Single Assignment C (SAC) High Productivity Meets High Performance

We present the ins and outs of the purely functional, data parallel programming language SaC (Single Assignment C). SaC defines stateand side-effect-free semantics on top of a syntax resembling that of imperative languages like C/C++/C# or Java: functional programming with curly brackets. In contrast to other functional languages data aggregation in SaC is not based on lists and trees, but puts stateless arrays into the focus. SaC implements an abstract calculus of truly multidimensional arrays that is adopted from interpreted array languages like Apl. Arrays are abstract values with certain structural properties. They are treated in a holistic way, not as loose collections of data cells or indexed memory address ranges. Programs can and should be written in a mostly index-free style. Functions consume array values as arguments and produce array values as results. The array type system of SaC allows such functions to abstract not only from the size of vectors or matrices but likewise from the number of array dimensions, supporting a highly generic programming style. The design of SaC aims at reconciling high productivity in software engineering of compute-intensive applications with high performance in program execution on modern multiand many-core computing systems. While SaC competes with other functional and declarative languages on the productivity aspect, it competes with hand-parallelised C and Fortran code on the performance aspect. We achieve our goal through stringent co-design of programming language and compilation technology. The focus on arrays in general and the abstract view of arrays in particular combined with a functional state-free semantics are key ingredients in the design of SaC. In conjunction they allow for far-reaching program transformations and fully compiler-directed parallelisation. From literally the same source code SaC currently supports symmetric multisocket, multi-core, hyperthreaded server systems, CUDA-enables graphics accelerators and the MicroGrid, an innovative general-purpose manycore architecture. The CEFP lecture provides an introduction into the language design of SaC, followed by an illustration of how these concepts can be harnessed to write highly abstract, reusable and elegant code. We conclude with outlining the major compiler technologies for achieving runtime performance levels that are competitive with low-level machine-oriented programming environments.