[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
      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:                    |
-------------------------------------+-------------------------------------
Description changed by snowleopard:

Old description:

> 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

New description:

 A recent Hadrian issue [1] led me to finding a rather embarrassing bug:

 {{{#!hs
 -- 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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15569#comment:2>
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