<<
operations of the final four swaps are incorrect, they should be shift-right >>
operations and their shift values would also need to be corrected.// Swap endian (big to little) or (little to big)
uint32_t num = 0x12345678;
uint32_t res =
((num & 0x000000FF) << 24) |
((num & 0x0000FF00) << 8) |
((num & 0x00FF0000) >> 8) |
((num & 0xFF000000) >> 24);
printf("%0x\n", res);
The result is represented here in both binary and hex, notice how the bytes have swapped:
?0111 1000 0101 0110 0011 0100 0001 0010?
78563412
In terms of performance, leave it to the compiler to optimize your code when possible. You should avoid unnecessary data structures like arrays for simple algorithms like this, doing so will usually cause different instruction behavior such as accessing RAM instead of using CPU registers.