Compiling tree transforms to operate on packed representations
When written idiomatically in most programming languages, programs that traverse and construct trees operate over pointer-based data structures, with one heap object per-leaf and node. While this may seem tautological, we show that common tree traversals—found in compiler passes, space-partitioning trees, and elsewhere—can instead be automatically compiled to operate on pointer-free pre-order serializations of trees. On current x86 architectures such programs can run up to several times faster than their pointer-based counterparts.
We present a prototype compiler for a small first-order, purely functional language of tree traversals. The output language includes mutable cursors into input and output buffers for packed data. We propose a compilation technique with an effect system for capturing traversal behavior, combined with a lightweight analysis inferring data flow, and a program synthesis step for creating missing traversals.