Compiling code that returns doubles with -finstrument-functions is broken.
This does not depend on -O or -g flags.

This program:
double function (void)
{
  return 10.0;
}
int main(void)
{
  double d = function ();
  printf ("res: %f\n", d);
}

outputs:

res: nan

Here is the generated code:
function:
	pushl	%ebp
	movl	%esp, %ebp
	pushl	%esi
	pushl	%ebx
	subl	$16, %esp
	subl	$8, %esp
	pushl	4(%ebp)
	pushl	$function
	call	__cyg_profile_func_enter
	addl	$16, %esp
	movl	$0, %ebx
	movl	$1076101120, %esi
	subl	$8, %esp
	pushl	4(%ebp)
	pushl	$function
	call	__cyg_profile_func_exit
	addl	$16, %esp
	movl	%ebx, -16(%ebp)
	movl	%esi, -12(%ebp)
	fldl	-16(%ebp)
	leal	-8(%ebp), %esp
	popl	%ebx
	popl	%esi
	popl	%ebp
	fstp	%st(0)
	ret

Everything seems peachy, except the last "fstp %st(0)" which just throws
away the calculated result.

This is pretty bad. I regulary use -finstrument-functions to profile
Mozilla and other things.