E-Path: Equality Saturation for Control-Flow Graphs
This work addresses the problem of applying equality saturation to arbitrary control-flow graphs without requiring normalization, benefiting compiler optimization researchers seeking to avoid phase-ordering issues.
The paper introduces E-Path, a data structure for equality-saturation-style optimization over control-flow graphs, enabling non-destructive optimization of loops and control-flow structures by treating instruction sequences as congruence units. The prototype is instantiated over an ANF-based CFG and shows potential for expressing classical CFG optimizations as rewrite-driven transformations.
Modern equality saturation systems excel at expression-level rewrites by exploring large spaces of equivalent programs without suffering from the phase-ordering problem. How- ever, these systems struggle to represent equivalence directly over arbitrary control-flow graphs, often requiring normal- ization into structured or tree-like forms before optimization can occur. We present the E-Path data structure, a prototype frame- work for equality-saturation-style optimization over control- flow graphs. Instead of representing congruence between individual expressions, the E-Path records equivalence be- tween instruction sequences embedded within a compiler intermediate representation. In this prototype, E-Path is in- stantiated over a restricted ANF-based control-flow graph used in a compiler backend, but the model itself is intended to be IR-agnostic. By treating instruction sequences as the fundamental unit of congruence, the E-Path enables non-destructive optimiza- tion of loops and other control-flow structures while preserv- ing multiple equivalent program variants simultaneously. This allows classical CFG optimizations to be expressed as rewrite-driven transformations without destructive mutation of the underlying graph.