assign_reg PtrArg (v:vs, fs, ds, ls) = Just (VanillaReg v VGcPtr, (vs, fs, ds, ls))
assign_reg NonPtrArg (v:vs, fs, ds, ls) = Just (VanillaReg v VNonGcPtr, (vs, fs, ds, ls))
-- PtrArg and NonPtrArg both go in a vanilla register
assign_reg PtrArg (v:vs, fs, ds, ls) = Just (VanillaReg v VGcPtr, (vs, fs, ds, ls))
assign_reg NonPtrArg (v:vs, fs, ds, ls) = Just (VanillaReg v VNonGcPtr, (vs, fs, ds, ls))
-- PtrArg and NonPtrArg both go in a vanilla register