Code generation for partially vectorizable loops in the vectorizing Pascal-XT compiler

Abstract

Automatic vectorization of loops by investigation of data access dependences is known from literature. But the algorithms almost only describe the vectorization of fully vectorizable loops. A loop is judged to be nonvectorizable, if some data access dependences form a cycle. However, practical programs often contain loops with cyclic data access dependences where not all variables are included in the cyclic dependence. If cyclic dependences can be isolated, variables not encountered in the cyclic dependence are vectorizable. Thus, in vectorizing compilers not only full loop vectorization but also partial vectorization should be done. Here, the code generation for partially vectorizable loops is described for the vectorizing Pascal-XT compiler. The effect is compared to the vectorizing Fortran compilers of CDC, Cray and Fujitsu.