Section: New Results

Revisiting out-of-SSA translation for correctness, efficiency, and speed

Participants : Benoit Boissinot, Alain Darte, Benoit Dupont-de-Dinechin [ stm icroelectronics ] , Christophe Guillon [ stm icroelectronics ] , Fabrice Rastello.

Static single assignment (SSA) form is an intermediate program representation in which many code optimizations can be performed with fast and easy-to-implement algorithms. However, some of these optimizations create situations where SSA variables arising from the same original variable now have overlapping live ranges. This complicates the translation out of SSA code into standard code. There are three issues: correctness, code quality (elimination of useless copies), and algorithm efficiency (speed and memory footprint). Briggs et al. proposed patches to correct the initial approach of Cytron et al. A cleaner and more general approach was then proposed by Sreedhar et al., along with techniques to reduce the number of generated copies. We went one step further. We proposed a conceptually simpler approach, based on coalescing and a more precise view of interferences, in which correctness and optimizations are separated. Our approach is provably correct and simpler to implement, with no patches or particular cases as in previous solutions, while reducing the number of generated copies. Also, experiments with SPEC CINT2000 show that it is 2x faster and 10x less memory-consuming than the Method III of Sreedhar et al., which makes it suitable for just-in-time compilation.

These results have been presented at CGO'09 [6] and were acknowledged by the best paper award.