A Study on the Impact of Compiler Optimizations on High-Level Synthesis

High-level synthesis is a design process that takes an untimed, behavioral description in a high-level language like C and produces register-transfer-level (RTL) code that implements the same behavior in hardware. In this design flow, the quality of the generated RTL is greatly influenced by the high-level description of the language. Hence it follows that both source-level and IR-level compiler optimizations could either improve or hurt the quality of the generated RTL. The problem of ordering compiler optimization passes, also known as the phase-ordering problem, has been an area of active research over the past decade. In this paper, we explore the effects of both source-level and IR optimizations and phase ordering on high-level synthesis. The parameters of the generated RTL are very sensitive to high-level optimizations. We study three commonly used source-level optimizations in isolation and then propose simple yet effective heuristics to apply them to obtain a reasonable latency-area tradeoff. We also study the phase-ordering problem for IR-level optimizations from a HLS perspective and compare it to a CPU-based setting. Our initial results show that an input-specific order can achieve a significant reduction in the latency of the generated RTL, and opens up this technology for future research.