The ISO C standard does not specify the semantics of many valid programs that use non-portable idioms such as integer-pointer casts. Recent efforts at formal definitions and verified implementation of the C language inherit this feature. By adopting high-level abstract memory models, they validate common optimizations. On the other hand, this prevents reasoning about much low-level code relying on the behavior of common implementations, where formal verification has many applications.We present the first formal memory model that allows many common optimizations and fully supports operations on the representation of pointers. All arithmetic operations are well-defined for pointers that have been cast to integers. Crucially, our model is also simple to understand and program with. All our results are fully formalized in Coq.

Formalizations of concurrent memory models often represent memory behavior in terms of sequences of operations, where operations are either reads, writes, or synchronizations. More concrete models of (sequential) memory behavior may include allocation and free operations, but also include details of memory layout or data representation. We present an abstract specification for sequential memory models with allocation and free operations, in the form of a set of axioms that provide enough information to reason about memory without overly constraining the behavior of implementations. We characterize a set of "well-behaved" programs that behave uniformly on all instances of the specification. We show that the specification is both feasible-the CompCert memory model implements it-and usable-we can use the axioms to prove the correctness of an optimization that changes the memory behavior of programs in an LLVM-like language.

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