CppCon 2018

Although expression templates have been around for many years and are a great example of compile-time code generation, they have typically been limited to relatively short expressions who's grammar can be expressed as a tree. What do you do when you want to express something more like an assembly-language program, with loops (cycles) in it? This was the problem that I encountered when trying to identify certain loop patterns in an LLVM compiler back end. The LLVM infrastructure provides a pattern matching library, but it has some limitations -- most notably the inability to express cycles in the code it matches.

Taking the idea of expression templates and kicking it up a notch, I created a library that allows an input pattern to be expressed directly in C++ using a syntax that resembles assembly language then generate, at compile time, an actual pattern-matching subroutine customized for that pattern. In the process, I deepened my knowledge of constexpr expressions, and built an interesting infrastructure that I will now share with you.

This talk is for the advanced C++ programmer. An understanding of templates, type traits, and basic metaprogramming is assumed. No prior knowledge of LLVM or compilers is required, though compiler back-end implementers will find this talk especially interesting.