Constraint-Aware E-Graph Rewriting for Hardware Performance Optimization

Samuel Coward, Theo Drane, George A. Constantinides

Abstract: Data-dependent constraints commonly occur across hardware and software, often in the form of code branches or input constraints. Expert designers exploit these constraints to realize new optimization opportunities. Numerical hardware designers exploit this aggressively, as if a particular module input value can never be seen, then there is no need to dedicate any circuit area to handling that input value. Floating-point hardware designers have gone further, specifically introducing carefully constructed case-splits to exploit underutilized critical paths. To automate constraint-aware optimization, we developed a theoretical framework, based on the e-graph data structure, that localizes constraint reasoning tasks and makes it simple to realize optimizations exploiting the underlying control structures. The theory introduced here provides an approach to encode multiple equivalence relations within a single e-graph. To demonstrate the value of the theoretical developments, we extend an existing register transfer level (RTL) optimization tool, ROVER, allowing it to exploit constraints present in the RTL itself. We combine this new constraint-awareness with a new RTL value range analysis, allowing ROVER to understand what values each intermediate signal can take. We further add to ROVER by developing a model for circuit delay, allowing ROVER to explore the tradeoffs between performance and circuit area. With these latest developments, ROVER is capable of fully automatically discovering known floating-point architectures from the computer arithmetic literature. The designs generated by constraint-aware ROVER are, on average, 30% faster and 1% smaller than those generated by state-of-the-art EDA tools.