Static conflict analysis for multi-threaded object-oriented programs

Abstract

A compiler for multi-threaded object-oriented programs needs information about the sharing of objects for a variety of reasons: to implement optimizations, to issue warnings, to add instrumentation to detect access violations that occur at runtime. An Object Use Graph (OUG) statically captures accesses from different threads to objects. An OUG extends the Heap Shape Graph (HSG), which is a compile-time abstraction for runtime objects (nodes) and their reference relations (edges). An OUG specifies for a specific node in the HSG a partial order of events relevant to the corresponding runtime object(s). Relevant events include read and write access, object escape, thread start and join.OUGs have been implemented in a Java compiler. Initial experience shows that OUGs are effective to identify object accesses that potentially conflict at runtime and isolate accesses that never cause a problem at runtime. The capabilities of OUGs are compared with an advanced program analysis that has been used for lock elimination. For the set of benchmarks investigated here, OUGs report only a fraction of shared objects as conflicting and reduce the number of compile-time reports in terms of allocation sites of conflicting objects by 28--92% (average 64%). For benchmarks of up to 30 KLOC, the time taken to construct OUGs is, with one exception, in the order of seconds.The information collected in the OUG has been used to instrument Java programs with checks for object races. OUGs provide precise information about object sharing and static protection, so runtime instrumentation that checks those cases that cannot be disambiguated at compile-time is sparse, and the total runtime overhead of checking for object races is only 3--86% (average 47%).