Improvements to the Psi-SSA representation

Modern compiler implementations use the Static Single Assignment representation [5] as a way to efficiently implement optimizing algorithms. However this representation is not well adapted to architectures with a predicated instruction set. The ψ-SSA representation was first introduced in [11] as an extension to the Static Single Assignment representation. The ψ-SSA representation extends the SSA representation such that standard SSA algorithms can be easily adapted to an architecture with a fully predicated instruction set. A new pseudo operation, the ψ operation, is introduced to merge several conditional definitions into a unique definition.

This paper presents an adaptation of the ψ-SSA representation to support architectures with a partially predicated instruction set. The definition of the ψ operation is extended to support the generation and the optimization of partially predicated code. In particular, a predicate promotion transformation is introduced to reduce the number of predicated operations, as well as the number of operations used to compute guard registers. An out of ψ-SSA algorithm is also described, which fixes and improves the algorithm described in [11]. This algorithm is derived from the out of SSA algorithm from Sreedhar et al. [10], where the definitions of liveness and interferences have been extended for the ψ operations. This algorithm inserts predicated copy operations to restore the correct semantics in the program in a non-SSA form.

The ψ-SSA representation is used in our production compilers, based on the Open64 technology, for the ST200 family processors. In this compiler, predicated code is generated by an if-conversion algorithm performed under the ψ-SSA representation [12, 1].