Efficiency in ILP processing by using orthogonality

摘要

For the next generations of Processor-Arrays-on-Chip (e. g., coarse-grained reconfigurable or programmable arrays)-including more than 100s to 1000s of processing elements-it is very important to keep the on-chip configuration/instruction memories as small as possible. Hence, compilers must take into account the scarceness of available instruction memory and create the code as compact as possible [1]. However, Very Long Instruction Word (VLIW) processors have the well-known problem that compilers typically produce lengthy codes. A lot of unnecessary code is produced due to unused Functional Units (FUs) or repeating operations for single FUs in instruction sequences. Techniques like software pipelining can be used to improve the utilization of the FUs, yet with the risk of code explosion [2] due to the overlapped scheduling of multiple loop iterations or other control flow statements. This is, where our proposed Orthogonal Instruction Processing (OIP) architecture (see Fig. 1) shows benefits in reducing the code size of compute-intensive loop programs. The idea is, contrary to lightweight VLIW processors used in arrays like Tightly Coupled Processor Arrays (TCPAs) [4], to equip each FU with its own instruction memory, branch unit, and program counter, but still let the FUs share the register files as well as input and output signals. This enables a processor to orthogonally execute a loop program. Each FU can execute its own sub-program while exchanging data over the register files. The branch unit and its instruction format have to be slightly changed by introducing a counter to each instruction that determines how often the instruction is repeated until the specified branch is executed. This enables repeating instructions without repeating them in the code. Those kind of processors have to be carefully programmed, e. g., to not run into data dependency problems while optimizing throughput. For solving this resource-constrained modulo scheduling problem, we use techniques based on mixed integer linear programming [5], [3].