Forwarding Unit Generation for Loop Pipelining in High-level Synthesis

In the loop pipelining of high-level synthesis, the reduction of initiation intervals (IIs) is very important. Existing loop pipelining techniques, however, pessimistically assumes that dependences whose occurrences can be determined only at runtime always occur, resulting in increased IIs. To address this issue, recent work achieves reduced II by a source code transformation which introduces runtime dependence analysis and performs pipeline stalls when the dependences actually occur. Unfortunately, the recent work suffers from the increased execution cycles by frequent pipeline stalls under the frequent occurrences of the dependences. In this paper, we propose a technique to reduce IIs in which data written to memories are also written to registers for such dependences of read-after-write (RAW) type. In our technique, registers which are faster than memories are accessed when the RAW dependences occur. Since the proposed technique achieved the reduction of the execution cycles by 34% with 15% gate count increase on average for three examples compared to the state-of-the-art technique, the proposed technique is effective for synthesizing high-speed circuits with loop pipelining.