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Inline Functions question

Mon, 28 Dec 2009 13:16:11 -0700

Learning about inline functions and very confused.

When I use an inline function am I not pushing another activation record onto the stack? If not then how does the inline function know how to return to the function that called it? And where is it exactly if its not on the stack? When the function is copied where is it copied to, RAM? I don't understand this.

If I declare a function inline and then call it multiple times in my program then I will have multiple copies of the function code in my program- I really have no idea what this means. Where does the compiler put all of these copies when it builds the program?

Thanks for any responses.

Re: Inline Functions question

Tue, 29 Dec 2009 05:09:53 -0700

Compiler simply inserts the code (copies it) into the place of a function call. A simple example:

int SUM (int a, int b)
{
	return a+b;
}
//
// Without INLINE
//
int main ()
{
	int n1,n2;

	printf ("ENTER TWO INTEGER VALUES: ");
	scanf ("%d %d", &n1, &n2);
	printf ("\nSUM OF THESE VALUES: %d", SUM (n1, n2));
	return 0;
}
//
// With INLINE (a simple replacement)
//
int main ()
{
	int n1,n2;

	printf ("ENTER TWO INTEGER VALUES: ");
	scanf ("%d %d", &n1, &n2);
	printf ("\nSUM OF THESE VALUES: %d", n1+n2);
	return 0;
}

Also, inline keyword works mostly in RELEASE build - compiler does it on RELEASE build and not all is inlined. Compiler measures the SIZE of functions and inlines only the functions, which are not large. Also, if a function is very SMALL (like SUM in my example) - you do not need to write inline for it - it will be inlined automatically, by size. All small functions are inlined this way, just by looking at the size of a function.

Re: Inline Functions question

Wed, 30 Dec 2009 12:43:29 -0700

Hmm maybe my question comes from a deeper ignorance of how a program is compiled.

If I have:

inline int SUM (int a, int b)
{
	return a+b;
}

int main ()
{
	int n1,n2;

	printf ("ENTER TWO INTEGER VALUES: ");
	scanf ("%d %d", &n1, &n2);
	printf ("\nSUM OF THESE VALUES: %d", SUM (n1, n2));
	return 0;
}

The compiler takes "SUM (n1, n2)" and replaces it with "n1+n2"? How is that replacement made? And how does it make my program bigger?

Re: Inline Functions question

Wed, 30 Dec 2009 12:45:20 -0700

Hmm maybe my question comes from a deeper ignorance of how a program is compiled.

If I have:

inline int SUM (int a, int b)
{
	return a+b;
}

int main ()
{
	int n1,n2;

	printf ("ENTER TWO INTEGER VALUES: ");
	scanf ("%d %d", &n1, &n2);
	printf ("\nSUM OF THESE VALUES: %d", SUM (n1, n2));
	return 0;
}

The compiler takes "SUM (n1, n2)" and replaces it with "n1+n2"? How is that replacement made? And how does it make my program bigger?

Re: Inline Functions question

Thu, 31 Dec 2009 05:44:34 -0700

The replacement is made when compiler links all together - only at this moment it can see the function size. Then the Assembly code of the function is placed instead of a function call. When code runs - it is already been replaced. The program gets larger in the case you are using a lot of calls to SUM (inlined) function. Just one call will not make it much larger - maybe a few bytes. But with inline functions you get faster code, because there is no CALL/RETURN codes. However, modern CPUs are very fast on first 16 levels of CALL/RET instructions and if code goes deeper in calls, then it will slow down, but not in very significant way - few CPU clocks (and they come 3.2 billion per second!).

Re: Inline Functions question

Thu, 31 Dec 2009 06:58:31 -0700

Well that makes things a bit clearer, but it seems like I still have to study this a bit more myself to fully understand it.

Thanks.