From e748006355d85fcddd17ba206873b0051219abb1 Mon Sep 17 00:00:00 2001
From: buggymcbugfix <4444-buggymcbugfix@users.noreply.gitlab.haskell.org>
Date: Sat, 16 May 2020 21:55:57 +0300
Subject: Piggyback on Enum Word methods for Word64

If we are on a 64 bit platform, we can use the efficient Enum Word
methods for the Enum Word64 instance.
---
 libraries/base/GHC/Word.hs | 42 ++++++++++++++++++++++++++++++++++++++----
 1 file changed, 38 insertions(+), 4 deletions(-)

diff --git a/libraries/base/GHC/Word.hs b/libraries/base/GHC/Word.hs
index d68fd464b9..81415b8872 100644
--- a/libraries/base/GHC/Word.hs
+++ b/libraries/base/GHC/Word.hs
@@ -892,10 +892,44 @@ instance Enum Word64 where
         | x <= fromIntegral (maxBound::Int)
                         = I# (word2Int# x#)
         | otherwise     = fromEnumError "Word64" x
-    enumFrom            = integralEnumFrom
-    enumFromThen        = integralEnumFromThen
-    enumFromTo          = integralEnumFromTo
-    enumFromThenTo      = integralEnumFromThenTo
+
+#if WORD_SIZE_IN_BITS < 64
+    enumFrom       = integralEnumFrom
+    enumFromThen   = integralEnumFromThen
+    enumFromTo     = integralEnumFromTo
+    enumFromThenTo = integralEnumFromThenTo
+#else
+    -- See Note [Stable Unfolding for list producers] in GHC.Enum
+    {-# INLINABLE enumFrom #-}
+    enumFrom w
+        = map wordToWord64
+        $ enumFrom (word64ToWord w)
+
+    -- See Note [Stable Unfolding for list producers] in GHC.Enum
+    {-# INLINABLE enumFromThen #-}
+    enumFromThen w s
+        = map wordToWord64
+        $ enumFromThen (word64ToWord w) (word64ToWord s)
+
+    -- See Note [Stable Unfolding for list producers] in GHC.Enum
+    {-# INLINABLE enumFromTo #-}
+    enumFromTo w1 w2
+        = map wordToWord64
+        $ enumFromTo (word64ToWord w1) (word64ToWord w2)
+
+    -- See Note [Stable Unfolding for list producers] in GHC.Enum
+    {-# INLINABLE enumFromThenTo #-}
+    enumFromThenTo w1 s w2
+        = map wordToWord64
+        $ enumFromThenTo (word64ToWord w1) (word64ToWord s) (word64ToWord w2)
+
+word64ToWord :: Word64 -> Word
+word64ToWord (W64# w#) = (W# w#)
+
+wordToWord64 :: Word -> Word64
+wordToWord64 (W# w#) = (W64# w#)
+#endif
+
 
 -- | @since 2.01
 instance Integral Word64 where
-- 
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