Symbolic Loop Compilation for Tightly Coupled Processor Arrays

Abstract: Loop compilation for Tightly Coupled Processor Arrays (TCPAs), a class of massively parallel loop accelerators, entails solving NP-hard problems, yet depends on the loop bounds and number of available processing elements (PEs), parameters known only at runtime because of dynamic resource management and input sizes. Therefore, this article proposes a two-phase approach called symbolic loop compilation: At compile time, the necessary NP-complete problems are solved and the solutions compiled into a space-efficient symbolic configuration. At runtime, a concrete configuration is generated from the symbolic configuration according to the parameters values. We show that the latter phase, called instantiation, runs in polynomial time with its most complex step, program instantiation, not depending on the number of PEs. As validation, we performed symbolic loop compilation on real-world loops and measured time and space requirements. Our experiments confirm that a symbolic configuration is space-efficient and suited for systems with little memory -- often, a symbolic configuration is smaller than a single concrete configuration -- and that program instantiation scales well with the number of PEs -- for example, when instantiating a symbolic configuration of a matrix-matrix multiplication, the execution time is similar for $4\times 4$ and $32\times 32$ PEs.