Building with link time optimization requires cooperation with the +
Building with link time optimization requires cooperation from the system linker. LTO support on Linux systems requires that you use -gold which supports for -LTO via plugins. This is the same system used by the upcoming +the gold linker which supports +LTO via plugins. This is the same mechanism used by the GCC LTO -support.
-The LLVMgold plugin implements the gold -plugin interface on -top of +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. @@ -37,25 +41,32 @@ The same plugin can also be used by other tools such as ar and
You need to build gold with plugin support and build the LLVMgold -plugin.
+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.
-
-
- Build gold 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 src +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. - - - Build LLVMgold. Configure LLVM with + 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.
You can produce bitcode files from llvm-gcc using - -emit-llvm or -flto or -O4 which is equivalent - to -O3 -flto.
+ -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. - It will not look for an alternate linker, which is why you need gold to be - the installed system linker in your path.
+ 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. +as much as gold could without the plugin.
The LLVM Compiler Infrastructure
- Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $ + Last modified: $Date: 2010-04-16 23:58:21 -0800 (Fri, 16 Apr 2010) $