#9476: Implement late lambda-lifting -------------------------------------+------------------------------------- Reporter: simonpj | Owner: sgraf Type: feature request | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: LateLamLift Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #8763 #13286 | Differential Rev(s): Phab:D5224 Wiki Page: LateLamLift | -------------------------------------+------------------------------------- Comment (by sgraf): I realised that while the heuristics currently employed work quite well, we are too pessimistic wrt. unsaturated applications. Currently, we don't lift binders that have undersaturated calls [https://github.com/sgraf812/ghc/blob/c4b8c04f26b5045da8015dd7289cd176a4addff... at all]. But there's nothing wrong with lifting those, as long as allocations don't get worse. So I set out to get rid of that heuristic and replaced it with a check that disallows occurrences in argument position instead (previously, this would be subsumed by the undersaturated call check). The results are inconclusive, with some significant slow-downs which I could pin-point to a bug in the heuristic that detects known calls. Currently, we say that a call to some variable would be lowered as a known call when `idArity bndr > 0`. But there actually are thunks (e.g. arity 0) of function type, which count as known calls. Abstracting over these thunks makes for an increase in allocation and runtime. I'm investigating more robust ways of detecting known calls. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9476#comment:46 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler