(arg_ty, res_ty) = splitFunTy fun_ty
-----------
mk_val_app :: CoreExpr -> CoreExpr -> Type -> Type -> CoreExpr
+mk_val_app (Var f `App` Type ty1 `App` Type _ `App` arg1) arg2 _ res_ty
+ | f == seqId -- Note [Desugaring seq (1), (2)]
+ = Case arg1 case_bndr res_ty [(DEFAULT,[],arg2)]
+ where
+ case_bndr = case arg1 of
+ Var v1 -> v1 -- Note [Desugaring seq (2)]
+ _ -> mkWildId ty1
+
mk_val_app fun arg arg_ty _ -- See Note [CoreSyn let/app invariant]
| not (isUnLiftedType arg_ty) || exprOkForSpeculation arg
= App fun arg -- The vastly common case
-mk_val_app (Var f `App` Type ty1 `App` Type _ `App` arg1) arg2 _ res_ty
- | f == seqId -- Note [Desugaring seq]
- = Case arg1 (mkWildId ty1) res_ty [(DEFAULT,[],arg2)]
-
mk_val_app fun arg arg_ty res_ty
= Case arg (mkWildId arg_ty) res_ty [(DEFAULT,[],App fun (Var arg_id))]
where
-- because 'fun ' should not have a free wild-id
\end{code}
-Note [Desugaring seq] cf Trac #1031
-~~~~~~~~~~~~~~~~~~~~~
+Note [Desugaring seq (1)] cf Trac #1031
+~~~~~~~~~~~~~~~~~~~~~~~~~
f x y = x `seq` (y `seq` (# x,y #))
The [CoreSyn let/app invariant] means that, other things being equal, because
Seq is very, very special! So we recognise it right here, and desugar to
case x of _ -> case y of _ -> (# x,y #)
-The special case would be valid for all calls to 'seq', but it's only *necessary*
-for ones whose second argument has an unlifted type. So we only catch the latter
-case here, to avoid unnecessary tests.
-
+Note [Desugaring seq (2)] cf Trac #2231
+~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+ let chp = case b of { True -> fst x; False -> 0 }
+ in chp `seq` ...chp...
+Here the seq is designed to plug the space leak of retaining (snd x)
+for too long.
+
+If we rely on the ordinary inlining of seq, we'll get
+ let chp = case b of { True -> fst x; False -> 0 }
+ case chp of _ { I# -> ...chp... }
+
+But since chp is cheap, and the case is an alluring contet, we'll
+inline chp into the case scrutinee. Now there is only one use of chp,
+so we'll inline a second copy. Alas, we've now ruined the purpose of
+the seq, by re-introducing the space leak:
+ case (case b of {True -> fst x; False -> 0}) of
+ I# _ -> ...case b of {True -> fst x; False -> 0}...
+
+We can try to avoid doing this by ensuring that the binder-swap in the
+case happens, so we get his at an early stage:
+ case chp of chp2 { I# -> ...chp2... }
+But this is fragile. The real culprit is the source program. Perhpas we
+should have said explicitly
+ let !chp2 = chp in ...chp2...
+
+But that's painful. So the code here does a little hack to make seq
+more robust: a saturated application of 'seq' is turned *directly* into
+the case expression. So we desugar to:
+ let chp = case b of { True -> fst x; False -> 0 }
+ case chp of chp { I# -> ...chp... }
+Notice the shadowing of the case binder! And now all is well.
+
+The reason it's a hack is because if you define mySeq=seq, the hack
+won't work on mySeq.
%************************************************************************
%* *