The claim was that this is the one case where goto isn't evil around minute 33:31 and that it performs better than switch. Well, does it? And if so, by how much?
This was the example:
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
#define DISPATCH() \
{ goto *op_table[*((s)++) - 'a']; }
static void interp_old(const char* s)
{
static void *op_table[] = {
&&op_a, &&op_b, &&op_c, &&op_d, &&op_e };
DISPATCH();
op_a: printf("Hell"); DISPATCH();
op_b: printf("o "); DISPATCH();
op_c: printf("wo"); DISPATCH();
op_d: printf("rld!\n"); DISPATCH();
op_e: return;
}
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Switch is pretty switched on, so let's see how this compares!
The code that does the comparison can be found here. Results(on my ancient 32 bit Dell):
With gcc -g:
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Using default value of 100000. Usage: ./dispatch # Sample size = 100000 chars * 100000 iterations. Dispatch method: 0.002152 Dispatch shuffled method: 0.002144 Goto switch method: 0.002641 Goto switch method 2: 0.002351 For loop Switch method: 0.002667 For loop Switch shuffled method: 0.002660 Recursive switch method: 0.010798 Recursive shuffled Switch method: 0.006913 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
With gcc -O3:
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Using default value of 100000. Usage: ./dispatch # Sample size = 100000 chars * 100000 iterations. Dispatch method: 0.001083 Dispatch shuffled method: 0.001108 Goto switch method: 0.001338 Goto switch method 2: 0.001381 For loop Switch method: 0.001465 For loop Switch shuffled method: 0.001385 Recursive switch method: 0.001437 Recursive shuffled Switch method: 0.001385 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
So, there is a small difference, but it's really not that big a deal, unless you really are pressed for time and space.
What if you have a huge array of a struct that maps variable A to variable B and also has an optional function pointer F? What you need is a 'fake_switch' generator, here is my version. It should be easily adjustable for your taste.
Goto it ;-)
Ps.: For extra fun, try this C to Assembler translator. I really like the hover feature they built, but another thing that would be useful is showing the size of the generated code.
