Eliminating exception constraints of java programs for IA-64

Abstract

Java exception checks are designed to ensure that any faulting instruction causing a hardware exception does not terminate the program abnormally. These checks, however, impose some constraints upon the execution order between an instruction potentially raising a Java exception and a faulting instruction causing a hardware exception. This reduces the effectiveness of instruction reordering optimizations for Java programs. Although some effective techniques have been proposed to eliminate exception checks, many exception checks still remain. We propose a new framework to perform speculation effectively for Java programs using a direct acyclic graph representation (DAG) based on the SSA form. Using this framework, we apply a well-known speculation technique to a faulting load instruction to eliminate such constraints. We use edges to represent exception constraints. This allows us to estimate the potential reduction of the critical path length accurately for applying speculation. The new framework also allows us to avoid extra copy instructions and to generate efficient code with minimum register pressure. We have implemented it in the IBM Java Just-In-Time compiler, and observed performance improvements of 30% for one of the micro-benchmark programs, up to 13% (with an average of 1.7%) for the Java Grande Benchmark Suite, and up to 12% (with an average of 1.2%) for SPECjvm98 on an Itanium processor. Our speculation technique was particularly effective for those programs that access multidimensional arrays.