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

You should profile what your code does, before worrying about optimising it.

On my compiler locally, your code gets entirely inlined, unrolled and turned into 8 constant test + or instructions when the value is unknown, and turned into a constant when the value is known at compile time. I could probably marginally improve it by removing a few branches, but the compiler is doing a reasonable job on its own.

Optimising the loop is then a bit pointless. A table lookup might be more efficient, but would probably prevent the compiler from making optimisations itself.