egg: Fast and Extensible E-graphs

Preprint, July 2020
    title={egg: Fast and Extensible E-graphs},
    author={Max Willsey and Yisu Remy Wang and Oliver Flatt and Chandrakana Nandi and Pavel Panchekha and Zachary Tatlock},
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An e-graph efficiently represents a congruence relation over many expressions. Although they were originally developed in the late 1970s for use in automated theorem provers, a more recent technique known as equality saturation repurposes e-graphs to implement state-of-the-art, rewrite-driven compiler optimizations and program synthesizers. However, e-graphs remain unspecialized for this newer use case. Equality saturation workloads exhibit distinct characteristics and often require ad hoc e-graph extensions to incorporate transformations beyond purely syntactic rewrites.

This work contributes two techniques that make e-graphs fast and extensible, specializing them to equality saturation. A new amortized congruence closure algorithm called rebuilding takes advantage of equality saturation's distinct workload, providing asymptotic speedups over current techniques in practice. A general mechanism called e-class analyses integrates domain-specific analyses into the e-graph, reducing the need for ad hoc manipulation.

We implemented these techniques in a new open-source library called egg. Our case studies on three previously published applications of equality saturation highlight how the flexibility of e-class analyses supports diverse domains and how egg can provide up to 3000× speed ups.