Hi, I have analyzed the branch taken rate of the Linux kernel compiled with GCC (using localyesconfig from Debian config) and found something strange.
Hardware: Intel Core i7-4770, 32G RAM, 10GbE Software: Linux 3.16.7, GCC 4.9.3 20121201, Debian sid I use perf with rbf88:k,rff88:k events (Haswell specific) to profile the taken rate of conditional branches in the kernel. Here are the results: Application Taken (rbf88:k) Total (rff88:k) Taken rate Apache 3.91e9 6.35e9 62% nginx 6.00e9 1.03e10 58% MySQL 1.11e9 1.81e9 61% PostgreSQL 2.95e8 4.79e8 62% memcached 3.69e9 6.39e9 58% Redis 1.77e9 3.06e9 58% The results are very strange because all the taken rates are greater than 50%. Why not reverse the basic block reordering heuristics to make them under 50%? Is there anything wrong with GCC? Regards, YUAN, Pengfei Peking University
