The LLVM gold plugin

Introduction

Building with link time optimization requires cooperation fromthe system linker. LTO support on Linux systems is available via thegold linker which supports LTO via plugins. This is the same mechanismused by the GCC LTO project.

The LLVM gold plugin implements the gold plugin interface on top oflibLTO. The same plugin can also be used by other tools such asar and nm. Note that ld.bfd from binutils version 2.21.51.0.2and above also supports LTO via plugins. However, usage of the LLVMgold plugin with ld.bfd is not tested and therefore not officiallysupported or recommended.

How to build it

You need to have gold with plugin support and build the LLVMgold plugin.The gold linker is installed as ld.gold. To see whether gold is the defaulton your system, run /usr/bin/ld -v. It will report “GNUgold” or else “GNU ld” if not. If gold is already installed at/usr/bin/ld.gold, one option is to simply make that the default bybacking up your existing /usr/bin/ld and creating a symbolic linkwith ln -s /usr/bin/ld.gold /usr/bin/ld. Alternatively, you can buildwith clang’s -fuse-ld=gold or add -fuse-ld=gold to LDFLAGS, which willcause the clang driver to invoke /usr/bin/ld.gold directly.

If you have gold installed, check for plugin support by running/usr/bin/ld.gold -plugin. If it complains “missing argument” thenyou have plugin support. If not, and you get an error such as “unknown option”,then you will either need to build gold or install a version with pluginsupport.

  • Download, configure and build gold with plugin support:
  1. $ git clone --depth 1 git://sourceware.org/git/binutils-gdb.git binutils
  2. $ mkdir build
  3. $ cd build
  4. $ ../binutils/configure --enable-gold --enable-plugins --disable-werror
  5. $ make all-gold

That should leave you with build/gold/ld-new which supportsthe -plugin option. Running make will additionally buildbuild/binutils/ar and nm-new binaries supporting plugins.

Once you’re ready to switch to using gold, backup your existing/usr/bin/ld then replace it with ld-new. Alternatively, installin /usr/bin/ld.gold and use -fuse-ld=gold as described earlier.

Optionally, add —enable-gold=default to the above configure invocationto automatically install the newly built gold as the default linker withmake install.

  • Build the LLVMgold plugin. Run CMake with-DLLVM_BINUTILS_INCDIR=/path/to/binutils/include. The correct includepath will contain the file plugin-api.h.

Usage

You should produce bitcode files from clang with the option-flto. This flag will also cause clang to look for the gold plugin inthe lib directory under its prefix and pass the -plugin option told. It will not look for an alternate linker without -fuse-ld=gold,which is why you otherwise need gold to be the installed system linker inyour path.

ar and nm also accept the -plugin option and it’s possible toto install LLVMgold.so to /usr/lib/bfd-plugins for a seamless setup.If you built your own gold, be sure to install the ar and nm-new youbuilt to /usr/bin.

Example of link time optimization

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

  1. --- a.c ---
  2. #include <stdio.h>
  3.  
  4. extern void foo1(void);
  5. extern void foo4(void);
  6.  
  7. void foo2(void) {
  8.   printf("Foo2\n");
  9. }
  10.  
  11. void foo3(void) {
  12.   foo4();
  13. }
  14.  
  15. int main(void) {
  16.   foo1();
  17. }
  18.  
  19. --- b.c ---
  20. #include <stdio.h>
  21.  
  22. extern void foo2(void);
  23.  
  24. void foo1(void) {
  25.   foo2();
  26. }
  27.  
  28. void foo4(void) {
  29.   printf("Foo4");
  30. }
  1. --- command lines ---
  2. $ clang -flto a.c -c -o a.o      # <-- a.o is LLVM bitcode file
  3. $ ar q a.a a.o                   # <-- a.a is an archive with LLVM bitcode
  4. $ clang b.c -c -o b.o            # <-- b.o is native object file
  5. $ clang -flto 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 libLTOexample gold does not currently eliminate foo4.

Quickstart for using LTO with autotooled projects

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/lib/bfd-plugins/

  • Set environment variables ($PREFIX is where you installed clang andbinutils):

  1. export CC="$PREFIX/bin/clang -flto"
  2. export CXX="$PREFIX/bin/clang++ -flto"
  3. export AR="$PREFIX/bin/ar"
  4. export NM="$PREFIX/bin/nm"
  5. export RANLIB=/bin/true #ranlib is not needed, and doesn't support .bc files in .a
  • Or you can just set your path:
  1. export PATH="$PREFIX/bin:$PATH"
  2. export CC="clang -flto"
  3. export CXX="clang++ -flto"
  4. export RANLIB=/bin/true
  • Configure and build the project as usual:
  1. % ./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.

Licensing

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