Re: [Gimp-developer] gimp-plugin profiling

[Jon Nordby <jononor gmail com>]

On 10 March 2013 22:55, Tibor Bamhor <tiborb95 gmail com> wrote:
> Hi,
>
> Thank for you interest. Today I spent some time reading about perf - it is
> quite complex tool or rather it measures things that I am not familiar with.
> However I did some primitive testing and got this output:
>
> ____7729.437618_task-clock________________#____0.386_CPUs_utilized__________
> ___________8556_context-switches__________#____0.001_M/sec__________________
> ______________0_cpu-migrations____________#____0.000_K/sec__________________
> ____________899_page-faults_______________#____0.116_K/sec__________________
> ____20346686795_cycles____________________#____2.632_GHz_____________________[92.65%]
> ______________0_stalled-cycles-frontend___#____0.00%_frontend_cycles_idle____[99.92%]
> ______588881284_stalled-cycles-backend____#____2.89%_backend__cycles_idle____[67.69%]
> _____2234245557_instructions______________#____0.11__insns_per_cycle________
> __________________________________________#____0.26__stalled_cycles_per_insn_[71.21%]
> ______931561726_branches__________________#__120.521_M/sec___________________[77.62%]
> _____1877007958_branch-misses_____________#__201.49%_of_all_branches_________[84.88%]
>
> I dont dare to interpret it, but the "201.49%" in last line was in red so
> there is obviously a problem there. Here I would start.
>
> If you (or anybody else) can point me at some source on internet I would be
> thankfull. Or perhaps shortly explain how to mitigate the problem. But I
> understand this is not gimp-specific issue...

Yes, if you have branches in inner loops eliminating them can give
large speedups.
A quick google search gave this reference which looks like an OK start:
http://software.intel.com/en-us/articles/avoiding-the-cost-of-branch-misprediction
Less branching may also allow the compiler to auto-vectorize more.
If you are unable to remove the branches in your inner loop, you can
try to guide the branch predictor using
GCCs __builtin_expect: http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html

Note that not just if/case statements introduce branching,
do/while/for do as well.

-- 
Jon Nordby - www.jonnor.com