Question

I was bored one rainy afternoon and came up with this:

int ia_array[5][5][5]; //interger array called array

{
        int i = 0, j = 0, k = 0;//counters
        while( i < 5 )//loop conditions
        {
            ia_array[i][j][k] = 0;//do something
            __asm inc k;//++k;

            if( k > 4)
            {
                __asm inc j;  //++j;
                __asm mov k,0;///k = 0;
            }
            if( j > 4)
            {
                __asm inc i;  //++i;
                __asm mov j,0;//j = 0;
            }
        }//end of while
    }//i,j,k fall out of scope

its functionally equivalent to three nested for loops. However in a for loop you cannot use __asm statements. Also you have the option to not put the counters in a scope so you can reuse them for other loops. I have looked at the disassembly for both and my alternative has 15 opcodes and the nested for loops have 24. Therefore is it potentially faster? suppose I’m really asking is __asm inc i; faster then ++i;?

note: i don’t intent to use this code in any projects, just out of curiosity. thanks for your time.

Answer 1

First off, your compiler will likely store the values of i, j and k in registers.

It’s more efficient to do for (i = 4; i <=0; i--) than for(i = 0; i < 5; i++) as the cpu can determine if the result of the last operation it executed was zero for free – it doesn’t have to explicitly compare to 4 (see the cmovz instruction).

It’s the not the case for x86 that having to execute less instruction will lead to faster code. There are all sorts of issues to do with instruction pipelining that quickly get too much for a programmer to write by hand. Leave it to the compiler, they’re sufficiently efficient these days (though definitely not optimal… but who wants to wait hours for their code to compile).

You can check it out yourself by running your function a few hundred thousand times with each implementation and check which is faster. Check if you can write asm instructions in for loops with

__asm {
    inc j;
    mov k, 0;
}

(it’s been a while since I did this)

P.S. Have fun experimenting with asm, it can be very interesting and rewarding!