#9291: Don't reconstruct sum types if the type subtly changes -------------------------------------+------------------------------------- Reporter: schyler | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by rwbarton): Replying to [comment:36 nomeata]:
Is this related with typed-ness of the IR?
Yes! The type system of Core ensures that we cannot get the ST stuff wrong (to some extent. It still requires us to also not inline `runRW#`, for example).
Is this really true? Say I write {{{#!hs f :: Bool -> IORef Int -> IO () f b v = v `seq` when b (writeIORef v 3) }}} GHC produces {{{ f1 = \ b_auQ v_auR eta_B1 -> case v_auR `cast` ... of _ { STRef ipv_sPi -> case b_auQ of _ { False -> (# eta_B1, () #); True -> case writeMutVar# ipv_sPi f2 eta_B1 of s2#_aQd { __DEFAULT -> (# s2#_aQd, () #) } } } }}} What stops me from rewriting this via case-of-case to the inequivalent {{{ f1 = \ b_auQ v_auR eta_B1 -> case v_auR `cast` ... of _ { STRef ipv_sPi -> case writeMutVar# ipv_sPi f2 eta_B1 of s2#_aQd { __DEFAULT -> case b_auQ of _ { False -> (# eta_B1, () #); -- Haha, don't actually use s2#_aQd here. True -> (# s2#_aQd, () #) } } } }}} I think it is just the flag `has_side_effects = True` on `writeMutVar#`, and doesn't have to do with types. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9291#comment:37 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler