BuildIt: A Type-Based Multi-stage Programming Framework for Code Generation in C++

The simplest implementation of a domain-specific language is to embed it in an existing language using operator overloading. This way, the DSL can inherit parsing, syntax and type checking, error handling, and the toolchain of debuggers and IDEs from the host language. A natural host language choice for most high-performance DSLs is the de-facto highperformance language, C++. However, DSL designers quickly run into the problem of not being able to extract control flows due to a lack of introspection in C++ and have to resort to special functions with lambdas to represent loops and conditionals. This approach introduces unnecessary syntax and does not capture the side effects of updates inside the lambdas in a safe way. We present BuildIt, a type-based multi-stage execution framework that solves this problem by extracting all control flow operators like if-then-else conditionals and for and while loops using a pure library approach. BuildIt achieves this by repeated execution of the program to explore all control flow paths and construct the AST piece by piece. We show that BuildIt can do this without exponential blow-up in terms of output size and execution time. We apply BuildIt's staging capabilities to the state-of-the-art tensor compiler TACO to generate low-level IR for custom-level formats. Thus, BuildIt offers a way to get both generalization and programmability for the user while generating specialized and efficient code. We also demonstrate that BuildIt can generate rich control-flow from relatively simple code by using it to stage an interpreter for an esoteric language. BuildIt changes the way we think about multi-staging as a problem by reducing the PL complexity of a supposedly harder problem requiring features like introspection or specialized compiler support to a set of common features found in most languages.