Latency Sensitive FMA Design

Abstract

The implementation of merged floating-point multiply-add operations can be optimized in many ways. For latency sensitive applications, our cascade design reduces the accumulation dependent latency by 2x over a fused design, at a cost of a 13% increase in non-accumulation dependent latency. A simple in-order execution model shows this design is superior in most applications, providing 12% average reduction in FP stalls, and improves performance by up to 6%. Simulations of superscalar out-of-order machines show 4% average improvement in CPI in 2-way machines and 4.6% in 4-way machines. The cascade design has the same area and energy budget as a traditional fused multiple-add FMA.