Ten Ways to Check if an Integer Is a Power Of Two in C

Re:Ten Ways to Check if an Integer Is a Power Of Two in C by vilya on Tuesday July 26, 2011 @11:20AM (#2807728)

FWIW, in gcc/g++ there are compiler intrinsics which (should) map to that instruction on CPUs where it's available: __builtin_popcnt, __builtin_popcountl and __builtin_popcountll for unsigned ints, unsigned longs and unsigned long longs respectively. Visual C++ provides equivalent functions for Windows (but I can't remember what they're called).

It does seem odd that the article misses this approach out.

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Re:Ten Ways to Check if an Integer Is a Power Of Two in C by dasmoth on Tuesday July 26, 2011 @11:40AM (#2807829)

Unfortunately __builtin_popcnt isn't emitting a popcnt instruction with the GCC I've got here, even using -msse4.2. I believe that very recent GCC does get this right.

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Re:Ten Ways to Check if an Integer Is a Power Of Two in C by neckbeard on Tuesday July 26, 2011 @07:29PM (#2809961)

GCC 4.4 isn't very recent, but it generates a popcnt with -msse4.2.

GCC 4.5, using popcnt on my Core-i7 860 takes the trivial loop mentioned using "Complement and Compare" from ~10.5s to ~7.5s

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