[Git][ghc/ghc][master] We can't UNPACK multi-constructor GADTs
Marge Bot (@marge-bot)
gitlab at gitlab.haskell.org
Wed Mar 12 02:39:48 UTC 2025
Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC
Commits:
b6d5b091 by Simon Peyton Jones at 2025-03-11T22:39:23-04:00
We can't UNPACK multi-constructor GADTs
This MR fixes #25672
See Note [Unpacking GADTs and existentials] in GHC.Types.Id.Make
- - - - -
6 changed files:
- compiler/GHC/Core/DataCon.hs
- compiler/GHC/Tc/TyCl.hs
- compiler/GHC/Types/Id/Make.hs
- + testsuite/tests/simplCore/should_fail/T25672.hs
- + testsuite/tests/simplCore/should_fail/T25672.stderr
- testsuite/tests/simplCore/should_fail/all.T
Changes:
=====================================
compiler/GHC/Core/DataCon.hs
=====================================
@@ -1018,6 +1018,9 @@ instance Data.Data DataCon where
gunfold _ _ = error "gunfold"
dataTypeOf _ = mkNoRepType "DataCon"
+instance Outputable HsSrcBang where
+ ppr (HsSrcBang _source_text bang) = ppr bang
+
instance Outputable HsBang where
ppr (HsBang prag mark) = ppr prag <+> ppr mark
=====================================
compiler/GHC/Tc/TyCl.hs
=====================================
@@ -4817,6 +4817,8 @@ checkValidDataCon dflags existential_ok tc con
; traceTc "Done validity of data con" $
vcat [ ppr con
, text "Datacon wrapper type:" <+> ppr (dataConWrapperType con)
+ , text "Datacon src bangs:" <+> ppr (dataConSrcBangs con)
+ , text "Datacon impl bangs:" <+> ppr (dataConImplBangs con)
, text "Datacon rep type:" <+> ppr (dataConRepType con)
, text "Datacon display type:" <+> ppr data_con_display_type
, text "Rep typcon binders:" <+> ppr (tyConBinders (dataConTyCon con))
=====================================
compiler/GHC/Types/Id/Make.hs
=====================================
@@ -1540,39 +1540,87 @@ shouldUnpackArgTy bang_opts prag fam_envs arg_ty
--- Given a type already assumed to have been normalized by topNormaliseType,
--- unpackable_type_datacons ty = Just datacons
--- iff ty is of the form
--- T ty1 .. tyn
--- and T is an algebraic data type (not newtype), in which no data
--- constructors have existentials, and datacons is the list of data
--- constructors of T.
unpackable_type_datacons :: Type -> Maybe [DataCon]
+-- Given a type already assumed to have been normalized by topNormaliseType,
+-- unpackable_type_datacons (T ty1 .. tyn) = Just datacons
+-- iff the type can be unpacked (see Note [Unpacking GADTs and existentials])
+-- and `datacons` are the data constructors of T
unpackable_type_datacons ty
| Just (tc, _) <- splitTyConApp_maybe ty
- , not (isNewTyCon tc) -- Even though `ty` has been normalised, it could still
- -- be a /recursive/ newtype, so we must check for that
+ , not (isNewTyCon tc)
+ -- isNewTyCon: even though `ty` has been normalised, whic includes looking
+ -- through newtypes, it could still be a /recursive/ newtype, so we must
+ -- check for that case
, Just cons <- tyConDataCons_maybe tc
- , not (null cons) -- Don't upack nullary sums; no need.
- -- They already take zero bits
- , all (null . dataConExTyCoVars) cons
- = Just cons -- See Note [Unpacking GADTs and existentials]
+ , unpackable_cons cons
+ = Just cons
| otherwise
= Nothing
+ where
+ unpackable_cons :: [DataCon] -> Bool
+ -- True if we can unpack a value of type (T t1 .. tn),
+ -- where T is an algebraic data type with these constructors
+ -- See Note [Unpacking GADTs and existentials]
+ unpackable_cons [] -- Don't unpack nullary sums; no need.
