\end{code}
\begin{code}
+------------------------------------------------
-- unsafeCoerce# :: forall a b. a -> b
unsafeCoerceId
= pcMiscPrelId unsafeCoerceName ty info
rhs = mkLams [openAlphaTyVar,openBetaTyVar,x] $
Cast (Var x) (mkUnsafeCoercion openAlphaTy openBetaTy)
+------------------------------------------------
+nullAddrId :: Id
-- nullAddr# :: Addr#
-- The reason is is here is because we don't provide
-- a way to write this literal in Haskell.
-nullAddrId
- = pcMiscPrelId nullAddrName addrPrimTy info
+nullAddrId = pcMiscPrelId nullAddrName addrPrimTy info
where
info = noCafIdInfo `setUnfoldingInfo`
mkCompulsoryUnfolding (Lit nullAddrLit)
-seqId
- = pcMiscPrelId seqName ty info
+------------------------------------------------
+seqId :: Id
+-- 'seq' is very special. See notes with
+-- See DsUtils.lhs Note [Desugaring seq (1)] and
+-- Note [Desugaring seq (2)] and
+-- Fixity is set in LoadIface.ghcPrimIface
+seqId = pcMiscPrelId seqName ty info
where
info = noCafIdInfo `setUnfoldingInfo` mkCompulsoryUnfolding rhs
[x,y] = mkTemplateLocals [alphaTy, openBetaTy]
rhs = mkLams [alphaTyVar,openBetaTyVar,x,y] (Case (Var x) x openBetaTy [(DEFAULT, [], Var y)])
+------------------------------------------------
+lazyId :: Id
-- lazy :: forall a?. a? -> a? (i.e. works for unboxed types too)
-- Used to lazify pseq: pseq a b = a `seq` lazy b
--
-- (see WorkWrap.wwExpr)
-- We could use inline phases to do this, but that would be vulnerable to changes in
-- phase numbering....we must inline precisely after strictness analysis.
-lazyId
- = pcMiscPrelId lazyIdName ty info
+lazyId = pcMiscPrelId lazyIdName ty info
where
info = noCafIdInfo
ty = mkForAllTys [alphaTyVar] (mkFunTy alphaTy alphaTy)
(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.
%************************************************************************
%* *
Note [no-case-of-case]
~~~~~~~~~~~~~~~~~~~~~~
-There is a time we *don't* want to do that, namely when
--fno-case-of-case is on. This happens in the first simplifier pass,
-and enhances full laziness. Here's the bad case:
- f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
-If we eliminate the inner case, we trap it inside the I# v -> arm,
-which might prevent some full laziness happening. I've seen this
-in action in spectral/cichelli/Prog.hs:
- [(m,n) | m <- [1..max], n <- [1..max]]
-Hence the check for NoCaseOfCase.
+We *used* to suppress the binder-swap in case expressoins when
+-fno-case-of-case is on. Old remarks:
+ "This happens in the first simplifier pass,
+ and enhances full laziness. Here's the bad case:
+ f = \ y -> ...(case x of I# v -> ...(case x of ...) ... )
+ If we eliminate the inner case, we trap it inside the I# v -> arm,
+ which might prevent some full laziness happening. I've seen this
+ in action in spectral/cichelli/Prog.hs:
+ [(m,n) | m <- [1..max], n <- [1..max]]
+ Hence the check for NoCaseOfCase."
+However, now the full-laziness pass itself reverses the binder-swap, so this
+check is no longer necessary.
Note [Suppressing the case binder-swap]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
improve_case_bndr env scrut case_bndr
- | switchIsOn (getSwitchChecker env) NoCaseOfCase
- -- See Note [no-case-of-case]
- = (env, case_bndr)
+ -- See Note [no-case-of-case]
+ -- | switchIsOn (getSwitchChecker env) NoCaseOfCase
+ -- = (env, case_bndr)
| otherwise -- Failed try; see Note [Suppressing the case binder-swap]
-- not (isEvaldUnfolding (idUnfolding v))
projects / ghc-hetmet.git / commitdiff