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

[GHC]

#15569: Constant folding optimises 1 into 3
-------------------------------------+-------------------------------------
           Reporter:  snowleopard    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.6.1-beta1
           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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 A recent Hadrian issue [1] led me to finding a rather embarrassing bug:

 {{{#!hs
 {-# NOINLINE sub #-}
 -- This is just subtraction in disguise
 minus :: Int -> Int -> Int
 minus x y = (8 - y) - (8 - x)
 {-# NOINLINE minus #-}

 main :: IO ()
 main = print (2 `minus` 1)
 }}}

 When compiled using `ghc-8.6.0.20180810` without optimisation this prints
 `1`, as expected, but when compiled with `-O` this prints `3`.

 Here is the incorrect rewrite rule [2]:

 {{{#!hs
 (L y :-:   v) :-: (L x :-: w) -> return $ mkL (y-x)   `add` (w `add` v)
 }}}

 This should be changed to:

 {{{#!hs
 (L y :-:   v) :-: (L x :-: w) -> return $ mkL (y-x)   `add` (w `sub` v)
 }}}

 Happy to submit the fix, but I'm not yet fully convinced that there are no
 other lurking bugs. This whole constant folding business is very error-
 prone.

 [1] https://github.com/snowleopard/hadrian/issues/641

 [2]
 https://github.com/ghc/ghc/blob/master/compiler/prelude/PrelRules.hs#L1786

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