[commit: packages/base] ghc-7.8: Workaround failed constant-folding for zeroBits (591142f)

[git at git.haskell.org]

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

On branch  : ghc-7.8
Link       : http://ghc.haskell.org/trac/ghc/changeset/591142f8aead4c28bdaa5656d79e6dc38273e685/base

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

commit 591142f8aead4c28bdaa5656d79e6dc38273e685
Author: Herbert Valerio Riedel <hvr at gnu.org>
Date:   Sat Mar 1 14:45:48 2014 +0100

    Workaround failed constant-folding for zeroBits
    
    For some reason GHC fails to constant fold `zeroBits :: Int` and
    `zeroBits :: Integer`; `ghc -show-iface` shows
    
      $fBitsInt_$czeroBits :: GHC.Types.Int
         {- Strictness: m,
            Unfolding: (GHC.Types.I# (GHC.Prim.andI# 1 (GHC.Prim.notI# 1))) -}
    
    Otoh, constant-folding works as expected, reducing `zeroBits` to 0 constant
    for the other integer-types (= {Word,Int}{8,16,32,64}` and `Word`). So this
    quickfix is actually just treating the symptom rather than the cause.
    
    Signed-off-by: Herbert Valerio Riedel <hvr at gnu.org>
    (cherry picked from commit 2dbfcd70e53845d9119389cecc88411b47b70644)


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

591142f8aead4c28bdaa5656d79e6dc38273e685
 Data/Bits.hs |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/Data/Bits.hs b/Data/Bits.hs
index e771624..28cd024 100644
--- a/Data/Bits.hs
+++ b/Data/Bits.hs
@@ -363,6 +363,8 @@ instance Bits Int where
     {-# INLINE bit #-}
     {-# INLINE testBit #-}
 
+    zeroBits = 0
+
     bit     = bitDefault
 
     testBit = testBitDefault
@@ -437,6 +439,7 @@ instance Bits Integer where
                      | otherwise = shiftRInteger x (negateInt# i#)
    testBit x (I# i) = testBitInteger x i
 
+   zeroBits   = 0
    bit        = bitDefault
    popCount   = popCountDefault