[GHC] #12877: Constant propagation with basic unboxed arithmetic instructions

[GHC]

#12877: Constant propagation with basic unboxed arithmetic instructions
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           Reporter:  hsyl20         |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I have a program that generates the following core:

 {{{#!hs
 main = case t of t0
    0##     -> ...
    DEFAULT -> case t0 -# 1## of t1
       0##    -> ...
       DEFAUT -> case t1 -# 1## of t2
          0##     -> ...
          DEFAULT -> case t2 -# 1## of _
             0##     -> ...
             DEFAULT -> ...
 }}}

 I think it would be possible to implement constant propagation to get:

 {{{#!hs
 main = case t of _
    0## -> ...
    1## -> ...
    2## -> ...
    3## -> ...
    DEFAULT -> ...
 }}}

 If I'm not mistaken, to do that we could replace occurences of:

 {{{#!hs
 case t -# k# of _
    n0# -> ...
    n1# -> ...
    ...
    DEFAULT -> f
 }}}

 with:

 {{{#!hs
 case t of _
    (n0#+k#) -> ... -- (ni#+k#) statically computed
    (n1#+k#) -> ...
    ...
    DEFAULT -> f
 }}}

 Any guidance on how to implement that would be appreciated.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12877>
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