gslshell(lua based) returns 0/0 ==0/0 true

Question

In gsl-shell ( a luajit based interface to gsl), I got

> 0/0==0/0
true

while it is false from Lua 5.1. Why is it? Does this discrepancy come from Luajit vs Lua, GSL library, or the gsl-shell interface?

Answer 1

Quick testing indicates that this is gsl-shell. lua5.1 and lua5.2 (from debian), lua 5.1 (self-compiled on CentOS 5) and luajit (from git master, self-compiled on debian) all give the proper behaviour (and return nan when the result of 0/0 is printed directly. The current build of gsl-shell (for Windows) has the behaviour you indicated and returns -2147483648 when the result of 0/0 is printed.

I can't explain it (or even theorize about it really) but it certainly seems to be a gsl-shell problem.

Answer 2

Asked on gsl-shell mailing list. found that

That's because gsl-shell is compiled with the -ffast-math flag, which(surprisingly!) disables nans and infs.1

Per GCC manual, I guess this option should not be turned on for such a numerical application like GSL-Shell.

*-ffast-math Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and fcx-limited-range. This option causes the preprocessor macro FAST_MATH to be defined. This option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.*