On Mon, Jan 19, 2009 at 9:01 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Mon, 19 Jan 2009, Nick Piggin wrote: >> >> I want to know what is the problem with the restrict keyword? >> I'm sure I've read Linus ranting about how bad it is in the >> past... > > No, I don't think I've ranted about 'restrict'. I think it's a reasonable > solution for performance-critical code, and unlike the whole type-aliasing > model, it actually works for the _sane_ cases (ie doing some operation > over two arrays of the same type, and letting the compiler know that it > can access the arrays without fearing that writing to one would affect > reading from the other). > > The problem with 'restrict' is that almost nobody uses it, and it does > obviously require programmer input rather than the compiler doing it > automatically. But it should work well as a way to get Fortran-like > performance from HPC workloads written in C - which is where most of the > people are who really want the alias analysis. while working in my college thesis, a fortran HPC workload (10k lines of fortran), converted to C resulted in performance speedups. this was with gcc 3.4. A simple f2c conversion + adaptations, resulted in a considerable speedup. (20% IIRC). The conversion was not done by performance reasons, the extra performance was simply a unexpected, but quite nice outcome. Just to let you guys know, that even with gcc3.4, gcc of C code ran faster than gfortran of the equiv. fortran code. ... pushing the optimization engine further (-march tunning and -O3) resulted in bad data. But I can't swear by the correctness of some of the computations with REAL's made in the original fortran code. > >> it seems like a nice opt-in thing that can be used where the aliases are >> verified and the code is particularly performance critical... > > Yes. I think we could use it in the kernel, although I'm not sure how many > cases we would ever find where we really care. > > Linus > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- Miguel Sousa Filipe -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
