Optimization works for Word but not Word32 or Word64

GHC 7.6.1 fails to properly optimize some code when explicitly specifying word size, that optimizes properly for Word.

I have also tested this in 7.4.2 and it works properly.

An example program:

import Criterion.Main
import Control.Monad.Writer
import Data.Bits
import Data.List
import Data.Word

bitByBitCopy :: Word -> Word
bitByBitCopy w = foldl' f 0 [0 .. bitSize w - 1]
  where f acc n = if w `testBit` n then acc `setBit` n else acc

bitByBitCopy32 :: Word32 -> Word32
bitByBitCopy32 w = foldl' f 0 [0 .. bitSize w - 1]
  where f acc n = if w `testBit` n then acc `setBit` n else acc

bitByBitCopy64 :: Word64 -> Word64
bitByBitCopy64 w = foldl' f 0 [0 .. bitSize w - 1]
  where f acc n = if w `testBit` n then acc `setBit` n else acc

-- Bench
wbench :: Benchmarkable b => String -> b -> Writer [Benchmark] ()
wbench s b = tell [bench s b]

main = defaultMain . execWriter $ do
    wbench "Word" $ nf bitByBitCopy 0
    when (bitSize (0::Word) == 32) $
        wbench "Word32" $ nf bitByBitCopy32 0
    when (bitSize (0::Word) == 64) $
        wbench "Word64" $ nf bitByBitCopy64 0

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture