Add another gcdInteger rule

This one is better when the result is converted to an Int

1 files changed, 8 insertions, 3 deletions

diff --git a/libraries/integer-gmp/GHC/Integer/Type.lhs b/libraries/integer-gmp/GHC/Integer/Type.lhs
index a402f37e49..b9d9c3cd65 100644
--- a/libraries/integer-gmp/GHC/Integer/Type.lhs
+++ b/libraries/integer-gmp/GHC/Integer/Type.lhs
@@ -283,9 +283,14 @@ lcmInteger a b =      if a `eqInteger` S# 0# then S# 0#
         ab = absInteger b
 
 -- This rule needs to use absInteger so that it works correctly when
--- the result is minBound :: Int
-{-# RULES "gcdInteger/Int" forall a b.
-    gcdInteger (smallInteger a) (smallInteger b) = absInteger (smallInteger (gcdInt a b))
+-- the result is minBound :: Int. But that isn't necessary when the
+-- result is converted to an Int.
+{-# RULES
+"gcdInteger/Int" forall a b.
+    gcdInteger (smallInteger a) (smallInteger b)
+        = absInteger (smallInteger (gcdInt a b))
+"integerToInt/gcdInteger/Int" forall a b.
+    integerToInt (gcdInteger (smallInteger a) (smallInteger b)) = gcdInt a b
   #-}
 gcdInt :: Int# -> Int# -> Int#
 gcdInt 0# y  = absInt y