#13425: Semantically-equivalent expressions evaluating at different values with -O1 -------------------------------------+------------------------------------- Reporter: rbasso | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- The following code shows inconsistent results when compiled with -O1: {{{#!hs import Data.Bits ((.&.)) flags :: Int -> [Int] flags x = filter (\y -> x .&. y > 0) [1, 128, 129, 255] main :: IO () main = do putStrLn $ show $ flags $ 255 -- These lines should display putStrLn (show (flags 255)) -- the same value, right? -- It appears that this is related to something that started -- with GHC 7.10, but was also changed in GHC 8.0, as we can -- see from the following tests: -- -- Stack snapshot lts-0.7 (GHC 7.8.3), with -O2 -- [1,128,129,255] -- [1,128,129,255] -- -- Stack snapshot lts-2.22 (GHC 7.8.4) with -O2 -- [1,128,129,255] -- [1,128,129,255] -- -- Stack snapshot lts-3.22 (GHC 7.10.2) with -O2 -- [1] -- [1] -- -- Stack snapshot lts-6.30 (GHC 7.10.3) with -O2 -- [1] -- [1] -- -- Stack snapshot lts-7.20 (GHC 8.0.1) with -O2 -- [1] -- [1,128,129,255] -- -- Stack snapshot lts-8.5 (GHC 8.0.2) with -O2 -- [1] -- [1,128,129,255] -- -- In GHC 8.0.2, compiling with any of the following... -- -O0 -- -O2 -fno-cmm-sink -- -O2 -fno-enable-rewrite-rules -- -- ...we get the expected results: -- [1,128,129,255] -- [1,128,129,255] -- -- Using only -O1, we get the abnormal behavior again: -- [1] -- [1,128,129,255] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13425 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler