How do you recognize a for(i = 0; i < n; i++) loop with its counter in assembly?

Look for a counter that's zeroed before the loop, incremented by 1 each pass, and compared against a limit at the bottom with a backward conditional jump.

* The four for-loop parts in assembly: zero the counter, body, add $1, then a bottom cmp + backward conditional jump. *

    # i = 0
    xor  %ecx, %ecx
.L_loop:
    # Body using %ecx as index
    movl (%rdi,%rcx,4), %eax
    ...
    # i++
    add  $1, %ecx
    # i < n
    cmp  %esi, %ecx
    jl   .L_loop

Identifying the parts:

Counting down (for(i = n-1; i >= 0; i--)):

    mov  %esi, %ecx
.L_loop:
    sub  $1, %ecx
    ...
    test %ecx, %ecx
    jns  .L_loop

jns = "jump if not sign" = i >= 0.

Go deeper:

• For loop — the init / condition / increment three-part loop the compiler is lowering
• Control flow — how loops become conditional branches (jump) at the instruction level