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.

Uniflorous

Did you try g++ -flto -lrary main.cpp? – Ilka

@πάνταῥεῖ Also broken. But I found the answer as I was writing the question, it's posted below. – Uniflorous

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

Uniflorous

It is worth noting that there is also gcc-ranlib and gcc-nm if you need them. – Anglofrench

@Anglofrench Indeed - it seems that ar is only part of the picture. I also needed to override with gcc-ranlib, as the system default couldn't handle the LTO objects. So, since it seems difficult to find anything approaching a one-liner solution to this: I got LTO working as follows: ./configure --prefix=/opt/testmake AR=gcc-ar RANLIB=gcc-ranlib CXXFLAGS='-O3 -flto' – Pathoneurosis

@Pathoneurosis As man ranlib states, it's equivalent to ar s. I create all my static libraries with gcc-ar rcs, only one step. – Uniflorous

Answer 2

As a complimentary answer: with GCC there is also the possibility to use -ffat-lto-objects which adds classic object code to the files in the archive. This makes it possible to use the static library in code that you build without -flto.

Maintenon

The trouble is it's difficult to catch where LTO is failing when fat-objects are used – Magnetism