Thanks! The compile farm may be a good resource. I had access to it in the past. Maybe I still do.
Marc Glisse <marc.gli...@inria.fr> wrote: >On Wed, 23 Mar 2016, Victor Shoup wrote: > >> This may be a bit off topic, but I figure the people on this list >> might know something about this. >> >> In some code I've been developing lately (NTL related, of course), >> I've been making more use of the __uint128_t type that is available >> on gcc (and its clang and icc clones). It's all ifdef'd properly, so >I >> only use it when it actually works. >> >> Anyway, I find that on x86-64 machines and recent gcc's, the compiler >> does a pretty good job of code generation...much better than I recall >> some years ago. However, I was wondering about the 64-bit ARM >> machine. I don't have access to such a machine, but I tried some >code >> out at https://gcc.godbolt.org (which is a very convenient site, by >the way). >> I was somewhat surprised that the code generated there by gcc-4.8 for >> 64-bit ARM was terrible: a 64x64->128 mul gets mapped to >> a generic128x128->128 function call. > >You realize ARM64 barely existed at the time of gcc-4.8? If gcc-5, or >better yet a snapshot of gcc-6, still generates suboptimal code, please > >report to https://gcc.gnu.org/bugzilla/ with a testcase, and the asm >you >would like gcc to generate instead. > >> So I'm starting to question whether relying on __uint128_t is such a >good idea. >> Maybe it would be better for me to isolate all of that code so that I >can >> just drop in appropriate assembly (as in GMP's longlong.h), >> as an alternative. > >It is always a compromise... > >> I could also ask gcc people what their plans for future optimizations >> in this area are, but I don't know who or where to ask. > >You could ask on g...@gcc.gnu.org, but reporting bugs when you see >suboptimal code generated seems much more likely to get you answers, >and >by showing constructive interest it may spark further optimizations. > >If this is for the development of free software, the GCC compile farm >includes some aarch64 machines on which you could experiment. > >-- >Marc Glisse -- Sent from my Android device with K-9 Mail. Please excuse my brevity. _______________________________________________ gmp-devel mailing list gmp-devel@gmplib.org https://gmplib.org/mailman/listinfo/gmp-devel