High-productivity languages for parallel computing become more important as parallel environments including multicores become more common. Cilk is such a language. It provides good load balancing for many applications including irregular ones; that is, it keeps all workers busy by creating plenty of "logical" threads and adopting the oldest-first work stealing strategy. This paper proposes a "logical thread"-free framework called Tascell, which achieves a higher performance and supports a wider range of parallel environments including clusters without loss of productivity. A Tascell worker spawns a "real" task only when requested by another idle worker. The worker performs the spawning by temporarily "backtracking" and restoring its oldest task-spawnable state. Our approach eliminates the cost of spawning/managing logical threads. It also promotes the reuse of workspaces and improves the locality of reference since it does not need to prepare a workspace for each concurrently runnable logical thread. Furthermore, Tascell enables elegant and highly-efficient backtrack search algorithms with delayed workspace copying. For instance, our 16-queens problem solver is 1.86 times faster than Cilk on a system with two dual-core processors. Our approach also enables a single program to run in both shared and distributed memory environments with reasonable efficiency and scalability.

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Abstract. We propose a new language concept called "L-closures" for a running program to legitimately inspect/modify the contents of its execution stack. L-closures are lightweight lexical closures created by evaluating nested function definitions. A lexical closure can access the lexically-scoped variables in the creation-time environment and indirect calls to it provide legitimate stack access. By using an intermediate language extended with L-closures in high-level compilers, high-level services such as garbage collection, check-pointing, multithreading and load balancing can be implemented elegantly and efficiently. Each variable accessed by an L-closure uses private and shared locations for giving the private location a chance to get a register. Operations to keep coherency with shared locations as well as operations to initialize L-closures are delayed until an L-closure is actually invoked. Because most high-level services create L-closures very frequently but call them infrequently (e.g., to scan roots in garbage collection), the total overhead can be reduced significantly. Since the GNU C compiler provides nested functions, we enhanced GCC at relatively low implementation costs. The results of performance measurements exhibit quite low costs of creating and maintaining L-closures.

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The SC language system was developed to provide a transformation-based language extension scheme for SC languages (extended/plain C languages with an S-expression-based syntax). Using this system, many flexible extensions to the C language can be implemented by means of transformation rules over S-expressions at low cost, mainly because of the preexisting Common Lisp capabilities for manipulating S-expressions. This paper presents the LW-SC (Lightweight-SC) language as an important application of this system, featuring nested functions (i.e., functions defined inside other functions). A function can manipulate its caller's local variables (or local variables of its indirect callers) by indirectly calling a nested function of its callers. Thus, many high-level services with "stack walk" can be easily and elegantly implemented by using LW-SC as an intermediate language. Moreover, such services can be implemented efficiently because we designed and implemented LW-SC to provide "lightweight" nested functions by aggressively reducing the costs of creating and maintaining nested functions. The GNU C compiler also provides nested functions as an extension to C, but our sophisticated translator to standard C is more portable and efficient for occasional "stack walk."

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