#15717: Performance regression in for_ alternatives from GHC 8.2.2 to newer GHCs -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.6.1 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: -------------------------------------+------------------------------------- We are investigating various ways to make `for_` and `traverse_` not leak space by changing the type signature from `(a -> f b) -> t a -> f ()` to `(a -> f ()) -> t a -> f ()`. While doing so, we noticed a regression from GHC 8.2.2 to 8.4.3 and 8.6.1. The code: https://gist.github.com/nh2/b8f9f8e60443bdb30c1cd7e0acb8c8eb/bb1cc1a4987091f... Run against 3 different GHC releases: {{{ # 8.2.2 stack --resolver lts-11.22 ghc -- --make -O2 -rtsopts ./TraverseMaybePerformance.hs && /usr/bin/time ./TraverseMaybePerformance 8 +RTS -sstderr 460,309,368 bytes allocated in the heap # 8.4.3 stack --resolver lts-12.11 ghc -- --make -O2 -rtsopts ./TraverseMaybePerformance.hs && /usr/bin/time ./TraverseMaybePerformance 8 +RTS -sstderr 860,301,736 bytes allocated in the heap # 8.6.1 stack --resolver nightly-2018-10-06 ghc -- --make -O2 -rtsopts ./TraverseMaybePerformance.hs && /usr/bin/time ./TraverseMaybePerformance 8 +RTS -sstderr 860,301,784 bytes allocated in the heap }}} Allocations doubled starting with 8.4. All was run on Ubuntu 16.04 64-bit. We haven't investigated in detail yet (also whether it's a GHC or libraries problem) since we're actually trying to do something else and this came out on the side, but it looks important enough to share already. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15717 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler