JCilk extends the serial subset of the Java language by importing the
fork-join primitives spawn and sync from the Cilk multithreaded
language, thereby providing call-return semantics for multithreaded
subcomputations. In addition, JCilk transparently integrates Java's
exception handling with multithreading by extending the semantics of
Java's try and catch constructs, but without adding new keywords.
This extension is "faithful" in that it obeys Java's ordinary serial
semantics when executed on a single processor. When executed in
parallel, however, an exception thrown by a JCilk computation causes
its sibling computations to abort, which yields a clean semantics in
which the enclosing cilk try block need only handle a single
exception.
The exception semantics of JCilk allow programs with speculative
computations to be programmed easily. Speculation is essential in
order to parallelize programs such as branch-and-bound or heuristic
search. We show how JCilk's linguistic mechanisms can be used to
program the "queens" puzzle and a parallel alpha-beta search.
We have implemented JCilk's semantic model in a prototype compiler
and runtime system, called JCilk-1. The compiler implements
continuations in a novel fashion by introducing goto statements into
Java. The JCilk-1 runtime system shadows the dynamic hierarchy of
cilk try blocks using a "try tree," allowing the system to chase
down side computations that must be aborted. Performance studies
indicate that JCilk's exception mechanism incurs minimal overhead,
contributing at most a few percent on top of the cost for a
spawn/return.
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