If-statement vs function pointer

问题:

The goal is to change the behaviour in an event loop, depending on whether a checkbox is toggled on or off. The simplest way, that I can think of, is just to test the checkbox state each time the loop is run.

// if-statement

void action() { /* ... */ }


void someLoop() {

  if (checkboxTrue) {
    action();
  }
  // ... other stuff

}

Would the code be more performant and cleaner or in any other way better, if a function pointer was being used? Like this:

// function pointer

void action() { /* ... */ }
void empty() {}
void (*actionPtr)();


void checkboxChanged(int val) {

  if (val == 1)
    actionPtr = &realAction;
  else
    actionPtr = ∅

}

void someLoop() {

  (*actionPtr)();
  // ... other stuff

}

回答1:

1. One indirect function call is more expensive than one if condition.

2. Several if conditions are more expensive than an indirect function call.

3. Worrying about speed at this point is pointless:
   You are waiting on the latency of the user, and you are handling stuff he can look at (i. e. there won't be huge amounts of checkboxes). Optimizing code that is executed less than a million times per second on a detailed level like this is absolutely pointless.

So, my advise is: stop worrying about the cost of an if or function call while you are programming a user interface. Only think about such stuff inside your time consuming algorithms.

However, if you find that you are indeed using complex if/else ladders and/or switch statements inside your inner loop, you might optimize by replacing those with indirect function calls.

---

Edit:
You say that you have 600 checks per second. Assuming you have only one if case to handle (the situation where the if is faster), you "save" roughly 6 microseconds per second by not using function pointer indirection, that's 0.0006% of the runtime. Definitely not worth the effort...

回答2:

Not having the conditional branch will obviously save some time, of course it is simply branching around a branch so you are incurring a pipe flush, it is a case of maybe two flushes instead of one (barring processor optimization of course) plus the extra code to do the compare.

extern void fun0 ( unsigned int );
extern void fun1 ( unsigned int );

void (*fun(unsigned int));


void dofun0 ( unsigned int x, unsigned int y )
{
    if(x) fun0(y);
    else  fun1(y);
}

void dofun ( unsigned int y )
{
    fun(y);
}

gives something like this for example

Disassembly of section .text:

00000000 <dofun0>:
   0:   e3500000    cmp r0, #0
   4:   e92d4008    push    {r3, lr}
   8:   e1a00001    mov r0, r1
   c:   1a000002    bne 1c <dofun0+0x1c>
  10:   ebfffffe    bl  0 <fun1>
  14:   e8bd4008    pop {r3, lr}
  18:   e12fff1e    bx  lr
  1c:   ebfffffe    bl  0 <fun0>
  20:   e8bd4008    pop {r3, lr}
  24:   e12fff1e    bx  lr

00000028 <dofun>:
  28:   e92d4008    push    {r3, lr}
  2c:   ebfffffe    bl  0 <fun>
  30:   e8bd4008    pop {r3, lr}
  34:   e12fff1e    bx  lr

You should be able to measure that performance difference if you carefully craft your test. It is going to be a very small difference, but definitely measurable.

回答3:

Imho, pointers are a bit cleaner for calling routines having many arguments:

int good(int a, int b, int c, char *d) { ... }
int bad (int a, int b, int c, char *d) { ... }
int ugly(int a, int b, int c, char *d) { ... }

Direct calls:

if (is_good) {
   good(1,2,3,"fire!");
} else if (is_bad) {
   bad(1,2,3,"fire!");
} else {
   ugly(1,2,3,"fire!");
}

Indirect calls:

if (is_good) {
   f = good;
} else if (is_bad) {
   f = bad;
} else {
   f = ugly;
}

...

f(1,2,3,"fire!");

回答4:

To get exact results, time measurements would be needed, but:
I´m certain that a if has less overhead than a full function call.
->If is faster

回答5:

Your function pointer implementation is IMO not clear at all. It is not obvious what is going on there, so you should avoid this.

Maybe a different option is to put your check/action function pointers into an array. And check it inside a loop. You have a clean someLoop functionality and your check/action ("business logic") is inside this array.

回答6:

If you are only going to be checking for these things in one central place, I recommend using a switch if possible, if/else if not.

For a start that's actually easier to extend and add new cases if you need them than having to write a new function, check for things, and use a function pointer. The function pointer solution becomes easier to extend if you were otherwise tempted to check for the conditions in multiple places.

But secondly (and less importantly), it will tend to be more efficient, sometimes considerably so, and in ways that go beyond just comparing the cost of a dynamic dispatch vs. a local branch/jump. It's because you give the optimizer so much more information to work with.

I once found a case where GCC 5.3 managed to squash down a complex set of switch cases involving multiple function calls in each case to a single branchless look-up table of what data to load into registers. I expected a jump table but it skipped the whole jump table and just figured out what data needed to be loaded for each case without branching at all.

I was so impressed as I didn't even realize through all those cases calling functions that it could just boil down to loading data from a LUT. But I'm pretty sure it would not have been able to squash my code like this if there were indirect function calls involved. They end up serving as optimization barriers unless the optimizer is so smart that it can generate all the code paths for every possible type of function pointer that can be called while figuring out which functions are going to be called through a given FP.