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

Two minor optimizations:
One for testing the bits in the input (a will be destroyed but this doesn't matter)
The other for shifting the mask.

static uint64_t inflate(unsigned char a)
{
    uint64_t mask = 0xFF;
    uint64_t result = 0;
    for (int i = 0; i < 8; i++) {
        if (a & 1)
            result |= mask;
        mask <<= 8;    
        a >>= 1;
    }

    return result;
} 

Maybe you can also replace the 'for (int i = 0; i < 8; i++)'-loop by a 'while (a)'-loop. This works, however, only if the right shift a >>=1 works unsigned (As much as I know C standard allows the compiler to do it signed or unsigned). Otherwise you will have an infinite loop in some cases.

EDIT:
To see the result I compiled both variants with gcc -std=c99 -S source.c. A quick glance at the resulting assembler outputs shows that the optimization shown above yields ca. 1/3 viewer instructions, most of them inside the loop.