Branch taken rate of Linux kernel compiled with GCC 4.9

[Paul_Koning@Dell.com]

> On Jan 13, 2015, at 7:44 AM, Alexander Monakov <amonakov@ispras.ru> wrote:
> 
> 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
>> results:
> [...]
>> 
>> 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
> reverse them.

Depending on what the processor hardware can do, the data you need is the branch mispredict rate.  A lot of processors will predict forward branches not taken, reverse branches taken, and I believe GCC optimizes that way.  (Some processors are different: MC68040 predicts all branches taken no matter what direction!)  If the mispredict rate is unreasonably high, then that might indeed suggest missed optimizations.

	paul