How can I use lto with static libraries?

Question

When I try to build static libraries with -flto, I get undefined reference errors:

library.cpp:

#include <iostream>  void foo() {   std::cout << "Test!" << std::endl; } 

main.cpp:

void foo();  int main() {   foo();   return 0; } 

Compilation output:

$ g++ -flto -c library.cpp $ ar rcs library.a library.o $ g++ -flto main.cpp library.a /tmp/ccZIgxCY.ltrans0.ltrans.o: In function `main': ccZIgxCY.ltrans0.o:(.text+0x5): undefined reference to `foo()' collect2: error: ld returned 1 exit status 

It works fine if I link with library.o instead of library.a. What am I missing? This is with GCC 4.9.1 and binutils 2.24.

Answer 1

The answer, as I found out from this post by GCC developer Honza Hubička, is to use the gcc-ar wrapper instead of ar by itself:

$ gcc-ar rcs library.a library.o 

This invokes ar with the right plugin arguments, in my case were

--plugin /usr/lib/gcc/x86_64-unknown-linux-gnu/4.9.1/liblto_plugin.so