Which loop is faster in C? while loop or do-while loop

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?

Hadn't we had this discussion: http://stackoverflow.com/q/3347001/694576 ? — alk, Mar 15 '12 at 07:24
please check the site for duplicates yourself before posting — Jens Gustedt, Mar 15 '12 at 07:42

score 6 · Answer 1 · answered Mar 15 '12 at 07:21

6

You are comparing apples and peaches.

A do-while and while loops are different in what functionality they provide.
do-while always exectures once, while only executes if condition is true.

In situations where you can use either there is no performance difference.
And if you have a doubt then profiling it yourself on your platform or analysing the assembly output of both is what you should do.

answered Mar 15 '12 at 07:21

Alok Save

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@Ais Just read my question once again. I said SOME SITUATIONS we can use not ALL SITUATIONS. – Mohammed H Mar 15 '12 at 07:24
@HabeebPerwad: Just read my answer once again. The THIRD PARAGRAPH to be specific. – Alok Save Mar 15 '12 at 07:27

MByD · Accepted Answer · 2012-03-15T07:27:24.927

6

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

edited Mar 15 '12 at 07:27

answered Mar 15 '12 at 07:22

MByD

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The code shows that while using one extra instruction 'jmp L2'. Even if it is only one instruction, we can conclude that do-while is faster than while, right? – Mohammed H Mar 15 '12 at 07:34
2

One branch that executes once is NOT something you should think about anyway. It is WAY TOO MINOR to be considered as a slow factor. It only happens in the first time. – MByD Mar 15 '12 at 07:45
1

And this is so minor change, which is MUCH LESS important than the code readability – MByD Mar 15 '12 at 07:47

score 2 · Answer 3 · answered Mar 15 '12 at 07:22

2

No. The C standard does not provide requirements for execution speed, so it is impossible to prove in general.

answered Mar 15 '12 at 07:22

Mankarse

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There are obvious situations that won't be optimized, like external function calls not in the C library (since they may have side effects that the compiler can't assume anything about). If your loop guard is an external call, then in general, calling it twice will be about twice as expensive as calling it once. We can have a lot of basically correct intution about program performance, optimization effects notwithstanding. – Kaz Mar 15 '12 at 07:43
@Kaz: I'm not sure I see your point. I agree that for certain programs and certain compilers it is possible to sometimes predict performance; however, this hardly constitutes a "proof" (as HabeebPerwad asked for). – Mankarse Mar 15 '12 at 08:31

score 2 · Answer 4 · answered Mar 15 '12 at 07:22

2

The difference between while and do-while is that do-while always executes the first time, without checking the condition. So assuming that the condition is actually true the first time, do-while saves this one check of the condition.

This is a micro-optimization at best.

answered Mar 15 '12 at 07:22

Mark Byers

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There is no upper bound on how much of an optimization this can be because a loop guard can be arbitrarily expensive relative to the body of the loop and number of iterations. – Kaz Mar 15 '12 at 07:34
I believe it is more than only 1 instruction. since it only has one branch while the other loops have two (one conditional at the begging of the branch and one unconditional at the end), it give the compiler more freedom to optimize the code. – Mehdi Aug 21 '17 at 18:26

score 1 · Answer 5 · answered Mar 15 '12 at 07:22

1

If you dont care about the different semantic (at least once vs. maybe none), there is no difference.

answered Mar 15 '12 at 07:22

Matthias

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score 1 · Answer 6 · answered Mar 15 '12 at 07:36

1

C semantic describes the behavior of an abstract machine in which issues of optimisation are irrelevant. (see C99 Standard, 5.1.2.3p1)

answered Mar 15 '12 at 07:36

ouah

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score 1 · Answer 7 · answered Mar 15 '12 at 07:40

If it's a situation where the compiler can optimize it well, stick with the while loop. Top-of-loop testing is clearer. The reason is rooted in formal verification. The while loop tests the preconditions for the correct execution of the body. The do loop executes a body once without testing anything, and then has to deal with the aftermath, so to speak. It's more tricky to reason about. We have to think about what the situation is as a result of the conditions that existed before the loop plus the effect of the unguarded execution of the first iteration.

If you have a loop guard which is expensive (like a call to some external function that can do I/O or who knows what else) then think about moving it down, if possible.

Which loop is faster in C? while loop or do-while loop

7 Answers7