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 +LTO via plugins. This is the same mechanism used by the GCC LTO project.
The LLVM gold plugin implements the @@ -37,24 +41,31 @@ 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.
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.
-
+ 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.
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:
++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" ++
+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" ++
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.