Exposed Datapath optimizations for Loop Scheduling

Abstract

Transport Triggered Architecture (TTA) processors allow unique low level compiler optimizations such as software bypassing and operand sharing. Previously, these optimizations have mostly been performed inside single basic blocks, leaving much of their potential unused. In this work, software bypassing and operand sharing are integrated with loop scheduling, allowing optimizations over loop iteration boundaries. This considerably further reduces register file accesses and immediate value transfers on tight loops – in some cases even eliminating all register file accesses from the loop body. In the benchmarked 12 small loops, compared to traditional VLIW-style processors, on average 63% of register file reads and 77% of register file writes could be eliminated. Compared to a compiler which performs these optimizations only inside a basic block, on average 58% of register file reads, 28% of register file writes could be eliminated. The additional register access reductions allow both direct energy savings from fewer register accesses and indirect energy savings by allowing the use of simpler register files with less read and write ports and a simpler interconnect network with less transport buses.