Shift Register Initialization in Scalar Replacement for Reducing Code Size

Abstract

Scalar replacement is an effective technique to improve the performance of the RTL code generated by high-level synthesis (HLS) from C programs with intensive array accesses. In scalar replacement, data accessed from arrays are stored into shift registers, and later array accesses on the same data are replaced with the accesses to the shift registers instead of the arrays. Namely, scalar replacement replaces array accesses with shift register accesses. Since arrays in C programs are usually mapped to RAMs with limited numbers of ports, reducing array accesses with scalar replacement leads to the memory access reduction, which in turn improves the performance of the resulting RTL code. In real-life C programs, sometimes, shift registers must be initialized conditionally using multiple array accesses, which increases the number of array accesses in main loops. To reduce the conditional array access in the main loops, the previous scalar replacement method proposed the use of a loop transformation called loop peeling. Loop peeling brings significant increase in code size, leading to the negative impacts on performance or circuit area of the synthesized hardware. In this paper, we propose a new method to initialize shift registers without loop peeling. The proposed method works as a preprocessing of the input C program prior to scalar replacement. With experimental results, we demonstrate the proposed method reduces the numbers of execution cycles of the synthesized hardware compared to the previous method.