Is there a more efficient way of expanding a char to an uint64_t?

Question

I want to inflate an unsigned char to an uint64_t by repeating each bit 8 times. E.g.

char -> uint64_t
0x00 -> 0x00
0x01 ->         


        

Answer 1

If you're willing to spend 256 * 8 = 2kB of memory on this (i.e. become less efficient in terms of memory, but more efficient in terms of CPU cycles needed), the most efficient way would be to pre-compute a lookup table:

static uint64_t inflate(unsigned char a) {
    static const uint64_t charToUInt64[256] = {
        0x0000000000000000, 0x00000000000000FF, 0x000000000000FF00, 0x000000000000FFFF,
        // ...
    };

    return charToUInt64[a];
}