#7994: Make foldl into a good consumer -------------------------------------+------------------------------------ Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by nomeata): I have come up with a stand-alone analysis that tries to find out the “caller arity” of a function. Quite a bit like the usage demand, but different from the demand analyser in a few aspects: It does not bother about the demand put on function arguments by a function (so much less fixed-pointing due to that), and it currently looks only for tail-calls (or almost-tail-calls; some post-processing of return values a after the “tail call” is ok, as long as it does not do any relevant calls). It is an analysis phase that puts the result in the IdInfo, and the simplifier will eta-expand functions where the „caller arity” is higher than the manifest arity. It has zero effect on the current code base. If I implement `foldl` via `foldr` (but ''without'' the use of `oneShot`), I get essentially the same as the result as without the analysis, but with the explicit `oneShot`: {{{ Min -0.2% -74.5% -3.0% -3.0% -50.0% Max +0.9% +1.8% +2.5% +2.5% +14.8% Geometric Mean -0.0% -3.7% -0.3% -0.3% -0.5% }}} It still does not help with `bernoulli`, and probably nothing will: Implementing `foldl` as `foldr` will always yield bad code if list fusion does not work all the way and the `build` (or in this case, `build2`) is not completely gone. But if list fusion works, the results can be very good The `oneShot` change is much simpler, has little risk of introducing bugs and does not increase compilation times. The “caller arity” pass, OTOH, may optimise other code as well (but at least nofib does not seem to contain much such code), and can likely be made more precise. But the conclusion is that `foldl` can be made a good consumer! -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7994#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler