The program runs 3 times slower when compiled with g++ 5.3.1 than the same program compiled with g++ 4.8.4, the same command

Question

Recently, I\'ve started to use Ubuntu 16.04 with g++ 5.3.1 and checked that my program runs 3 times slower. Before that I\'ve used Ubuntu 14.04, g++ 4.8.4. I bu

Answer 1

This is a bug in glibc that affects versions 2.23 (in use in Ubuntu 16.04) and early versions of 2.24 (e.g. Fedora and Debian already include the patched versions that are no longer affected, Ubuntu 16.10 and 17.04 do not yet).

The slowdown stems from the SSE to AVX register transition penalty. See the glibc bug report here: https://sourceware.org/bugzilla/show_bug.cgi?id=20495

Oleg Strikov wrote up a quite extensive analysis in his Ubuntu bug report: https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1663280

Without the patch, there are various possible workarounds: you can compile your problem statically (i.e. add -static) or you can disable lazy binding by setting the environment variable LD_BIND_NOW during the program's execution. Again, more details in the above bug reports.