efficient switch statement when using enums

Question

I have an enum and a switch statement using some of the enum entries but not all and they are currently out of order too, i.e. I have the following:

enum prot_tun_stat_e    {
    STAT_A = 0,     
    STAT_B,     
    STAT_C, 
    STAT_D,
    STAT_E,
    STAT_F, //5
    STAT_G,
    STAT_H,
    STAT_I,
    STAT_Y,
    STAT_K,     //10
    STAT_COUNT      //must be last
} __attribute__((packed));

and then I have a switch using the following entries:

switch(var) {
case C:
break;
case D:
break
case F:
break
case G:
break
default
}

and I was wondering if I better rearranged the items in the enum to be C=1,D=2,F=3&G=4 ? Would that be more efficient?

Thanks, Ron

Platform: PowerPC, compiler diab

Answer 1

If the compiler can determine that the parameter to the switch statement is limited to a small number then it can create a jump table. This table will take up less room if the values are contiguous but the difference between 4 entries or the 10 required is unlikely to matter. (And note that 0-3 is a better range than 1-4 - although the compiler can deal with this by jumping to offset n - 1).

You can check the output of the compiler to see if a jump table is being created (assuming you can read assembly!). And of course the answer to all performance questions: profile it!