The tight loop in SS's bitwise-and contains the following MIR: > 23 LoadSlot slots136 (Int32) > ... > 29 LoadSlot slots136 (Double) > 33 TruncateToInt32 LoadSlot29 Obviously if the first LoadSlot succeeds, then the second LoadSlot must also be an Int32. There are two independent ways to go about fixing this: 1) A Double LoadSlot with no other uses other than a truncation should be replaced with an instruction that passes through ints (rather than converting them) and converts doubles (rather than passing them through). 2) We should just emit a LoadSlot(Int32) in the first place. Avoids the branch implicit in #1.