VaLHALLA: Variable latency history aware local-carry lazy adder

Current low-energy adder designs fail to address subtraction or signed operations, are imprecise, or have large overheads. These overheads include overlapping sub-adders, relatively complex carry speculation mechanisms and bulky error correction circuits, all of which consume considerable amount of energy. In this paper, we address all these problems and introduce Variable Latency History Aware Local-carry Lazy Adder (VaLHALLA). VaLHALLA employs sliced non-overlapping sub-adders and speculates the carry-ins for each slice by exploiting the temporal operand correlation of each instruction location (i.e., it considers both operand history and instruction locale). VaLHALLA achieves accurate results at lower energy-delay-product than competing designs.