[commit: ghc] master: Do not generate a data-con wrapper for !Int# (900cfdc)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/900cfdc2700ad9e8c7a12dd25bb0396e5e7651df/ghc

>---------------------------------------------------------------

commit 900cfdc2700ad9e8c7a12dd25bb0396e5e7651df
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Thu Mar 9 15:58:32 2017 +0000

    Do not generate a data-con wrapper for !Int#
    
    See Note [Data con wrappers and unlifted types] in MkId.
    We were being totally stupid!
    
    See Trac #1600 comment:66


>---------------------------------------------------------------

900cfdc2700ad9e8c7a12dd25bb0396e5e7651df
 compiler/basicTypes/DataCon.hs |  2 +-
 compiler/basicTypes/MkId.hs    | 20 ++++++++++++++++++--
 2 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/compiler/basicTypes/DataCon.hs b/compiler/basicTypes/DataCon.hs
index 43bcf75..c6bb8eb 100644
--- a/compiler/basicTypes/DataCon.hs
+++ b/compiler/basicTypes/DataCon.hs
@@ -504,7 +504,7 @@ data HsSrcBang =
 -- Bangs of data constructor arguments as generated by the compiler
 -- after consulting HsSrcBang, flags, etc.
 data HsImplBang
-  = HsLazy  -- ^ Lazy field
+  = HsLazy    -- ^ Lazy field, or one with an unlifted type
   | HsStrict  -- ^ Strict but not unpacked field
   | HsUnpack (Maybe Coercion)
     -- ^ Strict and unpacked field
diff --git a/compiler/basicTypes/MkId.hs b/compiler/basicTypes/MkId.hs
index 890a4bf..6be2b5c 100644
--- a/compiler/basicTypes/MkId.hs
+++ b/compiler/basicTypes/MkId.hs
@@ -657,6 +657,18 @@ the interface file.
 The HsImplBangs passed are in 1-1 correspondence with the
 dataConOrigArgTys of the DataCon.
 
+Note [Data con wrappers and unlifted types]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Consider
+   data T = MkT !Int#
+
+We certainly do not want to make a wrapper
+   $WMkT x = case x of y { DEFAULT -> MkT y }
+
+For a start, it's still to generate a no-op.  But worse, since wrappers
+are currently injected at TidyCore, we don't even optimise it away!
+So the stupid case expression stays there.  This actually happened for
+the Integer data type (see Trac #1600 comment:66)!
 -}
 
 -------------------------
@@ -673,7 +685,7 @@ dataConSrcToImplBang
    -> HsImplBang
 
 dataConSrcToImplBang dflags fam_envs arg_ty
-              (HsSrcBang ann unpk NoSrcStrict)
+                     (HsSrcBang ann unpk NoSrcStrict)
   | xopt LangExt.StrictData dflags -- StrictData => strict field
   = dataConSrcToImplBang dflags fam_envs arg_ty
                   (HsSrcBang ann unpk SrcStrict)
@@ -684,7 +696,11 @@ dataConSrcToImplBang _ _ _ (HsSrcBang _ _ SrcLazy)
   = HsLazy
 
 dataConSrcToImplBang dflags fam_envs arg_ty
-    (HsSrcBang _ unpk_prag SrcStrict)
+                     (HsSrcBang _ unpk_prag SrcStrict)
+  | isUnliftedType arg_ty
+  = HsLazy  -- For !Int#, say, use HsLazy
+            -- See Note [Data con wrappers and unlifted types]
+
   | not (gopt Opt_OmitInterfacePragmas dflags) -- Don't unpack if -fomit-iface-pragmas
           -- Don't unpack if we aren't optimising; rather arbitrarily,
           -- we use -fomit-iface-pragmas as the indication