#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): Heh, in the previous numbers, I passed the arguments to `nofib-analyze` in the wrong order. So please swap signs. I now made sure that the `oneShot` makes it into the unfolding for `foldl`, which is a clear win. Now, relative to `master` again, I get this very nice result: {{{ -------------------------------------------------------------------------------- Program Size Allocs Runtime Elapsed TotalMem bernouilli -0.1% +1.5% 0.14 0.14 +0.0% cryptarithm2 -0.0% -0.5% 0.01 0.01 +0.0% fem -0.0% -2.8% 0.02 0.02 +0.0% fft2 -0.1% -55.9% 0.04 0.04 +0.0% gen_regexps -0.0% -33.6% 0.00 0.00 +0.0% integrate -0.1% -59.4% 0.13 0.13 -1.0% minimax -0.0% -15.6% 0.00 0.00 +0.0% nucleic2 +0.9% +1.7% 0.05 0.05 +0.0% scs -0.0% -0.9% +1.0% +0.5% +0.0% simple -0.1% -9.4% 0.14 0.14 +0.0% x2n1 -0.1% -74.5% 0.01 0.01 +0.0% -------------------------------------------------------------------------------- Min -0.2% -74.5% -3.0% -3.0% -50.0% Max +0.9% +1.7% +4.0% +3.4% +10.4% Geometric Mean -0.0% -3.7% -0.2% -0.3% -0.6% }}} So it seems to be well worth turning `foldl` into a good consumer, even if the resulting code is not perfect for complicated cases like `foldl .. .. $ concat $ ..`. The increase for `bernouilli` comes from this code line: {{{ sum $ zipWith (*) powers (tail $ tail combs) }}} where originally, because `sum` is not a good consumer, no list fusion happens and a `sum` specialized to `Integer` is used, while after the change, `sum` is fused with `zipWith`, but not further, so now we have {{{ foldr2 (\x y r eta -> r (eta + (x * y))) id powers (tail $ tail combs) 0. }}} This means that we are have elimiated one intermediate list, but we are allocating PAP ’s on each iteration, which is more expensive (three arguments instead of two!). This is verified by looking at ticky numbers. Maybe foldr2 should have been inlined here. Or we just live with it. Is this (explicit `oneShot`ness-annotations using a magic function) a path we want to go? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7994#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler