Optimizing translation out of SSA using renaming constraints

Static Single Assignment form is an intermediate representation that uses Φ instructions to merge values at each confluent point of the control flow graph. Φ instructions are not machine instructions and must be renamed back to move instructions when translating out of SSA form. Without a coalescing algorithm, the out of SSA translation generates many move instructions. Leung and George [A. L. Leung et al., (1999)] use a SSA form for programs represented as native machine instructions, including the use of machine dedicated registers. For this purpose, they handle renaming constraints thanks to a pinning mechanism. Pinning Φ arguments and their corresponding definition to a common resource is also a very attractive technique for coalescing variables. Extending this idea, we propose a method to reduce the Φ-related copies during the out of SSA translation, thanks to a pinning-based coalescing algorithm that is aware of renaming constraints. We implemented our algorithm in the STMicroelectronics Linear Assembly Optimizer [B. Dupont de Dinechin et al., (2000)]. Our experiments show interesting results when comparing to the existing approaches of Leung and George [A. L. Leung et al., (1999)], Sreedhar et al. [V. Sreedhar et al., (1999)], and Appel and George for register coalescing [L. George et al., (1996)].