addFloat (Floats ok_to_spec floats) new_float
= Floats (combine ok_to_spec (check new_float)) (floats `snocOL` new_float)
where
- check (FloatLet _) = OkToSpec
+ check (FloatLet _) = OkToSpec
check (FloatCase _ _ ok_for_spec)
| ok_for_spec = IfUnboxedOk
| otherwise = NotOkToSpec
-> UniqSM (Floats, CoreArg)
corePrepArg env arg dem = do
(floats, arg') <- corePrepExprFloat env arg
- if exprIsTrivial arg'
+ if exprIsTrivial arg' && allLazy NotTopLevel NonRecursive floats
+ -- Note [Floating unlifted arguments]
then return (floats, arg')
else do v <- newVar (exprType arg')
(floats', v') <- mkLocalNonRec v dem floats arg'
exprIsTrivial (Cast e _) = exprIsTrivial e
exprIsTrivial (Lam b body) | isTyVar b = exprIsTrivial body
exprIsTrivial _ = False
+\end{code}
+
+Note [Floating unlifted arguments]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider C (let v* = expensive in v)
+
+where the "*" indicates "will be demanded". Usually v will have been
+inlined by now, but let's suppose it hasn't (see Trac #2756). Then we
+do *not* want to get
+
+ let v* = expensive in C v
+because that has different strictness. Hence the use of 'allLazy'.
+(NB: the let v* turns into a FloatCase, in mkLocalNonRec.)
+
+
+\begin{code}
-- ---------------------------------------------------------------------------
-- Dealing with expressions
-- ---------------------------------------------------------------------------
| isStrict dem
-- It's a strict let so we definitely float all the bindings
- = let -- Don't make a case for a value binding,
+ = let -- Don't make a case for a value binding,
-- even if it's strict. Otherwise we get
-- case (\x -> e) of ...!
float | exprIsHNF rhs = FloatLet (NonRec bndr rhs)
- | otherwise = FloatCase bndr rhs (exprOkForSpeculation rhs)
+ | otherwise = FloatCase bndr rhs (exprOkForSpeculation rhs)
in
return (addFloat floats float, evald_bndr)