[GHC] #15569: Constant folding optimises 1 into 3

[GHC]

#15569: Constant folding optimises 1 into 3
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        Reporter:  snowleopard       |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.6.1-beta1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #9136             |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by snowleopard):

 In an attempt to find a safer implementation for constant folding I came
 up with this:

 https://gist.github.com/snowleopard/2dd93951cfd42e03aa04a4aa696ca029

 This may be an overkill, but does shift most of the verification load to
 the compiler: we still need to manually verify a few key functions like
 `eval`, but the constant folding rules can't be written incorrectly.

 NB: I covered most of the rules, but not all. Also I haven't really shown
 what to do with negative variables (non-literals) in the end, but this bit
 doesn't look too complicated.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15569#comment:8>
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