Monday, February 17, 2014

Devirtualization in C++, part 4 (analyzing the type inheritance graph for fun and profit)

Last time I described the construction of the type inheritance graph, a main datastructure used by special purpose devirtualization algorithms. Now it is time for some real optimization fun! In this post I will review the most elementary transformations and will show some real world data.

Sunday, February 9, 2014

Devirtualization in C++, part 3 (building the type inheritance graph)

Having covered the basic optimizations done by the C++ frontend and the way how generic middle-end optimizations may devirtualize. It is a time to dive into an wonderful world if special-purpose middle-end devirtualization techniques.

Topic of optimization of polymorphic calls is seemingly well covered by the literature with active research since the advent of object oriented programming (see, for example, DF Bacon, PF Sweeney: Fast static analysis of C++ virtual function calls and the numerous papers citing thisone). Pretty much all of the techniques are based on constructing and analyzing the type inheritance graph.

Unfortunately most of the research papers make assumptions that are not met in modern C++ programs. Here the situation is complicated by many C++ features including interesting implementation details of multiple and virtual inheritance partly described in this post and ability to make dynamic type changes. Important is also the fact that the compiler hardly ever see the whole program at once. Even with the link-time optimization one has to take into account the presence of shared libraries. Because shared libraries are supposed to be upgradeable without recompiling programs that use them, the compiler can not make any analysis beyond what it is given in the header files and as explained last time sometimes not even that. Moreover shared libraries are also often dynamically loaded as plugins. Contemporary C++ programs are really a lot more dynamic than most of the research assume.

Today I thus cover the seemingly simple topic of building and walking the type inheritance graph and show details one has to think about. It may seem exhausting, but so was my way to gain all this wisdom. :) I promise that next time I finally get to some real optimization fun.