#15698: SingleEntry update flag for Stg bindings is not used -------------------------------------+------------------------------------- Reporter: osa1 | Owner: (none) Type: task | Status: new Priority: lowest | Milestone: 8.6.1 Component: Compiler | Version: 8.6.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I was looking at code generation for bindings with different update flags. Update flag type is defined as: {{{ data UpdateFlag = ReEntrant | Updatable | SingleEntry }}} I realized that we don't care about the difference between `ReEntrant` and `SingleEntry`, we only care about whether a binding is updatable or not, which is defined as {{{ isUpdatable :: UpdateFlag -> Bool isUpdatable ReEntrant = False isUpdatable SingleEntry = False isUpdatable Updatable = True }}} So we could remove `SingleEntry` and replace all uses of it with `ReEntrant` and everything would work the same. This raises the question of whether we're missing an optimisation in the code generator. Looking at code generation differences of updatable and non-updatable bindings, it seems like for a thunk (a binding with no arguments) we generate a thunk header and push an update frame regardless of the update flag. As far as I can see, update flag is only used when generating AP and selector thunks (we don't generate AP or selector thunks if the binding is not updatable). My question is: it seems to me that if a thunk is single entry then we should be able to give it a non-thunk type (maybe FUN?) and avoid pushing an update frame in `closureCodeBody`. Am I missing anything or is this possible? Is this worth trying? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15698 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler