[GHC] #8832: Constant-folding regression wrt `clearBit (bit 0) 0 `

[GHC]

#8832: Constant-folding regression wrt `clearBit (bit 0) 0 `
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        Reporter:  hvr               |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  high              |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.8.1-rc2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Runtime           |               Test Case:
  performance bug                    |  simplCore/should_compile/T8832
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by bgamari):

 The problem here is that the `Bits` instance for `Integer` overrides
 `bit`, using it's own `bitInteger` function in place of the usual
 `bitDefault`. `bitDefault` is constant folded by `PrelRules` by virtue of
 being implemented in terms of `shiftL`.

 This is presumably done to optimize the case of construction of a `BigNat`
 for large arguments, but `bitBigNat#`, which is what handles this case, is
 currently just the naive implementation with a TODO.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8832#comment:23>
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