Re: g++ Bug? Why w/ and w/o "O3" options gives different result(g++3.3.3) for this program?

[Ulrich Eckhardt]

Peng Yu wrote:
> I used the following command lines to compile the progrogram below.
> However, the output of the exe files is different. The w/o O3 exe
> produced the corrected result.
> 
> Is this indicate that there is a bug with g++3.3.3's O3 option? Or I
> shouldn't use O3 in this case?

[snipped floating point math]

Two possible reasons:
- the optimization affects when and where intermediate results are stored
in registers and when they are stored in memory. Those storages could have
different precisions, thus leading to more or less loss of precision.
- you rely on undefined behaviour somewhere. Unless you provide a minimal
example (which you should have anyway), I'm too lazy to analyse each and
every line of your program - sorry.

> g++-3.3 -c  -O3  -o main.o main.cc
> g++-3.3   main.o   -o main
> result:
> temp = (0.0134963,0)
> temp = (0.0132417,0)
> 
> g++-3.3 -c   -o main.o main.cc
> g++-3.3   main.o   -o main
> result:
> temp = (0.0140056,0)
> temp = (0.0140056,0)

This looks like normal floating point rounding errors.

> #define N        18//34 //should be greater than (1+s)*2 / Delta_f
> 
> #define L_x      5//10 //in the unit of \lambda\mathit{NA}
> #define L_y      5//10 //in the unit of \lambda\mathit{NA}

Huh? I first thought you were using integer divisions here, but those are
really obfuscating comments, if you ask me. Anyway, use constants and not
defines in C++.

Uli

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