Avoiding gcc function prologue overhead?

Question

I've lately encountered a lot of functions where gcc generates really bad code on x86. They all fit a pattern of:

if (some_condition) {
    /* do something really simple and return */
} else {
    /* something complex that needs lots of registers */
}

Think of simple case as something so small that half or more of the work is spent pushing and popping registers that won't be modified at all. If I were writing the asm by hand, I would save and restore the saved-across-calls registers inside the complex case, and avoid touching the stack pointer at all in the simple case.

Is there any way to get gcc to be a little bit smarter and do this itself? Preferably with command line options rather than ugly hacks in the source...

Edit: To make it concrete, here's something very close to some of the functions I'm dealing with:

if (buf->pos < buf->end) {
    return *buf->pos++;
} else {
    /* fill buffer */
}

and another one:

if (!initialized) {
    /* complex initialization procedure */
}
return &initialized_object;

and another:

if (mutex->type == SIMPLE) {
    return atomic_swap(&mutex->lock, 1);
} else {
    /* deal with ownership, etc. */
}

Edit 2: I should have mentioned to begin with: these functions cannot be inlined. They have external linkage and they're library code. Allowing them to be inlined in the application would result in all kinds of problems.

Answer 1

I would do it like this:

static void complex_function() {}

void foo()
{
    if(simple_case) {
        // do whatever
        return;
    } else {
        complex_function();
    }
}

The compiler my insist on inlining complex_function(), in which case you can use the noinline attribute on it.

Answer 2

Update

To explicitely suppress inlining for a single function in gcc, use:

void foo() __attribute__ ((noinline))
{
  ...
}

See also How can I tell gcc not to inline a function?

---

Functions like this will regularly be inlined automatically unless compiled -O0 (disable optimization).

In C++ you can hint the compiler using the inline keyword

If the compiler won't take your hint you are probably using too many registers/branches inside the function. The situation is almost certainly resolved by extracting the 'complicated' block into it's own function.

---

Update i noticed you added the fact that they are extern symbols. (Please update the question with that crucial info). Well, in a sense, with external functions, all bets are off. I cannot really believe that gcc will by definition inline all of a complex function into a tiny caller simply because it is only called from there. Perhaps you can give some sample code that demonstrates the behaviour and we can find the proper optimization flags to remedy that?

Also, is this C or C++? In C++ I know it is common place to include the trivial decision functions inline (mostly as members defined in the class declaration). This won't give a linkage conflict like with simple (extern) C functions.

Also you can have template functions defined that will inline perfectly in all compilation modules without resulting in link conflicts.

I hope you are using C++ because it will give you a ton of options here.

Answer 3

Perhaps upgrade your version of gcc? 4.6 has just been released. As far as I understand, it has the possibility of "partial inline". That is, an easily integratable outer part of a function is inlined and the expensive part is transformed into a call. But I have to admit that I didn't try it myself, yet.

Edit: The statement I was referring to from the ChangeLog:

Partial inlining is now supported and enabled by default at -O2 and greater. The feature can be controlled via -fpartial-inlining.

Partial inlining splits functions with short hot path to return. This allows more aggressive inlining of the hot path leading to better performance and often to code size reductions (because cold parts of functions are not duplicated).

...

Inlining when optimizing for size (either in cold regions of a program or when compiling with -Os) was improved to better handle C++ programs with larger abstraction penalty, leading to smaller and faster code.

Answer 4

I would probably refactor the code to encourage inlining of the simple case. That said, you can use -finline-limit to make gcc consider inlining larger functions, or -fomit-frame-pointer -fno-exceptions to minimize the stack frame. (Note that the latter may break debugging and cause C++ exceptions to misbehave badly.)

Probably you won't be able to get much from tweaking compiler options, though, and will have to refactor.

Answer 5

Seeing as these are external calls, it might be possible the gcc is treating them as unsafe and preserving registers for the function call(hard to know without seeing the registers that it preserves, including the ones you say 'aren't used'). Out of curiousity, does this excessive register spilling still occur with all optimizations disabled?