Variable Latency Speculative Parallel Prefix Adders for Unsigned and Signed Operands

Abstract

A variable latency adder (VLA) reduces average addition time by using speculation: the exact arithmetic function is replaced by an approximated one, that is faster and gives correct results most of the times. When speculation fails, an error detection and correction circuit gives the correct result in the following clock cycle. Previous papers investigate VLAs based on Kogge-Stone, Han-Carlson or carry select topologies, speculating that carry propagation involves only a few consecutive bits. In several applications using 2's complement representation, however, operands have a Gaussian distribution and a nontrivial portion of carry chains can be as long as the adder size. In this paper we propose five novel VLA architectures, based on Brent-Kung, Ladner-Fisher, Sklansky, Hybrid Han-Carlson, and Carry increment parallel-prefix topologies. Moreover, we present a new efficient error detection and correction technique, that makes proposed VLAs suitable for applications using 2's complement representation. In order to investigate VLAs performances, proposed architectures have been synthesized using the UMC 65 nm library, for operand lengths ranging from 32 to 128 bits. Obtained results show that proposed VLAs outperform previous speculative architectures and standard (non-speculative) adders when high-speed is required.