Setting a single bit

val |= 1u << n;

Clearing a bit

val &= ~(1u << n);

Toggling a bit

val ^= 1u << n;

Setting a bit to a specified value

One strategy is to clear the bit first and then set it to x in a second step if necessary.

val &= ~(1u << n);
val |= x << n;

It’s possible to combine these two lines.

val = (val & ~(1u << n)) | (x << n);

Checking a bit

bit = (val >> n) & 1u;

Swapping bytes

Swapping bytes is very useful to convert between Little- and Big-Endian. Here’s an example for a byte swap on a 64bit integer.

uint64 bswap(uint64 x)
{
    return    ((x & 0xff00000000000000u) >> 56u)
            | ((x & 0x00ff000000000000u) >> 40u)
            | ((x & 0x0000ff0000000000u) >> 24u)
            | ((x & 0x000000ff00000000u) >>  8u)
            | ((x & 0x00000000ff000000u) <<  8u)
            | ((x & 0x0000000000ff0000u) << 24u)
            | ((x & 0x000000000000ff00u) << 40u)
            | ((x & 0x00000000000000ffu) << 56u);
}

Interestingly most compilers are able to compile this code with a single machine instruction, so no need for assembler to speed this up.

bswap:
        mov     rax, rdi
        bswap   rax
        ret