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

[Simon Peyton Jones]

Herbert would you like to open a ticket, saying what's wrong and how to reproduce it?

Simon

| -----Original Message-----
| From: ghc-commits [mailto:ghc-commits-bounces at haskell.org] On Behalf Of
| git at git.haskell.org
| Sent: 01 March 2014 13:53
| To: ghc-commits at haskell.org
| Subject: [commit: packages/base] ghc-7.8: Workaround failed constant-
| folding for zeroBits (591142f)
| 
| Repository : ssh://git@git.haskell.org/base
| 
| On branch  : ghc-7.8
| Link       :
| http://ghc.haskell.org/trac/ghc/changeset/591142f8aead4c28bdaa5656d79e6
| dc38273e685/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
| 
| 
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