Author: Vincent Diepeveen
Date: 13:01:51 07/05/01
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On July 05, 2001 at 08:23:47, Robert Hyatt wrote: >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? i feel it has to do with that it is using those MMX registers/ SSE somehow. on paper the round off in this testset should be as the default optimizations show it. It is giving an result which is far from it. also casting could cause this round off error. most important is not the value it gives, but the fact that other optimizations give different values! >Another explanation is a program bug that has an unitialized variable. No it is not that. we talk about 5 lines of code here which produce the round off error. y = ax^2 + bx + c, where the parameters of the function are giving already a few of those variables. >High levels of optimization often cause such bugs to produce non-deterministic >results. I know i have intensively checked the code again, but it wasn't a bug. it's only a few lines anyway. > > > >> >>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... it is bad because m$ in all those years only got 2% faster for me, whereas intel c++ won less as that. So from 486 code to IA32 model code it's now like 1% faster as it used to be for my chessprogram? 460kb code size. Huge number of registers are in existance now on all x86 processors, can't compiler assume register renaming a bit more? Well i'm no compiler expert of course, lucky i am not, but i find a speedup of 1% over all those years pretty bad! The size of the executable is also way bigger as gcc and visual produce exe. In fact gcc produces the smallest executable which i like a lot! If a function must be inlined then i can put inline in front of it, no problem! > > > > >> >>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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