[Bug c/40528] Add a new ifunc attribute

["agner at agner dot org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40528

--- Comment #16 from Agner Fog <agner at agner dot org> 2011-07-08 08:52:32 UTC ---
(In reply to comment #15)
> (In reply to comment #14)
> > (In reply to comment #13)
> > > What is the status of this issue?
> > 
> > It is implemented on ifunc branch.
> > 
> > > Is option 3 implemented?
> > 
> > Yes, on ifunc branch.
> > 
> > > Which versions of Linux and binutils support IFUNC?
> Still doesn't work. 
> "warning: âifuncâ attribute directive ignored"
> GNU Binutils for Ubuntu 2.21.0.20110327

The ifunc attribute is described in
http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html but it doesn't work
(see my previous comment). After some experimentation I found that the method
described below works. Either the compiler should be fixed or the onlinedocs
should be changed.

// Example of gnu indirect function
#include <stdio.h>
#include <time.h>

// Define different versions of my function
int myfunc1() {
   return 1;
}

int myfunc2() {
   return 2;
}

// Type definition for pointer to my function
typedef int (*MyFunctionPointer)(void);

// Prototype for the common entry point 
extern "C"                                  // remove this line if not C++
int myfunc();
__asm__ (".type myfunc, @gnu_indirect_function");

// Make the dispatcher function
MyFunctionPointer myfunc_dispatch (void) __asm__ ("myfunc");
MyFunctionPointer myfunc_dispatch (void)  {

   if (time(0) & 1) {
      // If time is odd at first call, use version 1
      return myfunc1;
   }
   else {
      // else use version 2
      return myfunc2;
   }
}

int main() {
   // Test the call to myfunc
   printf("\nCalled function number %i\n", myfunc());
   return 0;
}