Search code examples
cmicro-optimization

What is the fastest way to find if a number is even or odd?

What is the fastest way to find if a number is even or odd?

Solution

  • It is pretty well known that

    static inline int is_odd_A(int x) { return x & 1; }

    is more efficient than 

    static inline int is_odd_B(int x) { return x % 2; }

    But with the optimizer on, will is_odd_B be no different from is_odd_A? No — with gcc-4.2 -O2, we get, (in ARM assembly):

    _is_odd_A:
    and r0, r0, #1
    bx  lr

_is_odd_B:
    mov r3, r0, lsr #31
    add r0, r0, r3
    and r0, r0, #1
    rsb r0, r3, r0
    bx  lr

    We see that is_odd_B takes 3 more instructions than is_odd_A, the main reason is because

    ((-1) % 2) == -1
((-1) & 1) ==  1

    However, all the following versions will generate the same code as is_odd_A:

    #include <stdbool.h>
static inline bool is_odd_D(int x) { return x % 2; }      // note the bool
static inline int  is_odd_E(int x) { return x % 2 != 0; } // note the !=

    What does this mean? The optimizer is usually sophisticated enough that, for these simple stuff, the clearest code is enough to guarantee best efficiency.