Hi, I'm trying to implement word2Double#, which I represent as a MachOp I call MO_UF_Conv (which is very similar to MO_SF_Conv). On some platform I need to implement this MachOp as a call to a C function. Here's what I did in the PPC backend (PPC/CodeGen.hs) for example: getRegister' dflags (CmmMachOp mop [x]) -- unary MachOps = case mop of MO_UF_Conv from to -> coerceWord2FP from to x coerceWord2FP :: Width -> Width -> CmmExpr -> NatM Register coerceWord2FP fromRep toRep x = do dflags <- getDynFlags (src, src_code) <- getSomeReg x -- This is not the type of register I need! dst <- getNewRegNat (floatSize toRep) targetExpr <- cmmMakeDynamicReference dflags addImportNat CallReference lbl let target = ForeignTarget targetExpr (ForeignConvention CCallConv [NoHint] [NoHint] CmmMayReturn) call_code <- genCCall target [src] [dst] let code' = src_code `appOL` call_code return (Fixed (floatSize toRep) dst code') where lbl = mkCmmCodeLabel primPackageId (fsLit $ word2FloatLabel toRep) This doesn't work. The issue is that to make a C call I need CmmFormals and CmmActuals. I cannot figure out how to get the former, as I need to get hold of a LocalReg, which I don't see any way of creating. I could try to make MO_UF_Conv a CallishMachOp, but then I'll have a more difficult time implementing this MachOp in e.g. the LLVM backend, where I don't want to generate a C call but instead output some instructions. I could use some pointers. Cheers, Johan