Speculative optimization using hardware-monitored guarded regions for java virtual machines

Aggressive dynamic optimization in high-performance Java Virtual Machines can be hampered by language features like Java's exception model, which requires precise detection and handling of program-generated exceptions. Furthermore, the compile-time overhead of guaranteeing correctness of code transformations precludes many effective optimizations from consideration. This paper describes a novel approach for circumventing the optimization-crippling effects of exception semantics and streamlining the implementation of aggressive optimizations at run time. Under a hardware-software hybrid model, the runtime system delineates guarded regions of code and specifies a contract--in the simplest case, one that requires exception-free execution--that must be adhered to in order to ensure that the aggressively optimized code within that region will behave as the programmer expects. The contracted runtime condition is assumed to be true, and code within a guarded region is aggressively optimized based on this assumption. Hardware monitors for exceptions throughout the region execution, and undoes the effects of the guarded region if an exception occurs, re-executing the region with a conventionally optimized version. Since exceptions are very rare, code can be optimized as if optimization-crippling conditions did not exist, leading to compile time reduction, code quality improvement, and potential performance improvement up to 67.7% and averaging 15.9% in our limit study of a set of Java benchmarks.