Predicated Static Single Assignment

Lori Carter, Beth Simon, Brad Calder, Larry Carter, and Jeanne Ferrante

In proceedings of the International Conference on Parallel Architectures and Compilation Techniques, October 1999.


Increases in instruction level parallelism are needed to exploit the potential parallelism available in future wide issue architectures. Predicated execution is an architectural mechanism that increases instruction level parallelism by removing branches and allowing simultaneous execution of multiple paths of control, only committing instructions from the correct path. In order for the compiler to expose such parallelism, traditional compiler data-flow analysis needs to be extended to predicated code.

In this paper, we present Predicated Static Single Assignment (PSSA) to enable aggressive predicated optimization and instruction scheduling. PSSA removes false dependences by exploiting renaming and information about the multiple control paths. We demonstrate the usefulness of PSSA for Predicated Speculation and Control Height Reduction. These two predicated code optimizations used during instruction scheduling reduce the dependence length of the critical paths through a predicated region. Our results show that using PSSA to enable speculation and control height reduction reduces execution time from 10% to 58%.