This is the mail archive of the
mailing list for the GCC project.
Re: Branch taken rate of Linux kernel compiled with GCC 4.9
- From: Alexander Monakov <amonakov at ispras dot ru>
- To: Pengfei Yuan <0xcoolypf at gmail dot com>
- Cc: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Tue, 13 Jan 2015 15:44:52 +0300 (MSK)
- Subject: Re: Branch taken rate of Linux kernel compiled with GCC 4.9
- Authentication-results: sourceware.org; auth=none
- References: <CACmZjJLQK6y6+X-57BWmC9W-hfmaBe57ZVXrhqHKEUyW=WHatQ at mail dot gmail dot com>
On Tue, 13 Jan 2015, Pengfei Yuan wrote:
> 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
> 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?
Your measurement includes the conditional branches at the end of loop bodies.
When loops iterate, those branches are taken, and it doesn't make sense to