On 13/10/15 21:44 -0400, Nathan Froyd wrote:
Including <algorithm> in C++11 mode (typically done for std::{min,max,swap}) includes <random>, for std::uniform_int_distribution. On x86 platforms, <random> manages to drag in <x86intrin.h> through x86's opt_random.h header, and <x86intrin.h> has gotten rather large recently with the addition of AVX intrinsics. The comparison between C++03 mode and C++11 mode is not quite exact, but it gives an idea of the penalty we're talking about here:froydnj@thor:~/src$ echo '#include <algorithm>' | g++ -x c++ - -o - -E -std=c++11 | wc 53460 127553 1401268 froydnj@thor:~/src$ echo '#include <algorithm>' | g++ -x c++ - -o - -E -std=c++03 | wc 9202 18933 218189 That's approximately a 7x penalty in C++11 mode (granted, C++11 includes more than just <x86intrin.h>) with GCC 4.9.2 on a Debian system; current mainline is somewhat worse: froydnj@thor: gcc-build$ echo '#include <algorithm>' | xgcc [...] -std=c++11 | wc 84851 210475 2369616 froydnj@thor: gcc-build$ echo '#include <algorithm>' | xgcc [...] -std=c++03 | wc 9383 19402 239676 <x86intrin.h> itself clocks in at 1.3MB+ of preprocessed text.
Yep, that's been bothering me for a while.
This patch aims to reduce that size penalty by recognizing that both of the places that #include <x86intrin.h> do not need the full set of x86 intrinsics, but can get by with a smaller, more focused header in each case. <ext/random> needs only <emmintrin.h> to declare __m128i, while x86's opt_random.h must include <pmmintrin.h> for declarations of various intrinsic functions. The net result is that the size of mainline's <algorithm> is significantly reduced: froydnj@thor: gcc-build$ echo '#include <algorithm>' | xgcc [...] -std=c++11 | wc 39174 88538 1015281 which seems like a win.
Indeed!
Bootstrapped on x86_64-pc-linux-gnu with --enable-languages=c,c++, tested with check-target-libstdc++-v3, no regressions. Also verified that <algorithm> and <ext/random> pass -fsyntax-check with -march=native (on a recent Haswell chip); if an -march=native bootstrap is necessary, I am happy to do that if somebody instructs me in getting everything properly set up. OK?
OK, thanks.
