Question

Some situation, we can use while loop or do-while loop interchangeably.
One of my friend told me that such situation we should use do-while loop.
Because it is faster than while.
Can anyone give me proof for it?

Answer 1

Both loops are pretty much the same (except of the fact that a do-while is executed at least once), and in many cases compile to the same instructions, at least on x86.

The following code:

int main(int argv, char *argc[]) {
    int a = 0;
    while (a < 100) {
        a++;
    }
    a = 0;
    do {
        a++;
    }while(a < 100);
    return 0;
}

Generates this assembly (using gcc -S -o temp.as temp.c):

_main:
    pushl   %ebp
    movl    %esp, %ebp
    andl    $-16, %esp
    subl    $16, %esp
    call    ___main
    movl    $0, 12(%esp) >> while
    jmp L2  
L3:
    addl    $1, 12(%esp)
L2:
    cmpl    $99, 12(%esp)
    jle L3
    movl    $0, 12(%esp)  >> do-while
L4:
    addl    $1, 12(%esp) 
    cmpl    $99, 12(%esp)
    jle L4
    movl    $0, %eax
    leave
    ret