SSA Elimination after Register Allocation

The SSA-form uses a notational abstractions called φ-functions. These instructions have no analogous in actual machine instruction sets, and they must be replaced by ordinary instructions at some point of the compilation path. This process is called SSA elimination. Compilers usually performs SSA elimination before register allocation. But the order could as well be the opposite: our puzzle based register allocator performs SSA elimination after register allocation. SSA elimination before register allocation is straightforward and standard, while the state-of-the-art approaches to SSA elimination after register allocation have several shortcomings. In this report we present spill-free SSA elimination, a simple and efficient algorithm for SSA elimination after register allocation that avoids increasing the number of spilled variables. We also present three optimizations that enhance the quality of the code produced by the core algorithm. Our experiments show that spill-free SSA elimination takes less than five percent of the total compilation time of a JIT compiler. Our optimizations reduce the number of memory accesses by more than 9% and improve the program execution time by more than 1.8%.