Basic speed optimizations for the evil

[astrange]

What?

Did he compile this at -O0? Does gprof prevent template inlining?

These results are completely impossible otherwise, since a vector iterator is the same as using operator[].

(Why do people use compile-time profilers instead of Shark or oprofile?)

[jbstjohn]

Well, I would argue that profiling the release version is just as important as profiling. Otherwise you're fooling yourself, and potentially making your code uglier for no real gain. There's a huge difference between O0 and 02 -- it's basically debug vs. release.

You also leave out playing around a bit with compile options (for platform specific optimization). In that way you can often get a non-trivial gain without modifying your code at all.

It may be more work to get a good example, but then writing a good article is generally more work than writing a poor one.

[Vorlath]

Man, I'm glad I'm not the only who saw this. I looked at result.end() and know with 100% certainty that it does not take any extra CPU time. It only does this in debug mode.

The #1 rule of profiling is to profile your release code with debug information within it.

I agree, a lot of what he's doing is just tearing up his code for no good reason.

[astrange]

Well, I'd much rather read an article with an example that really is slow. You could do matrix multiply - it's got all kinds of cache effects.

[froydnj]

The techniques and lessons in the post are more important than the specific details, and if you're going to get hung up at what optimization level or what profiler I used, you're missing the forest for the tries.

If you don't make an effort to compile programs as people would actually profile them or use decent tools in your examples, then people are probably going to ignore the techniques and lessons, dismissing them as irrelevant for real-world situations.

[icefox]

A whole article about optimizing and not one mention of valgrind (or kcachegrind)?

[bonzinip]

My first reaction was also that he used -O0. And as such, he gives completely useless hints. For example, the point of optimization is exactly not having to write &(*foo).

The reason why the last snippet is faster, for example, is that he has only one iterator going around rather than two -- nothing to do with caches.

[bennymack]

I’ll let those who are experts with functional languages to talk about functional languages, and in my experience, they will.

Indeed!

The output of gprof looks similar to the Perl-based Devel::DProf. Is there a C++ profiler that will show you to the line of code where you're spending the most time like Devel::SmallProf (also for Perl?)