVC to make money. That is an incentive to produce the best compiler they can. Intel isn't competing with that at all... > >Using all those instructions it's NOT giving deterministic results. >If i turn off those P3 instructions, then it is of course going to >be hell slower as visual. > >Nevertheless it's a good thing that it no longer crashes like version 5.00 > >Best regards, >Vincent
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