#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 -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15569#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler