[Haskell-beginners] let x = x in x (GHC.Prim)

Hello Haskellers, I'm trying to write a faster popCount function for x86 systems. I tried cloning the ghc-prim package and repurposing it for my own needs, but it isn't working as hoped. In particular, popCnt64# was implemented in GHC.Prim as: popCnt64# = let x = x in x Which shouldn't terminate. Yet when I call it, it magically finds the C implementation in hs_popcnt64 and returns the correct value. My cloned project doesn't behave that way. Instead it doesn't terminate as I would expect. Anyone know what's happening here, if there is a way to make this work or tell me if I'm going about this completely the wrong way? Cheers, -John