Enhance Data instance generation

Use `mkConstrTag` to explicitly pass the constructor tag instead of
using `mkConstr` which queries the tag at runtime by querying the index
of the constructor name (a string) in the list of constructor names.

Perf improvement:

    T16577(normal) ghc/alloc 11325573876.0  9249786992.0 -18.3% GOOD

Thanks to @sgraf812 for suggesting an additional list fusion fix during
reviews.

Metric Decrease:
    T16577

1 files changed, 12 insertions, 5 deletions

diff --git a/libraries/base/Data/Data.hs b/libraries/base/Data/Data.hs
index 4ede199e39..0d4ef944a1 100644
--- a/libraries/base/Data/Data.hs
+++ b/libraries/base/Data/Data.hs
@@ -85,6 +85,7 @@ module Data.Data (
         Fixity(..),
         -- ** Constructors
         mkConstr,
+        mkConstrTag,
         mkIntegralConstr,
         mkRealConstr,
         mkCharConstr,
@@ -120,6 +121,7 @@ import Data.Eq
 import Data.Maybe
 import Data.Monoid
 import Data.Ord
+import Data.List (findIndex)
 import Data.Typeable
 import Data.Version( Version(..) )
 import GHC.Base hiding (Any, IntRep, FloatRep)
@@ -628,10 +630,9 @@ mkDataType str cs = DataType
                         , datarep = AlgRep cs
                         }
 
-
 -- | Constructs a constructor
-mkConstr :: DataType -> String -> [String] -> Fixity -> Constr
-mkConstr dt str fields fix =
+mkConstrTag :: DataType -> String -> Int -> [String] -> Fixity -> Constr
+mkConstrTag dt str idx fields fix =
         Constr
                 { conrep    = AlgConstr idx
                 , constring = str
@@ -639,9 +640,15 @@ mkConstr dt str fields fix =
                 , confixity = fix
                 , datatype  = dt
                 }
+
+-- | Constructs a constructor
+mkConstr :: DataType -> String -> [String] -> Fixity -> Constr
+mkConstr dt str fields fix = mkConstrTag dt str idx fields fix
   where
-    idx = head [ i | (c,i) <- dataTypeConstrs dt `zip` [1..],
-                     showConstr c == str ]
+    idx = case findIndex (\c -> showConstr c == str) (dataTypeConstrs dt) of
+            Just i  -> i+1 -- ConTag starts at 1
+            Nothing -> errorWithoutStackTrace $
+                        "Data.Data.mkConstr: couldn't find constructor " ++ str
 
 
 -- | Gets the constructors of an algebraic datatype