Agner`s CPU blog

This feature, called "Gnu indirect function" was introduced two years ago (link). Since then, I have waited impatiently for an implementation that works. Now I have discovered that this feature is actually used for a few functions in the standard function library (glibc v. 2.13). The official documentation (link) says that you can use __attribute__ ((ifunc("name_of_dispatch_function"))), but this doesn't work.

After some experimentation, I found that the method shown below actually works:

// 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;
}

// Prototype for the common entry point
extern "C" int myfunc();
__asm__ (".type myfunc, @gnu_indirect_function");

// Make the dispatcher function. This returns a pointer to the desired function version
typeof(myfunc) * myfunc_dispatch (void) __asm__ ("myfunc");
typeof(myfunc) * 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;
}

The "Gnu indirect function" feature requires support in the assembler, linker and loader, which is found in binutils version 2.20 and later.

The Gnu standard library glibc uses this feature to implement multiple versions of a few memory and string functions, including memmove, memset, memcmp, strcmp, strstr, but - strangely - not the most important one: memcpy.

This feature can be useful for anybody who wants to make a highly optimized function library for Linux. It is not possible in Windows, but it may be implemented in BSD and Mac systems. See my manual Optimizing software in C++ for a method that works on all platforms.