bearophile, el 23 de diciembre a las 12:02 me escribiste: > Leandro Lucarella: > > > bearophile, el 23 de diciembre a las 00:13 me escribiste: > > > Compared to GCC LLVM lacks vectorization (this can be important for > > > certain heavy numerical computing code), profile-guided optimization > > > (this is usually less important, it's uncommon that it gives more than > > > 5-25% performance improvement) > > > > I don't know if that are accurate numbers, but 5-25% looks like a *lot* to > > me. > > Vectorization can improve 2X or 3X+ the performance of certain code > (typical example: matrix multiplication done right). > > Performance differences start to matter in practice when they are 2X or > more. In most situations users aren't able to appreciate a 20% > performance improvement of an application. (But small improvements are > important for the compiler devs because they are cumulative, so many > small improvements may eventually lead some a significant difference).
Well, you are talking about a single user, but for servers, if you have to provide a minimum quality of service, a 20% difference means you can serve 20% more people, for example (not that people would have to wait 0.2 secs more, because that is not an option). > > > but it has a link-time optimizations that gcc lacks (about as important > > > as profile-guided optimization or a little more). > > > > And GCC have LTO too, see: > > http://gcc.gnu.org/wiki/LinkTimeOptimization > > Oh, nice, I have not tried this yet. Is this going in Gcc 4.5? > LTO of LLVM is pretty good, I don't know if GCC implements it equally > well (I fear that the answer is negative. I think it will be in GCC 4.5 but I don't know the details. -- Leandro Lucarella (AKA luca) http://llucax.com.ar/ ---------------------------------------------------------------------- GPG Key: 5F5A8D05 (F8CD F9A7 BF00 5431 4145 104C 949E BFB6 5F5A 8D05) ---------------------------------------------------------------------- Se va a licitar un sistema de vuelos espaciales mendiante el cual, desde una plataforma que quizás se instale en la provincia de Córdoba. Esas naves espaciales va a salir de la atmósfera, va a remontar la estratósfera y desde ahí elegir el lugar donde quieran ir de tal forma que en una hora y media podamos desde Argentina estar en Japón, en Corea o en cualquier parte. -- Carlos Saúl Menem (sic)