On Thu, Apr 29, 2010 at 2:27 PM, Jan Hubicka <hubi...@ucw.cz> wrote: >> Thanks for the comments. FDO will probably improve SPEC2000 score. >> Although it is not obvious for some tests because the train data sets >> for them are different from the reference data sets and it might >> actually mislead the compiler. > > There are several studies on the topic and it is not that bad in practice. > In wast majority of cases even pretty bad training runs gets significant > portion of improvement you can get from training on the final benchmark > data. In SPEC case FDO improves pretty much all benchmarks.
Agree. > > I think the FDO is relatively little used because it is relatively hard > to use (i.e. user has to modify makefiles and learn how the feature works) > and also because there is very little support for it (i.e. in automake and > such) >> As for vortex FDO improvement, vortex contains a moderate size loop in >> which most of time is spent. The loop has if-then-else on the top loop >> level. On all SPEC2000 data sets, one if-branch is taken practically >> always (like 1 to 1,000,000). So it is not amazing for me that FDO >> gives such improvement for vortex. > > It would be interesting to know if same improvement happens with LTO and if > not what LIPO does. I will unbreak vortex on our tester. > Vortex needs -fno-strict-aliasing. It casts between two record types with one record being a 'prefix' of another. David > Honza >
