precision - sine result depends on C++ compiler used

Question

I use the two following C++ compilers:

• cl.exe : Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24210 for x86
• g++ : g++ (Ubuntu 5.2.1-22ubuntu2) 5.2.1 20151010

When using the built-in sine function, I get different results. This is not critical, but sometimes results are too significants for my use. Here is an example with a 'hard-coded' value:

printf("%f
", sin(5451939907183506432.0));

Result with cl.exe:

0.528463

Result with g++:

0.522491

I know that g++'s result is more accurate and that I could use an additional library to get this same result, but that's not my point here. I would really understand what happens here: why is cl.exe that wrong?

Funny thing, if I apply a modulo of (2 * pi) on the param, then I get the same result than g++...

[EDIT] Just because my example looks crazy for some of you: this is a part of a pseudorandom number generator. It is not important to know if the result of the sine is accurate or not: we just need it to give some result.

question from:https://stackoverflow.com/questions/46711285/sine-result-depends-on-c-compiler-used

Answer 1

I think Sam's comment is closest to the mark. Whereas you're using a recentish version of GCC/glibc, which implements sin() in software (calculated at compile time for the literal in question), cl.exe for x86 likely uses the fsin instruction. The latter can be very imprecise, as described in the Random ASCII blog post, "Intel Underestimates Error Bounds by 1.3 quintillion".

Part of the problem with your example in particular is that Intel uses an imprecise approximation of pi when doing range reduction:

When doing range reduction from double-precision (53-bit mantissa) pi the results will have about 13 bits of precision (66 minus 53), for an error of up to 2^40 ULPs (53 minus 13).