Optimal compilation for exposed datapath architectures with buffered processing units by SAT solvers

Abstract

Conventional processor architectures are restricted in exploiting instruction level parallelism (ILP) due to the limited number of available registers in their instruction sets. Therefore, recent processor architectures expose their datapaths so that the compiler not only schedules instructions to functional units, but also takes care of directly moving values between functional units avoiding the need of registers at all. However, the current compiler technology is still based on classic register architectures where a nearly optimal register mapping is the key for the quality of the generated assembly code. The Synchronous Control Asynchronous Dataflow (SCAD) architecture is a new exposed datapath architecture where processing units (PUs) are equipped with first-in first-out (FIFO) buffers at their inputs and outputs. Code generation for SCAD machines can be done as known for classic queue machines to completely eliminate the use of registers, and to improve the degree of exploited ILP. However, the SCAD code generated this way is not optimal since compared to queue machines, SCAD machines can contain many PUs and buffers which offers the compiler more freedom to reduce unnecessary computational overhead. In this paper, we map the SCAD code generation problem to a satisfiability problem, and then use SAT solvers to generate code without overhead that works with the minimal number of PUs. The generated optimal code will serve as a reference to judge the quality of heuristics that will be finally used in SCAD compilers.