+ = False -- They already take zero bits; see (UC0)
+
+ unpackable_cons [con] -- Exactly one data constructor; see (UC1)
+ = null (dataConExTyCoVars con)
+
+ unpackable_cons cons -- More than one data constructor; see (UC2)
+ = all isVanillaDataCon cons
{-
Note [Unpacking GADTs and existentials]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-There is nothing stopping us unpacking a data type with equality
-components, like
- data Equal a b where
- Equal :: Equal a a
-
-And it'd be fine to unpack a product type with existential components
-too, but that would require a bit more plumbing, so currently we don't.
+Can we unpack a value of an algebraic data type T? For example
+ data D a = MkD {-# UNPACK #-} (T a)
+Can we unpack that (T a) field?
+
+Three cases to consider in `unpackable_cons`
+
+(UC0) No data constructors; a nullary sum type. This already takes zero
+ bits so there is no point in unpacking it.
+
+(UC1) Single-constructor types (products). We can just represent it by
+ its fields. For example, if `T` is defined as:
+ data T a = MkT a a Int
+ then we can unpack it as follows. The worker for MkD takes three unpacked fields:
+ data D a = MkD a a Int
+ $MkD :: T a -> D a
+ $MkD (MkT a1 a2 i) = MkD a1 a2 i
+
+ We currently /can't/ do this if T has existentially-bound type variables,
+ hence: null (dataConExTyCoVars con) in `unpackable_cons`.
+ But see also (UC3) below.
+
+ But we /can/ do it for (some) GADTs, such as:
+ data Equal a b where { Equal :: Equal a a }
+ data Wom a where { Wom1 :: Int -> Wom Bool }
+ We will get a MkD constructor that includes some coercion arguments,
+ but that is fine. See #14978. We still can't accommodate existentials,
+ but these particular examples don't use existentials.
+
+(UC2) Multi-constructor types, e.g.
+ data T a = T1 a | T2 Int a
+ Here we unpack the field to an unboxed sum type, thus:
+ data D a = MkD (# a | (# Int, a #) #)
+
+ However, now we can't deal with GADTs at all, because we'd need an
+ unboxed sum whose component was a unboxed tuple, whose component(s)
+ have kind (CONSTRAINT r); and that's not well-kinded. Hence the
+ all isVanillaDataCon
+ condition in `unpackable_cons`. See #25672.
+
+(UC3) For single-constructor types, with some more plumbing we could
+ allow existentials. e.g.
+ data T a = forall b. MkT a (b->Int) b
+ could unpack to
+ data D a = forall b. MkD a (b->Int) b
+ $MkD :: T a -> D a
+ $MkD (MkT @b x f y) = MkD @b x f y
+ Eminently possible, but more plumbing needed.
-So for now we require: null (dataConExTyCoVars data_con)
-See #14978
Note [Unpack one-wide fields]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
=====================================
testsuite/tests/simplCore/should_fail/T25672.hs
=====================================
@@ -0,0 +1,15 @@
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE GADTs #-}
+
+module T25672 where
+
+data IntOrWord (isInt :: Bool) where
+ Int :: !Int -> IntOrWord True
+ Word :: !Word -> IntOrWord False
+
+data WrapIntOrWord (isInt :: Bool)
+ = WrapIntOrWord {lit :: {-# UNPACK #-} !(IntOrWord isInt)}
+
+boom :: WrapIntOrWord True
+boom = WrapIntOrWord (Int 1)
=====================================
testsuite/tests/simplCore/should_fail/T25672.stderr
=====================================
@@ -0,0 +1,6 @@
+T25672.hs:12:7: warning: [GHC-40091]
+ • Ignoring unusable UNPACK pragma
+ on the first argument of ‘WrapIntOrWord’
+ • In the definition of data constructor ‘WrapIntOrWord’
+ In the data type declaration for ‘WrapIntOrWord’
+
=====================================
testsuite/tests/simplCore/should_fail/all.T
=====================================
@@ -1,3 +1,6 @@
test('T7411', [expect_broken_for(7411, ['optasm', 'optllvm',
'threaded2', 'dyn']),
exit_code(1)], compile_and_run, [''])
+
+# This one produces a warning
+test('T25672', normal, compile, ['-O'])
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b6d5b09103dea97351774c5ab34082165504b997
--
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/b6d5b09103dea97351774c5ab34082165504b997
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