int mytest2(SV* sv_vec, unsigned int bit) {

    STRLEN vecbytes; // Length of vector in bytes
    unsigned int *vec = (unsigned int*) SvPV(sv_vec, vecbytes);

    if( bit / 8 >= vecbytes) return 0; // Check in range

    vec[ bit / 32 ] |= ( 1U << ( bit % 32 ) ); // Set bit (CHANGES $vector)

    return 1;
}


int mytest3(SV* sv_vec, unsigned int bit) {

    STRLEN vecbytes; // Length of vector in bytes
    unsigned __int64 *vec = (unsigned __int64 *) SvPV(sv_vec, vecbytes);

    if( bit / 8 >= vecbytes) return 0; // Check in range

    vec[ bit / 64] |= ( 1ULL << ( bit % 64 ) ); // Set bit (CHANGES $vector)

    return 1;
}

##</code><code>##

C:\test>903727.pl
         Rate qwords  bytes dwords    vec
qwords 3.05/s     --    -2%    -2%   -25%
bytes  3.13/s     2%     --    -0%   -23%
dwords 3.13/s     2%     0%     --   -23%
vec    7.70/s   151%   149%   147%     --

C:\test>903727.pl
         Rate qwords  bytes dwords    vec
dwords 3.10/s     --    -0%    -1%   -60%
bytes  3.11/s     0%     --    -0%   -60%
qwords 3.13/s     1%     0%     --   -59%
vec    7.69/s   148%   147%   146%     --