[GHC] #13334: Constant folding for repeated integer operation of unknown value

[GHC]

#13334: Constant folding for repeated integer operation of unknown value
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           Reporter:  bgamari        |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  low            |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           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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 While looking at Phab:D3197 I noticed that currently there is nothing to
 turn an expression of the form `n+n+...+n` into `n*m`. It seems like some
 rules like,
 {{{
 forall x. x +# x = 2*x
 forall n x. n*x + x = (n+1) * x
 forall n x. x + n*x = (n+1) * x
 }}}
 Might fix this. Of course, it's quite possible that a string of additions
 **is** sometimes faster than a multiplication, but it seems to me that we
 should (for integer types) perform the rewrite to multiplication and then
 teach the code generator to lower to addition if it deems it advantageous.

 Naturally, this also applies to other integer operators as well.
 Regardless, I doubt that this is hurting anyone too badly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13334>
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