On 14/03/11 22:22, Thomas Klimpel wrote:
My current workaround for
k = modulo(m,n);
in places where I'm unable to work around the missing "modulo" functionality of C/C++ is to write
k = ((m%n)+n)%n
Be careful; not only does that perform two divisions (which is slow), it also gives the wrong answer when m and n are big enough.
Based on the information from Kim Barrett,
k = m >= 0 ? m%n : (m%n) + n;
should also work (that's a bit longer than the old form, but looks more efficient, and I could write a "modulo" template encapsulating this functionality for my personal use). If I'm not mistaken, this would be the most efficient workaround in case of XInt.
I tend to use r = m%n; if (r < 0) r += n; which the compiler is more likely to implement using a conditional move, rather than a branch (but it does have the issue that it's not an expression). I did some tests with: int m1(int m, int n) { return ((m%n)+n)%n; } int m2(int m, int n) { return m >= 0 ? m%n : (m%n) + n; } int m3(int m, int n) { int r = m%n; if (r < 0) r += n; return r; } icc 11.1 compiles m2 and m3 to essentially the same code. gcc 4.5.2's version of m2 is longer than m3 and contains a branch (-O3 in all cases). For both compilers m1 contains two divisions (there's no way to optimize it down to one). John Bytheway