Linear Scan Register Allocation for the Java HotSpot Client Compiler

Register allocation is the task of assigning local variables and temporary values to physical registers of a processor. It is crucial for the efficiency of compiled code. The most commonly used algorithm treats the task of register allocation as a graph coloring problem. It generates code of good quality, but is too slow for just-in-time compilers because of its quadratic runtime complexity. For such compilers, the linear scan algorithm is an efficient alternative: It generates code of nearly the same quality, but is much faster than the graph coloring algorithm because it needs only a single pass over the lifetime intervals. The Java HotSpot Virtual Machine by Sun Microsystems uses a just-in-time compiler to generate native code for frequently executed methods. To achieve a high compilation speed and a low startup time, the HotSpot client compiler avoids time-consuming optimizations. The current product version assigns registers using a local heuristic. In the context of this master thesis, a research version of the compiler was extended with the linear scan algorithm for register allocation. The implemented variant improves the basic algorithm with more advanced optimizations: It makes use of lifetime holes, splits intervals if necessary and models register constraints of the target architecture with fixed intervals. Benchmark results prove that the linear scan algorithm is a good tradeoff if both compilation time and runtime of a program matter: The compilation time is only slightly higher in comparison with the old local heuristic for register allocation, but the resulting code executes about 30% faster. The benchmarks also indicate the high impact of the Intel SSE2 extensions on the speed of numeric Java applications.