Signal representation guided synthesis using carry-save adders for synchronous data-path circuits

Abstract

Arithmetic transformations using carry-save adders have been exploited recently in design automation but existing transformation approaches only optimize combinatorial functions. Most applications need synchronous circuits and it is known that techniques that move the positions of the registers, such as retiming, can significantly reduce the cycle time of a synchronous circuit. However, retiming disregards arithmetic transformations and its power is limited by the circuit topology. This work is the first to exploit carry-save arithmetic transformations together with the moving of the register positions. To enable such transformations, we first propose the use of a new multiple-vector signal representation. Next, we use multiple-vector signal representation as a common guide for all of our simultaneous carry-save arithmetic transformations with the moving of the register positions. Specifically, we propose, operation forward and operation backward carry-save transformations, which are transformations across register boundaries. We also propose operation duplicate and operation merge transformations to exploit the resource sharing and timing trade-offs in the implementation of a multiple-fanout network. Finally, we propose an efficient and effective heuristic that selectively applies a sequence of transformations to optimize the timing and the area of a synchronous circuit. Experimental results show that the proposed techniques significantly out-perform previous approaches.