Fast way to remove bits from a ulong

Question

I want to remove bits from a 64 bit string (represented by a unsigned long). I could do this with a sequence of mask and shift operations, or iterate over each bit as in the cod

Answer 1

There is a very good site that has been around for a long time called Bit Twiddling Hacks. It has a number of fast algorithms for bit manipulation. One you might want to look at (and I am copying verbatim here, this is not my own work) is this algorithm:

Conditionally set or clear bits without branching

bool f;         // conditional flag
unsigned int m; // the bit mask
unsigned int w; // the word to modify:  if (f) w |= m; else w &= ~m; 

w ^= (-f ^ w) & m;

// OR, for superscalar CPUs:
w = (w & ~m) | (-f & m);

On some architectures, the lack of branching can more than make up for what appears to be twice as many operations. For instance, informal speed tests on an AMD Athlon™ XP 2100+ indicated it was 5-10% faster. An Intel Core 2 Duo ran the superscalar version about 16% faster than the first. Glenn Slayden informed me of the first expression on December 11, 2003. Marco Yu shared the superscalar version with me on April 3, 2007 and alerted me to a typo 2 days later.