Biorthogonality, Step-Indexing and Compiler Correctness Nick Benton Chung-Kil Hur Microsoft Research University of Cambridge
[email protected] [email protected] Abstract to a deeper semantic one (‘does the observable behaviour of the We define logical relations between the denotational semantics of code satisfy this desirable property?’). In previous work (Benton a simply typed functional language with recursion and the opera- 2006; Benton and Zarfaty 2007; Benton and Tabareau 2009), we tional behaviour of low-level programs in a variant SECD machine. have looked at establishing type-safety in the latter, more semantic, The relations, which are defined using biorthogonality and step- sense. Our key notion is that a high-level type translates to a low- indexing, capture what it means for a piece of low-level code to level specification that should be satisfied by any code compiled implement a mathematical, domain-theoretic function and are used from a source language phrase of that type. These specifications are to prove correctness of a simple compiler. The results have been inherently relational, in the usual style of PER semantics, capturing formalized in the Coq proof assistant. the meaning of a type A as a predicate on low-level heaps, values or code fragments together with a notion of A-equality thereon. These Categories and Subject Descriptors F.3.1 [Logics and Mean- relations express what it means for a source-level abstractions (e.g. ings of Programs]: Specifying and Verifying and Reasoning about functions of type A ! B) to be respected by low-level code (e.g. Programs—Mechanical verification, Specification techniques; F.3.2 ‘taking A-equal arguments to B-equal results’).