#12191: 7% allocation regression in Haddock performance tests -------------------------------------+------------------------------------- Reporter: bgamari | Owner: niteria Type: task | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Compile-time | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #10482 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by niteria): This appears to be ordering related. Meaning that when things are ordered differently we do different kind of work and it costs more. When I changed `Ord UnitId` from my original diff to {{{ instance Ord UnitId where nm1 `compare` nm2 = -- do the same work, but return the old ordering if stableUnitIdCmp nm1 nm2 == LT then getKey (getUnique nm1) `compare` getKey (getUnique nm2) -- this was enough to stop GHC from being smart and optimize it out else getUnique nm1 `compare` getUnique nm2 }}} The difference went from `7%` to `0.21%`. I will try to narrow this down to functions that are ordering sensitive and see if I can make them stable. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12191#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler