#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: -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler