Building with link time optimization requires cooperation from the system linker. LTO support on Linux systems requires that you use the gold linker which supports LTO via plugins. This is the same mechanism used by the GCC LTO project.

The LLVM gold plugin implements the gold plugin interface on top of libLTO. The same plugin can also be used by other tools such as ar and nm.

You need to have gold with plugin support and build the LLVMgold plugin. Check whether you have gold running /usr/bin/ld -v. It will report “GNU gold” or else “GNU ld” if not. If you have gold, check for plugin support by running /usr/bin/ld -plugin. If it complains “missing argument” then you have plugin support. If not, such as an “unknown option” error then you will either need to build gold or install a version with plugin support.

• To build gold with plugin support:

  mkdir binutils
  cd binutils
  cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src login
  {enter "anoncvs" as the password}
  cvs -z 9 -d :pserver:anoncvs@sourceware.org:/cvs/src co binutils
  mkdir build
  cd build
  ../src/configure --enable-gold --enable-plugins
  make all-gold
  

  That should leave you with binutils/build/gold/ld-new which supports the -plugin option. It also built would have binutils/build/binutils/ar and nm-new which support plugins but don't have a visible -plugin option, instead relying on the gold plugin being present in ../lib/bfd-plugins relative to where the binaries are placed.
• Build the LLVMgold plugin: Configure LLVM with --with-binutils-include=/path/to/binutils/src/include and run make.

The linker takes a -plugin option that points to the path of the plugin .so file. To find out what link command gcc would run in a given situation, run gcc -v [...] and look for the line where it runs collect2. Replace that with ld-new -plugin /path/to/LLVMgold.so to test it out. Once you're ready to switch to using gold, backup your existing /usr/bin/ld then replace it with ld-new.

You can produce bitcode files from llvm-gcc using -emit-llvm or -flto, or the -O4 flag which is synonymous with -O3 -flto.

llvm-gcc has a -use-gold-plugin option which looks for the gold plugin in the same directories as it looks for cc1 and passes the -plugin option to ld. It will not look for an alternate linker, which is why you need gold to be the installed system linker in your path.

If you want ar and nm to work seamlessly as well, install LLVMgold.so to /usr/lib/bfd-plugins. If you built your own gold, be sure to install the ar and nm-new you built to /usr/bin.

The following example shows a worked example of the gold plugin mixing LLVM bitcode and native code.

--- a.c ---
#include <stdio.h>

extern void foo1(void);
extern void foo4(void);

void foo2(void) {
  printf("Foo2\n");
}

void foo3(void) {
  foo4();
}

int main(void) {
  foo1();
}

--- b.c ---
#include <stdio.h>

extern void foo2(void);

void foo1(void) {
  foo2();
}

void foo4(void) {
  printf("Foo4");
}

--- command lines ---
$ llvm-gcc -flto a.c -c -o a.o              # <-- a.o is LLVM bitcode file
$ ar q a.a a.o                              # <-- a.a is an archive with LLVM bitcode
$ llvm-gcc b.c -c -o b.o                    # <-- b.o is native object file
$ llvm-gcc -use-gold-plugin a.a b.o -o main # <-- link with LLVMgold plugin

Gold informs the plugin that foo3 is never referenced outside the IR, leading LLVM to delete that function. However, unlike in the libLTO example gold does not currently eliminate foo4.

Once your system ld, ar and nm all support LLVM bitcode, everything is in place for an easy to use LTO build of autotooled projects:

• Follow the instructions on how to build LLVMgold.so.
• Install the newly built binutils to $PREFIX
• Copy Release/lib/LLVMgold.so to $PREFIX/libexec/gcc/x86_64-unknown-linux-gnu/4.2.1/ and $PREFIX/lib/bfd-plugins/
• Set environment variables ($PREFIX is where you installed llvm-gcc and binutils):

  export CC="$PREFIX/bin/llvm-gcc -use-gold-plugin"
  export CXX="$PREFIX/bin/llvm-g++ -use-gold-plugin"
  export AR="$PREFIX/bin/ar"
  export NM="$PREFIX/bin/nm"
  export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
  export CFLAGS="-O4"
  

• Or you can just set your path:

  export PATH="$PREFIX/bin:$PATH"
  export CC="llvm-gcc -use-gold-plugin"
  export CXX="llvm-g++ -use-gold-plugin"
  export RANLIB=/bin/true
  export CFLAGS="-O4"
  

• Configure & build the project as usual: ./configure && make && make check

The environment variable settings may work for non-autotooled projects too, but you may need to set the LD environment variable as well.

Gold is licensed under the GPLv3. LLVMgold uses the interface file plugin-api.h from gold which means that the resulting LLVMgold.so binary is also GPLv3. This can still be used to link non-GPLv3 programs just as much as gold could without the plugin.