On 11/09/2009 12:03 AM, Basile STARYNKEVITCH wrote: > is gcc-trunk -flto -O2 aimed for medium sized programs (something like > bash), or for bigger ones (something like the linux kernel, the Xorg > server, the Qt or GTK graphical toolkit libraries, or bootstrapping GCC > itself.
My understanding is that LTO aims at both, but that one needs to use -fwhopr for really large systems as the otherwise e.g. the memory usage may exceed the available memory. I don't know whether one can really estimated how much memory compilation needs. It is surely not a simple function on the number of code lines. I tried -flto successfully for our 100 kLoC Fortran code and there lto1 needs <1/2 GB of RAM (370 MB if I recall correctly). (Thanks to especially Richard; initially more than 4 GB were needed and lto1 crashed thus). Toon also used LTO [1] for their HIRLAM weather forecasting program, which has according to [2] 1.2 MLoC in Fortran and O(10 kLoC) in C. If I recall correctly, bootstrapping GCC also works in principle, except for problems when comparing stage2 with stage3. > Perhaps the question is when not to use -flto and use -fwhopr instead? My rule of thumb is: Try -flto first, if it does not work (running out of memory), try -fwhopr. I think the advantage of -flto is also that it is better tested, while -fwhopr has known issues. Tobias [1] http://gcc.gnu.org/ml/gcc/2009-10/msg00122.html [2] http://moene.org/~toon/GCCSummit-2006.pdf
