These abstracts are for talks at this event.
NEPLS is a venue for ongoing research, so the abstract and supplemental material associated with each talk is necessarily temporal. The work presented here may be in a state of flux. In all cases, please consult the authors' Web pages for up-to-date information. Please don't refer to these pages as a definitive source.
Reductions, Intersection Types, and Explicit Substitutions
This paper is part of a general programme of treating explicit substitutions as the primary lambda-calculi from the point of view of foundations as well as applications. We work in a composition-free calculus of explicit substitutions and an augmented calculus obtained by adding explicit garbage-collection, and explore the relationship between intersection-types and reduction. We show that the terms which normalize by leftmost reduction and the terms which normalize by head reduction can each be characterized as the terms typable in a certain system. The relationship between typability and strong normalization is subtly different from the classical case.Our notions of leftmost- and head-reduction are non-deterministic, and our normalization theorems apply to any computations obeying these strategies. In this way we refine and strengthen the classical normalization theorems. The proofs require some new techniques in the presence of reductions involving explicit substitutions. Indeed, in our proofs we do not rely on results from classical lambda-calculus, which in our view is subordinate to the calculus of explicit substitution. The proof technique we employreadily yields the result that a term is strongly normalizing if and only if it is strongly normalizing in the calculus extended by garbage-collection.
Flow-Directed Lightweight CPS Conversion
Programming languages such as Scheme and SML/NJ support first-class continuations through a CALL/CC primitive. Without continuations, programs obey a last-in-first-out procedure-call discipline that allows stack-based allocation of activation records. In the presence of continuations, programs no longer obey a last-in-first-out procedure-call discipline and thus require heap-based allocation of activation records. In the past, two general strategies have been used to support CALL/CC. One involves converting the entire program to continuation-passing style (CPS). This process is called CPS conversion. In essence, this heap-allocates all activation records. The other involves leaving the program in direct style by implementing mechanisms to copy portions of the stack to the heap when creating continuations and copying saved continuations from the heap back to the stack when calling continuations. The former has the advantage that creating and calling continuations is cheap but the disadvantage that ordinary procedure call and return is expensive. The latter has the reverse set of tradeoffs. Lightweight CPS conversion is a novel scheme that gives the advantages of both approaches, without the disadvantages, in most situations.
First-Class Macros and Polymorphic Types: Progress Report
In my paper First-class Macros Have Types (in POPL00) I showed how to extend Scheme so that macros are first-class values -- although macros are still <EM>expanded</EM> at compile-time. The key idea is the addition of a simple type system to keep track of the expanders. The realization of this type system in a dynamically typed language such as Scheme or Dylan is straightforward, but its realization in a modern statically typed language (i.e., a language with type inference and polymorphism) presents some interesting challenges. In this talk, I will explain what the problems are, and what progress I have made towards solving them.
Making the Trains Run On Time
Automatic memory management, or garbage collection, is an important service for runtime systems supporting applications written in higher-level languages like ML or Java. The challenge in implementing garbage collectors is to make them as non-intrusive and scalable as possible. In particular, collectors should not require the application to be suspended for undue periods of time. Generational collectors improve collection pause times by segregating objects by age and by concentrating collection activity on recently allocated objects. This segregation of younger objects in a young generation works because, in practice, most objects die soon after allocation. However, the older generations must also be collected periodically--the challenge remains how to collect these regions without incurring long periods where the application is suspended. One approach to limiting pause times in the collection of older generations is to use an incremental collection technique where, at each collection pause, a portion of the older generation is collected. One example of this kind of technique is the train algorithm. Unfortunately, train-based collectors often fail to deliver on their promise of short, predictable pause times. In this talk, I will outline the reasons for these failures and present techniques that allow us to overcome these deficiencies through the use of better representations for remembered-sets and through better placement of objects. I will also offer experimental results supporting the idea that it is possible to constrain pause times, achieve good throughput, and still support incremental, copying garbage collection.
Equational Reasoning for Linking with First-Class Primitive Modules
Modules and linking are usually formalized by encodings which use the lambda-calculus, records (possibly dependent), and possibly some construct for recursion. In contrast, we introduce the m-calculus, a calculus where the primitive constructs are modules, linking, and the selection and hiding of module components. The m-calculus supports smooth encodings of software structuring tools such as functions (lambda-calculus), records, objects (varsigma-calculus), and mutually recursive definitions. The m-calculus can also express widely varying kinds of module systems as used in languages like C, Haskell, and ML. The m-calculus has been proven confluent, thereby showing that equational reasoning via the m-calculus is sensible and well behaved.
Program Analysis Techniques for Pointers and Accessed Memory Regions
Extracting information about what pieces of memory statements and procedures in a program access is the basis for the automatic detection of interactions between various code fragments in the program. This talk presents two analysis techniques that are able to deliver this kind of information for general programs, including programs which use recursion, multithreading, and manipulate pointers. A first step towards memory disambiguation is pointer analysis. This analysis computes, for each pointer and each program point, a conservative approximation of the memory locations to which that pointer may point. In this talk I will presents a novel interprocedural, flow-sensitive, and context-sensitive pointer analysis algorithm for multithreaded programs that may concurrently update shared pointers. Since pointer analysis treats arrays as monolithic objects, more sophisticated analyses are needed to extract access regions within arrays. This talk presents a novel framework for the symbolic bounds analysis of accessed memory regions. Our framework formulates each analysis problem as a system of inequality constraints between symbolic bound polynomials. It then reduces the constraint system to a linear program. The solution to the linear program provides symbolic lower and upper bounds for the regions of memory that each statement and procedure accesses. Experimental results from our implemented compiler show that the combination of pointer analysis and symbolic bounds analysis can solve several important problems, including static race detection, automatic parallelization, static detection of array bounds violations, elimination of array bounds checks, and reduction of the number of bits used to store computed values.
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