Algorithm for copying N bits at arbitrary position from one int to another

Question

An interesting problem I\'ve been pondering the past few days is how to copy one integer\'s bits into another integer at a given position in the destination integer. So, for exa

Answer 1

I don't think it can be done more efficient unless you write assembler.

You can improve the readability and solve your overflow problem changing some little things:

int setbits2(int destination, int source, int at, int numbits)
{
    // int mask = ((1LL<>(sizeof(int)*8-numbits))<

More efficient assembler version (VC++):

// 3rd aproach
#define INT_SIZE 32;
int setbits3(int destination, int source, int at, int numbits)
{ __asm {
    mov ecx, INT_SIZE
    sub ecx, numbits
    or  eax, -1
    shr eax, cl
    mov ecx, at
    shl eax, cl // mask == eax
    mov ebx, eax
    not eax
    and eax, destination
    mov edx, source
    shl edx, cl
    and edx, ebx
    or  eax, edx
}}

• 1st aproach: Slower on 32bit architecture
• 2nd aproach: (~0u) and (sizeof(int)*8) are calculated at compile time, so they don't charge any cost.
• 3rd aproach: You save 3 ops (memory accesses) writing it in assembler but you will need to write ifdefs if you want to make it portable.