optimizing wrapper functions

[Zack Weinberg <zack at rabi dot columbia dot edu>]

When writing code that loads dynamic modules, you might do something
like this:

int (*real_foo)(int, int);

int
foo(int a, int b)
{
  if(real_foo)
    return real_foo(a, b);

  try_to_load_real_foo();

  if(real_foo)
    return real_foo(a, b);

  return -1;
}

It would be nice if this could be optimized down to something like
this (or this but with a frame pointer):

foo:
	movl	real_foo, %eax
	testl	%eax, %eax
	jne	*%eax
	call	try_to_load_real_foo
	movl	real_foo, %eax
	testl	%eax, %eax
	jne	*%eax
	movl	$-1, %eax
	ret

Instead, I get this:

foo:
	pushl	%esi
	pushl	%ebx
	movl	real_foo,%eax
	movl	12(%esp),%esi
	movl	16(%esp),%ebx
	testl	%eax,%eax
	jne	call_it
	call	try_to_load_real_foo
	movl	real_foo,%eax
	testl	%eax,%eax
	je	fail
call_it:
	pushl	%ebx
	pushl	%esi
	call	*%eax
	addl	$8,%esp
	jmp	done
fail:
	movl $-1,%eax
done:
	popl %ebx
	popl %esi
	ret

I guess GCC would have to recognize that the called function had an
identical prototype to the wrapper, and was being called with the
wrapper's arguments, to pull this off.

zw