Towards Zero-Overhead Disambiguation of Deep Priority Conflicts

Eduardo de Souza Amorim

Context-free grammars are widely used for language prototyping and implementation. They allow formalizing the syntax of domain-specific or general-purpose programming languages concisely and declaratively. However, the natural and concise way of writing a context-free grammar is often ambiguous. Therefore, grammar formalisms support extensions in the form of declarative disambiguation rules to specify operator precedence and associativity, solving ambiguities that are caused by the subset of the grammar that corresponds to expressions.

Ambiguities with respect to operator precedence and associativity arise from combining the various operators of a language. While shallow conflicts can be resolved efficiently by one-level tree patterns, deep conflicts require more elaborate techniques, because they can occur arbitrarily nested in a tree.

Current state-of-the-art approaches to solving deep priority conflicts come with a severe performance overhead. In this talk, I will present a novel low-overhead implementation technique for disambiguating deep associativity and priority conflicts in scannerless generalized parsers with lightweight data-dependency. By parsing a corpus of popular open-source repositories written in Java and OCaml, we found that our approach yields speedups of up to 1.73 x over a grammar rewriting technique when parsing programs with deep priority conflicts — with a modest overhead of 1 % to 2 % when parsing programs without deep conflicts.

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