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 simulatio

Answer 1

There are four different types of rounding for floating-point numbers: round toward zero, round up, round down, and round to the nearest number. Depending upon compiler/operating system, the default may be different on different systems. For programmatically changing the rounding method, see fesetround. It is specified by C99 standard, but may be available to you.

You can also try -ffloat-store gcc option. This will try to prevent gcc from using 80-bit floating-point values in registers.

Also, if your results change depending upon the rounding method, and the differences are significant, it means that your calculations may not be stable. Please consider doing interval analysis, or using some other method to find the problem. For more information, see How Futile are Mindless Assessments of Roundoff in Floating-Point Computation? (pdf) and The pitfalls of verifying floating-point computations (ACM link, but you can get PDF from a lot of places if that doesn't work for you).