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 system used by the upcoming 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 build gold with plugin support and build the LLVMgold plugin.
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 src
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.
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.
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
$ llvm-gcc b.c -c -o b.o # <-- b.o is native object file
$ llvm-gcc -use-gold-plugin a.o 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.
gold, ar and nm all support plugins now, so everything should be in place for an easy to use LTO build of autotooled projects:
export CC="$PREFIX/bin/llvm-gcc -use-gold-plugin"
export AR="$PREFIX/bin/ar --plugin libLLVMgold.so"
export NM="$PREFIX/bin/nm --plugin libLLVMgold.so"
export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
export CFLAGS="-O4"
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.