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The C/C++11 memory model defines the semantics of concurrent memory accesses in C/C++, and in particular supports racy "atomic" accesses at a range of different consistency levels, from very weak consistency ("relaxed") to strong, sequential consistency ("SC"). Unfortunately, as we observe in this paper, the semantics of SC atomic accesses in C/C++11, as well as in all proposed strengthenings of the semantics, is flawed, in that (contrary to previously published results) both suggested compilati

Despite many years of research, it has proven very difficult to develop a memory model for concurrent programming languages that adequately balances the conflicting desiderata of programmers, compilers, and hardware. In this paper, we propose the first relaxed memory model that (1) accounts for a broad spectrum of features from the C++11 concurrency model, (2) is implementable, in the sense that it provably validates many standard compiler optimizations and reorderings, as well as standard compi

Interaction trees (ITrees) are a general-purpose data structure for representing the behaviors of recursive programs that interact with their environments. A coinductive variant of “free monads,” ITrees are built out of uninterpreted events and their continuations. They support compositional construction of interpreters from event handlers , which give meaning to events by defining their semantics as monadic actions. ITrees are expressive enough to represent impure and potentially nonterminating

We define logical relations between the denotational semantics of a simply typed functional language with recursion and the operational behaviour of low-level programs in a variant SECD machine. The relations, which are defined using biorthogonality and stepindexing, capture what it means for a piece of low-level code to implement a mathematical, domain-theoretic function and are used to prove correctness of a simple compiler. The results have been formalized in the Coq proof assistant.

Coinduction is one of the most basic concepts in computer science. It is therefore surprising that the commonly-known lattice-theoretic accounts of the principles underlying coinductive proofs are lacking in two key respects: they do not support compositional reasoning (i.e. breaking proofs into separate pieces that can be developed in isolation), and they do not support incremental reasoning (i.e. developing proofs interactively by starting from the goal and generalizing the coinduction hypothe

Compiler verification is essential for the construction of fully verified software, but most prior work (such as CompCert) has focused on verifying whole-program compilers. To support separate compilation and to enable linking of results from different verified compilers, it is important to develop a compositional notion of compiler correctness that is modular (preserved under linking), transitive (supports multi-pass compilation), and flexible (applicable to compilers that use different interme

We present a new Markov Chain Monte Carlo (MCMC) sampling algorithm for probabilistic programs. Our approach and tool, called R2, has the unique feature of employing program analysis in order to improve the efficiencyof MCMC sampling. Given an input program P, R2 propagates observations in P backwards to obtaina semantically equivalent program P' in which every probabilistic assignment is immediately followed by an observe statement. Inference is performed by a suitably modified version of the M

We designed, implemented, and deployed Alive2: a bounded translation validation tool for the LLVM compiler’s intermediate representation (IR). It limits resource consumption by, for example, unrolling loops up to some bound, which means there are circumstances in which it misses bugs. Alive2 is designed to avoid false alarms, is fully automatic through the use of an SMT solver, and requires no changes to LLVM. By running Alive2 over LLVM’s unit test suite, we discovered and reported 47 new bugs,

There has recently been great progress in proving the correctness of compilers for increasingly realistic languages with increasingly realistic runtime systems. Most work on this problem has focused on proving the correctness of a particular compiler, leaving open the question of how to verify the correctness of assembly code that is hand-optimized or linked together from the output of multiple compilers. This has led Benton and other researchers to propose more abstract, compositional notions o

There has recently been great progress in proving the correctness of compilers for increasingly realistic languages with increasingly realistic runtime systems. Most work on this problem has focused on proving the correctness of a particular compiler, leaving open the question of how to verify the correctness of assembly code that is hand-optimized or linked together from the output of multiple compilers. This has led Benton and other researchers to propose more abstract, compositional notions o

There has been great progress in recent years on developing effective techniques for reasoning about program equivalence in ML-like languages---that is, languages that combine features like higher-order functions, recursive types, abstract types, and general mutable references. Two of the most prominent types of techniques to have emerged are *bisimulations* and *Kripke logical relations (KLRs)*. While both approaches are powerful, their complementary advantages have led us and other researchers

Probabilistic programs use familiar notation of programming languages to specify probabilistic models. Suppose we are interested in estimating the distribution of the return expression r of a probabilistic program P. We are interested in slicing the probabilistic program P and obtaining a simpler program Sli(P) which retains only those parts of P that are relevant to estimating r, and elides those parts of P that are not relevant to estimating r. We desire that the Sli transformation be both cor

Major compiler verification efforts, such as the CompCert project, have traditionally simplified the verification problem by restricting attention to the correctness of whole-program compilation, leaving open the question of how to verify the correctness of separate compilation. Recently, a number of sophisticated techniques have been proposed for proving more flexible, compositional notions of compiler correctness, but these approaches tend to be quite heavyweight compared to the simple "closed

There has been great progress in recent years on developing effective techniques for reasoning about program equivalence in ML-like languages---that is, languages that combine features like higher-order functions, recursive types, abstract types, and general mutable references. Two of the most prominent types of techniques to have emerged are *bisimulations* and *Kripke logical relations (KLRs)*. While both approaches are powerful, their complementary advantages have led us and other researchers