No abstract

This paper describes the system overview of our Java Just-In-Time (JIT) compiler, which is the basis for the latest production version of IBM Java JIT compiler that supports a diversity of processor architectures including both 32-bit and 64-bit modes, CISC, RISC, and VLIW architectures. In particular, we focus on the design and evaluation of the cross-platform optimizations that are common across different architectures. We studied the effectiveness of each optimization by selectively disabling it in our JIT compiler on three different platforms: IA-32, IA-64, and PowerPC. Our detailed measurements allowed us to rank the optimizations in terms of the greatest performance improvements with the smallest compilation times. The identified set includes method inlining only for tiny methods, exception check eliminations using forward dataflow analysis and partial redundancy elimination, scalar replacement for instance and class fields using dataflow analysis, optimizations for type inclusion checks, and the elimination of merge points in the control flow graphs. These optimizations can achieve 90% of the peak performance for two industry-standard benchmark programs on these platforms with only 34% of the compilation time compared to the case for using all of the optimizations.

No abstract

Tracking the allocation site of every object at runtime is useful for reliable, optimized Java. To be used in production environments, the tracking must be accurate with minimal speed loss. Previous approaches suffer from performance degradation due to the additional field added to each object or track the allocation sites only probabilistically. We propose two novel approaches to track the allocation sites of every object in Java with only a 1.0% slowdown on average. Our first approach, the Allocation-Site-as-aHash-code (ASH) Tracker, encodes the allocation site ID of an object into the hash code field of its header by regarding the ID as part of the hash code. ASH Tracker avoids an excessive increase in hash code collisions by dynamically shrinking the bit-length of the ID as more and more objects are allocated at that site. For those Java VMs without the hash code field, our second approach, the Allocation-Site-via-a-Class-pointer (ASC) Tracker, makes the class pointer field in an object header refer to the allocation site structure of the object, which in turn points to the actual class structure. ASC Tracker mitigates the indirection overhead by constant-class-field duplication and allocation-site equality checks. While a previous approach of adding a 4-byte field caused up to 14.4% and an average 5% slowdown, both ASH and ASC Trackers incur at most a 2.0% and an average 1.0% loss. We demonstrate the usefulness of our low-overhead trackers by an allocation-site-aware memory leak detector and allocation-sitebased pretenuring in generational GC. Our pretenuring achieved on average 1.8% and up to 11.8% speedups in SPECjvm2008.

No abstract