#13227: Loss of full laziness in mapFB -------------------------------------+------------------------------------- Reporter: simonpj | Owner: nomeata Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3067 Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I fixed a small bug in the new one-shot stuff, in this patch {{{ commit fc9d152b058f21ab03986ea722d0c94688b9969f Author: Simon Peyton Jones < simonpj@microsoft.com > Date: Thu Feb 16 09:41:55 2017 +0000 Comments and tiny refactor only }}} Here's the critical bit {{{ --- a/compiler/simplCore/OccurAnal.hs +++ b/compiler/simplCore/OccurAnal.hs @@ -1867,17 +1867,17 @@ occAnalApp env (Var fun, args, ticks) -- This is the *whole point* of the isRhsEnv predicate -- See Note [Arguments of let-bound constructors] - n_val_args = valArgCount args + length (takeWhile isOneShotInfo (occ_one_shots env)) - -- See Note [Sources of one-shot information], bullet point A' - + n_val_args = valArgCount args n_args = length args fun_uds = mkOneOcc env fun (n_val_args > 0) n_args is_exp = isExpandableApp fun n_val_args -- See Note [CONLIKE pragma] in BasicTypes -- The definition of is_exp should match that in Simplify.prepareRhs - one_shots = argsOneShots (idStrictness fun) n_val_args - -- See Note [Sources of one-shot information] + one_shots = argsOneShots (idStrictness fun) guaranteed_val_args + guaranteed_val_args = n_val_args + length (takeWhile isOneShotInfo + (occ_one_shots env)) + -- See Note [Sources of one-shot information], bullet point A'] }}} Notice that `guaranteed_val_args` should be used only for the call to `argOneShots`, not in the calls to `isExpandableApp` or `mkOneOcc`. I thoght this was just cleanup. For example, `is_exp` only matters if `isRhsEnv` is true; and in that case I think `occ_one_shots` is empty (see `rhsCtxt`); so I doubt the change to `is_exp` makes any difference at all. Nevertheless it does: we observed a 7% reduction in allocation for `haddock.base` and `haddock.Cabal` after this one patch. Bonkers! I have no idea why. But I'm just recording it here. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13227#comment:21 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler