Using bitset in place of using hand written bit manipulation code?

Question

Is there any performance loss/gain using bitset in place where hand written?

How to build the following using a bitset at runtime

• make all the bits between 2 and 5 as zero i.e., 11110011.

Answer 1

For a simple example such as the above, a decent hand-coded solution will be faster than using bitset, but the performance differences in either case would be small.

Answer 2

The golden rule:

Don't optimise prematurely!

Bitset will in 99% of cases, be fast enough, and has the advantage of being a common concept such that it's both more readable, and less prone to implementation errors. Don't simply assume that your code will obviously need the speed increase; write the code using bitset, profile your application, and see if:

1. It's fast enough as it is; and
2. If it's not fast enough, does it really spend the majority of its time doing bit operations?

Per the 80-20 rule, chances are that you'll get a much greater return on making some other bit of code faster. And hey, if it turns out that you do need to improve the bit-twiddling speed, at least you have some decent baseline figures to use in order to show that you r solution really is faster than the default (which you'd need anyway if you wanted to optimise for performance).

Answer 3

The easiest solution to your second question would be to use another bitset.

void makebitszero(bitset<8>& b) {
  // Everything but bits 3 and 4 (between 2 and 5).
  static const bitset<8> mask = ~bitset<8>(12);
  b &= mask;
}

It takes a bit of math to come up with an expression for mask given two bit positions.

[edit] Ok, here's the math. The trick is that (1UL << X) -1 is a sequence of X ones. E.g. 3 => 00000111. Hence, (1<<5) - (1<<3) = 00011111 - 00000111 -1 + 1 = 00011000 (bits 3 and 4). Thus in code:

template<int i, int j, int N> 
void makeBitsZero(bitset<N>& b) {
  // Everything from bit i up to but not including bit j (i < j)
  static const bitset<N> mask = ~bitset<N>(1UL<<j) - (1UL<<i));
  b &= mask;
}