---------- Forwarded Message ---------- Subject: [CPUblog] Gnu support for CPU dispatching - sort of... Date: Friday 08 July 2011, 14:34:24 From: "Agner's CPU blog" <cpub...@agner.org> To: cpub...@agner.org
This message was sent from messageboard Agner`s CPU blog Subject: Gnu support for CPU dispatching - sort of... Author: Agner Go to message <http://agner.org/optimize/blog/read.php?i=167> The Gnu tools have added a new feature for automatic CPU dispatching. This means that you can have multiple versions of the same function, each optimized for a different CPU or a different instruction set. For example, you may want to have three different versions of an important library function: one that is compatible with any old CPU, a better one for CPUs with SSE2, and a still better one for CPUs with the AVX instruction set. This feature, called "Gnu indirect function" was introduced two years ago (link <http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40528>). 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 <http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html>) 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 #include // 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 function call is resolved via the normal procedure linkage table (PLT). The PLT entry is changed to point to the desired version of the function, either at load time or at the first call. The PLT initially points to |myfunc_dispatch|. This function is called only once, and the return value from |myfunc_dispatch| replaces its own entry in the PLT. 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++ <http://agner.org/optimize/#manuals> for a method that works on all platforms. ----------------------------------------- -- You received this message because you are subscribed to the Google Groups "mpir-devel" group. To post to this group, send email to mpir-devel@googlegroups.com. To unsubscribe from this group, send email to mpir-devel+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/mpir-devel?hl=en.
