Automatic Array Inlining in Java Virtual Machines

Array inlining expands the concepts of object inlining to arrays. Groups of objects and arrays that reference each other are placed consecutively in memory so that their relative offsets are fixed, i.e. they are colocated. This allows memory loads to be replaced by address arithmetic, which reduces the costs of field and array accesses. We implemented this optimization for Sun Microsystems' Java HotSpot™ VM. The optimization is performed automatically and requires no actions on the part of the programmer. Arrays are frequently used for the implementation of dynamic data structures. Therefore, the length of arrays often varies, and fields referencing such arrays have to be changed whenever the array is reallocated. We present an efficient code pattern that detects these changes and allows the optimized access of such array fields. It is integrated into the array bounds check. We also claim that inlining array element objects into an array is not possible without a global data flow analysis. The evaluation shows that our dynamic approach can optimize frequently accessed fields with a reasonable low compilation and analysis overhead. The peak performance of SPECjvm98 is improved by 10% on average, with a maximum of 25%.