[Bug c++/91875] Performance drop with mt19937 with -O2/-O3/-Ofast compared to -O1

[hehaochen at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91875

--- Comment #1 from hehaochen at hotmail dot com <hehaochen at hotmail dot com> ---
All the experiments are run on
gcc-docker(https://hub.docker.com/_/gcc?tab=description) on CentOS Linux
release 7.6.1810 Core.