#14208: Performance with O0 is much better than the default or with -O2, runghc performs the best -------------------------------------+------------------------------------- Reporter: harendra | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 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 harendra): It was hard to find in the manual the difference between O0 and the default. The manual says "O0 is the default", which seems to be incorrect (NEED TO BE FIXED). So I had to turn to the GHC code. Alright, so O0 seems to ignore or omit "interface-pragmas" whatever the heck they are, from DynFlags.hs: {{{#!hs , ([0], Opt_IgnoreInterfacePragmas) , ([0], Opt_OmitInterfacePragmas) }}} When I used "-fignore-interface-pragmas" I got the same improvement in performance. The GHC manual documents this flag but says nothing about what this really means (NEED TO BE FIXED). I can only guess. Since this has to do something with the interface, this also explains why the performance is good when the whole code is in the same module and bad when the code is split into two modules. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14208#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler