Bit reflect for uint64_t hand-optimized for size for AVR processors. https://godbolt.org/z/qjn8rb

avr_reflect_u64.c

#include <inttypes.h>

typedef union {
    uint64_t u64;
    uint8_t u8[8];
} uint64_union;

__attribute__((optimize("Os"))) // Avoid loop unrolling.
void reflect_u64(uint64_t* value)
{
    // Let's operate on memory instead of using registers.
    uint64_union *orig = (uint64_union *) value;

    // Reflect the bits of each byte of the 64-bit value.
    for (int i = 0; i < 8; i++) {
        uint8_t src = orig->u8[i];
        uint8_t tmp = 0;
        for (int j = 0; j < 8; j++) {
            tmp <<= 1;
            tmp |= src & 1;
            src >>= 1;
        }
        orig->u8[i] = tmp;
    }

    // Reflect the bytes (refl[0] = orig[7]; refl[1] = orig[6] &c)
    for (int i = 0; i < 4; i++) {
        uint8_t tmp = orig->u8[i];
        orig->u8[i] = orig->u8[7 - i];
        orig->u8[7 - i] = tmp;
    }
}