On Sat, Oct 3, 2009 at 11:12 PM, Diego Novillo <dnovi...@google.com> wrote: > The LTO branch has been merged into trunk at revision 152434. ... > To enable LTO, simply add the flag '-flto' to both compile and > link commands. It doesn't really matter whether you compile and > link in separate invocations or a single one. All that matters > is that -flto should be in both places: > > $ gcc -flto -O2 -o f f1.c f2.c > > or > > $ gcc -flto -c f1.c > $ gcc -flto -c f2.c > $ gcc -o f -flto -O2 f1.o f2.o
Remember though that options specified at compile time are _not_ automatically enabled at link time! Thus the 2nd example above would perform no initial optimization (effectively skipping all early optimizations until the IPA passes) while enabling optimization (starting with IPA passes and then remaining optimization passes) at link time. Thus the better canonical example would have been $ gcc -O2 -flto -c f1.c $ gcc -O2 -flto -c f2.c $ gcc -o f -flto -O2 f1.o f2.o where in particular enabling -fwhole-program is what should enable extra optimization (well, if you do actually link a final binary). Note that one missing feature is picking up entries from static library archives at link time. To not end up with unused LTO sections from static libraries in the final binary you need to have recent enough binutils or strip them manually. Richard.
