#16111: Inconsistent behavior of Data.Bits.shiftL with different optimization levels and -fllvm -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.3 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: -------------------------------------+------------------------------------- Consider this program: {{{#!hs module Main (main) where import Data.Bits import Data.Word main :: IO () main = print $ toInteger (shiftL 1 hm :: Word64) == toInteger (shiftL 1 hm :: Word64) hm :: Int hm = -1 {-# NOINLINE hm #-} }}} The result of this program depends greatly on what optimization levels are used, and whether `-fllvm` is used: {{{ $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O0 Bug.hs && ./Bug [1 of 1] Compiling Main ( Bug.hs, Bug.o ) Linking Bug ... True $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O1 Bug.hs && ./Bug [1 of 1] Compiling Main ( Bug.hs, Bug.o ) Linking Bug ... True $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O2 Bug.hs && ./Bug [1 of 1] Compiling Main ( Bug.hs, Bug.o ) Linking Bug ... Bug: Bad shift length-1 $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O0 -fllvm Bug.hs && ./Bug [1 of 1] Compiling Main ( Bug.hs, Bug.o ) Linking Bug ... True $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O1 -fllvm Bug.hs && ./Bug [1 of 1] Compiling Main ( Bug.hs, Bug.o ) Linking Bug ... False $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O2 -fllvm Bug.hs && ./Bug [1 of 1] Compiling Main ( Bug.hs, Bug.o ) Linking Bug ... Bug: Bad shift length-1 }}} This program manages to return all of `True`, `False`, and `Bad shift length-1` depending on which flags are used! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16111 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler