#5654: Profiling semantics bug -------------------------------------+------------------------------------- Reporter: simonmar | Owner: simonmar Type: bug | Status: new Priority: low | Milestone: Component: Profiling | Version: 7.2.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | profiling/should_run/scc004 Blocked By: | Blocking: Related Tickets: #10007 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by Ben Gamari <ben@…>): In [changeset:"394231b301efb6b56654b0a480ab794fe3b7e4db/ghc" 394231b3/ghc]: {{{ #!CommitTicketReference repository="ghc" revision="394231b301efb6b56654b0a480ab794fe3b7e4db" Fix cost-centre-stacks bug (#5654) This fixes some cases of wrong stacks being generated by the profiler. For background and details on the fix see `Note [Evaluating functions with profiling]` in `rts/Apply.cmm`. This does have an impact on allocations for some programs when profiling. nofib results: ``` k-nucleotide +0.0% +8.8% +11.0% +11.0% 0.0% puzzle +0.0% +12.5% 0.244 0.246 0.0% typecheck 0.0% +8.7% +16.1% +16.2% 0.0% ------------------------------------------------------------------------ -------- Min -0.0% -0.0% -34.4% -35.5% -25.0% Max +0.0% +12.5% +48.9% +49.4% +10.6% Geometric Mean +0.0% +0.6% +2.0% +1.8% -0.3% ``` But runtimes don't seem to be affected much, and the examples I looked at were completely legitimate. For example, in puzzle we have this: ``` position :: ItemType -> StateType -> BankType position Bono = bonoPos position Edge = edgePos position Larry = larryPos position Adam = adamPos ``` where the identifiers on the rhs are all record selectors. Previously the profiler gave a stack that looked like ``` position bonoPos ... ``` i.e. `bonoPos` was at the same level of the call stack as `position`, but now it looks like ``` position bonoPos ... ``` I used the normaliser from the testsuite to diff the profiling output from other nofib programs and they all looked better. Test Plan: * the broken test passes * validate * compiled and ran all of nofib, measured perf, diff'd several .prof files Reviewers: niteria, erikd, austin, scpmw, bgamari Reviewed By: bgamari Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D2804 GHC Trac Issues: #5654, #10007 }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/5654#comment:16> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler