General performance improvements

[diegohavenstein]

Hi Simon,

I have studied the code a bit. I found several spots where a conditional is inside a loop (or several loops), where this could be avoided by using a array to store possible values and accessing the right index by evaluating a boolean expression.

I have marked the spots in the code here. This is not a real "pull request", since I did not change the code logic yet. But if you agree with me that the improvement could be noticeable in the numbers afterwards, I can proceed to change the code accordingly and push the changes afterwards to create a useful pull request.

Regards,
Diego

[sdsllitebot]

Can one of the admins verify this patch?

[eloj]

I'm not Simon, but I'd be more compelled by seeing at least some benchmark first. It's very hard to look at code and come up with micro-optimization that actually turn out to be worthwhile optimizations in practice, doubly so when code quality must also be considered. But that's me.

[simongog]

Hi Diego,

wow, thanks for reviewing the code so carefully. As @eloj already mentioned, the code should be fast and intelligible. In the past I discovered that many of these micro-optimizations do not really improve execution time (assuming the code is compiled with optimizations) and sometimes the branch is explicitly there since reading and writing through proxy objects is actually more expensive than the branch. So we have to be careful. I think this would be an example for this:
https://github.com/diegohavenstein/sdsl-lite/blob/general_performance_improvements/include/sdsl/suffix_array_algorithm.hpp#L481

How would you replace this code?

Things like exchanging if (x > max_val) max_val=x by std::max are ok, since both versions are easy to understand. I will give you detailed comments on your proposals in a few days.

[diegohavenstein]

Hi Simon,

yes, reading the proxy object regardless of the outcome of the branch could be expensive. In these cases I will not do anything, since I don't know the exact properties of these objects. But there are also lots of spots, like
https://github.com/diegohavenstein/sdsl-lite/blob/general_performance_improvements/include/sdsl/construct_sa_se.hpp#L242

where the branch could hurt. Maybe it is better if I see some profiling results first, maybe these functions are not heavily used and the optimizations are useless at the end. I will have a look these days.