12.2.1. DSP Builder Memory and Multiplier Trade-Off Options
DSP Builder converts multipliers with a single constant input into balanced adder trees, which occurs automatically where the depth of the tree is not greater than 2. If the depth is greater than 2, DSP Builder compares the hard multiplier threshold with the estimated size of the adder tree, which is generally much lower than the size of a full soft multiplier. If DSP Builder combines two non-constant multipliers followed by an adder into a single DSP block, DSP Builder does not convert the multiplier into LEs, even if a large threshold is present.