x86 assembly programming loops with ecx and loop instruction versus jmp + j<condition>

Question

i'm currently learning x86 assembly language and wondered what is the better way for implementing loops. One way would be to mov a value to ecx register and use the loop instruction and the other way would be using a jmp instruction and then comes the loop body and then a conditional jumping eventually to the beginning of the loop body. I guess the first one will has a better readability but other then that i don't know why to use it.

Answer 1

When you mention jmp+body+test, I believe you are talking about the translation of a while loop in high-level languages. There is a reason for the second approach. Let's take a look.

Consider

x = N
while (x != 0) {
    BODY
    x--
}

The naive way is

    mov ecx, N      ; store var x in ecx register
top:
    cmp ecx, 0      ; test at top of loop
    je bottom       ; loop exit when while condition false
    BODY
    dec ecx
    jmp top
bottom:

This has N conditional jumps and N unconditional jumps.

The second way is:

    mov ecx, N 
    jmp bottom
top:
    BODY
    dec ecx
bottom:
    cmp ecx, 0
    jne top

Now we still do N conditional jumps but we only do ONE unconditional jump. A small savings but it just might matter, especially because it is in a loop.

Now you did mention the loop instruction which is essentially

dec ecx
cmp ecx, 0
je somewhere

How would you work that in? Probably like this:

    mov ecx, N
    cmp ecx, 0       ; Must guard against N==0
    je bottom
top:
    BODY
    loop top         ; built-in dec, test, and jump if not zero
bottom:

This is a pretty little solution typical of CISC processors. Is it faster than the second way above? That depends a great deal on the architecture. I suggest you do some research on the performance of the loop instruction in the IA-32 and Intel 64 processor architectures, if you really want to know more.