[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):  Phab:D5109
       Wiki Page:                    |
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Comment (by ChaiTRex):

 I've started reworking my code to generate a small subset of test cases
 that use one specific fixed constant (100^th^ prime, 200^th^ prime,
 ''etc''. to avoid coincidentally passing tests) for each spot where a
 literal or variable is expected, which should cut down significantly on
 the number of tests.

 I'd like to further cut down on the number of tests while still getting
 full coverage, but I've only skimmed some of the constant folding rules.
 Am I right in assuming that the following are true?

 * `Int`s are the only type we need to test (if this isn't true, my
 reworking will make the code take a type parameter, so generating tests
 for additional types will be simple).
 * The following expression shapes are the only ones required, where `∘` is
 multiplication, addition, or subtraction and where multiple occurrences in
 one expression can represent different operations. '''Note:''' negation
 stops after single-`∘` expressions:
   * `a`
   * `-a`
   * `a ∘ b`
   * `a ∘ -b`
   * `-a ∘ b`
   * `a ∘ (b ∘ c)`
   * `(a ∘ b) ∘ c`
   * `(a ∘ b) ∘ (c ∘ d)`

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15569#comment:18>
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