Different math rounding behaviour between Linux, Mac OS X and Windows

Question

HI,

I developed some mixed C/C++ code, with some intensive numerical calculations. When compiled in Linux and Mac OS X I get very similar results after the simulation ends. In Windows the program compiles as well but I get very different results and sometimes the program does not seem to work.

I used GNU compilers in all systems. Some friend recommend me to add -frounding-math and now the windows version seems to work more stable, but Linux and Os X, their results, do not change at all.

Could you recommend another options to get more concordance between Win and Linux/OSX versions?

Thanks

P.D. I also tried -O0 (no optimizations) and specified -m32

Answer 1

I can't speak to the implementation in Windows, but Intel chips contain 80-bit floating point registers, and can give greater precision than that specified in the IEEE-754 floating point standard. You can try calling this routine in the main() of your application (on Intel chip platforms):

inline void fpu_round_to_IEEE_double()
{
   unsigned short cw = 0;
   _FPU_GETCW(cw);        // Get the FPU control word
   cw &= ~_FPU_EXTENDED;  // mask out '80-bit' register precision
   cw |= _FPU_DOUBLE;     // Mask in '64-bit' register precision
   _FPU_SETCW(cw);        // Set the FPU control word
}

I think this is distinct from the rounding modes discussed by @Alok.