We prove a property of generalized LR (GLR) parsing -if the grammar is without right and hidden left recursions, then the number of consecutive reductions between the shifts of two adjacent symbols cannot be greater than a constant. Further, we show that this property can be used for constructing an optimized version of our GLR parser. Compared with a standard GLR parser, our optimized parser reads one symbol on every transition and performs significantly fewer stack operations. Our timings show that, especially for highly ambiguous grammars, our parser is significantly faster than a standard GLR parser.
To achieve maximum efficiency, modern embedded processors for media applications exploit single instruction multiple data (SIMD) instructions. SIMD instructions provide a form of vectorization where a large machine word is viewed as a vector of subwords and the same operation is performed on all subwords in parallel. Systematic usage of SIMD instructions can significantly improve program performance. With C becoming the dominant language for programming embedded devices, there is a clear need for C compilers that use SIMD instructions whenever appropriate. However, SIMD instructions typically require each memory access to be aligned with the instruction's data access size. Therefore an important problem in designing the compiler is to determine whether a C pointer is aligned, i.e. whether it refers to the beginning of a machine word. In this paper, we describe our SIMD generation algorithm and present an analysis method which determines the alignment of pointers at compile time. The alignment information is used to reduce the number of dynamic alignment checks and the overhead incurred by them. Our method uses an interprocedural analysis which propagates pointer alignment information in function bodies and through function calls. The effectiveness of our method is supported by experimental results which show that in typical programs the alignments of about 50% of the pointers can be statically determined. Copyright © 2006 John Wiley & Sons, Ltd.
Learning to code can be made more effective and sustainable if it is perceived as fun by the learner. Code Hunt uses puzzles that players have to explore by means of clues presented as test cases. Players iteratively modify their code to match the functional behaviour of secret solutions. This way of learning to code is very different to learning from a specification. It is essentially re-engineering from test cases. Code Hunt is based on the test/clue generation of Pex, a white-box test generation tool that uses dynamic symbolic execution. Pex performs a guided search to determine feasible execution paths. Conceptually, solving a puzzle is the manual process of conducting search-based test generation: the "test data" to be generated by the player is the player's code, and the "fitness values" that reflect the closeness of the player's code to the secret code are the clues (i.e., Pex-generated test cases). This paper is the first one to describe Code Hunt and its extensions over its precursor Pex4Fun. Code Hunt represents a high-impact educational gaming platform that not only internally leverages fitness values to guide test/clue generation but also externally offers fun user experiences where search-based test generation is manually emulated. Because the amount of data is growing all the time, the entire system runs in the cloud on Windows Azure.
This paper proposes that GUI development is as important as other aspects of programming, such as a sound understanding of control structures and object orientation. Far less attention has been paid to the programming structures for GUIs and certainly there are few cross language principles to aid the programmer. We propose that principles of GUIs can be extracted and learnt, and that they do enhance good programming practice. These principles have been implemented in our Views system which features an XML-based GUI description notation coupled with an engine that shields the programmer from much of the intricate complexity associated with events, listeners and handlers. The system is programmed primarily in C# for .NET, but is available in various forms for Java and for other platforms which support .NET through the SSCLI.
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