Lazy stateless incremental evaluation machinery for attribute grammars

Abstract

Many computer programs work with data that changes over time.Computations done over such data usually are repeated completely after a change in the data. For complex computations such repetitive recomputation can become too inefficient. When these recomputations take place on data which has only changed slightly, it often is possible to reformulate the computation to an incremental version which reuses the result of the computation on previous data. Such a situation typically occurs in compilers and editors for structured data (like a program) where program analyses and transformations (for example error checking) are done while editing.Although rewriting to incremental versions thus offers a solution to this problem, a manual rewrite of an already complex computation to its incremental counterpart is tedious, error prone, and inhibits further development of the original computation. We therefore intend to generate such incremental counterparts (semi)automatically by focusing on computations expressed using Attribute Grammars (AGs).In this paper we do groundwork for this goal and develop machinery for incremental attribute grammar evaluation based on change propagation and pure functions. We use pretty printing with free variable annotation to explain our techniques. Furthermore, our techniques also expose rules of conduct for a programmer desiring incrementality: the automatic translation of code to an incremental version does not always directly result in efficiency improvements because code often is written in a style unsuitable for automatic incrementalization. We show some common cases in which (small) code changes facilitating incrementality are required. We evaluate the effectiveness of the overall approach using a simple benchmark for the example, and a more extensive benchmark based on constraint-based type inference implemented with AGs.

Incremental Evaluation of Higher Order Attributes

“…Section 4 introduces earlier work on the representation of changes made to the input, and in Section 5 we describe the techniques for incremental attribute grammar evaluation without higher order attributes. This is similar to our earlier work (Bransen et al 2014) but adapted to support the main contribution of this paper in Section 6 where the support for higher order attributes is added. In Section 7 we evaluate the results with a benchmark and we discuss some shortcomings in Section 8.…”

“…Such an attribute is a simple counter which is increased every time a fresh variable is needed. As described in our previous work (Bransen et al 2014), such threaded attributes are bad for the effectiveness of the incremental evaluation. In previous work we have suggested several solutions for this, which could also be applied here, but to simplify the toy implementation we have chosen to implement fresh variable generation with a global mutable state instead.…”

“…In this paper, building upon our earlier work (Bransen et al 2014), we extend our techniques to support higher order attributes (Vogt et al 1989). A higher order attribute is an attribute value which itself is a tree structure over which attributes can be computed.…”

Generic representations of tree transformations

“…In fact, our work was motivated by the lack of an appropriate representation of transformations for the implementation of incremental evaluation of attribute grammars (Bransen et al 2013). As such, we plan to integrate our description of transformations in the Utrecht University Attribute Grammar system (UUAG, Swierstra et al 1999), and see if they can be put to good use in improving the performance of attribute grammars.…”

“…An example is the incremental evaluation of attribute grammars using change propagation (Bransen et al 2013): changing a value somewhere inside a tree might not require recomputing all the attributes. As such, incremental computations need information about how terms are transformed, and the quality of this information affects the recomputation avoidance mechanism.…”