[GHC] #15842: Exponentiation needs PrelRules

[GHC]

#15842: Exponentiation needs PrelRules
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           Reporter:  dfeuer         |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.6.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 When a user writes `a ^ lb`, where `lb` is an integer literal, the type of
 `lb` defaults to `Integer` unless it's annotated. If `lb` is very small,
 then `RULES` will expand the whole expression into a few multiplications.
 If not, the computation will pay the price of `Integer` arithmetic. If we
 want, we should be able to improve this with a special compilation rule.
 When `lb :: Integer`, and `0 <= lb <= maxBound :: Word`, we should be able
 to replace `lb` by the equivalent `Word`. The same problem appears with
 `^^` and `stimes`. Perhaps those can be fixed up as well, somehow.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15842>
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