On 2018-02-27 08:19 AM, Shao Cheng wrote:

Coming back to your use case, you may try avoid using raw lists and switch to unboxed vectors, turn on -O2 and rely on stream fusion of the vector package. That will result in a considerable speedup.

I looked at the core that’s generated, and there’s no need for vectors. Fusion happens, there’s no use of lists at all and unboxed types are used. The code boils down to a single recursive function:

let go i sum = case i of
        100000000 -> sum + 200000000
        _ -> go (i + 1) (sum + i * 2)
in go 1 0

except that the types are unboxed. The following complete program compiles down to almost identical core when compiled without optimization:

{-# LANGUAGE MagicHash #-}

import GHC.Exts

main = print $ I# value
  where
    value =
        let go :: Int# -> Int# -> Int#
            go i sum = case i of
                100000000# -> sum +# 200000000#
                _ -> go (i +# 1#) (sum +# i *# 2#)
        in go 1# 0#

I think that’s impressive even if it’s not a single number. Execution time on my lowly i5 is only 50ms.

BTW, GHC 8 seems to have removed the option for exporting core (-fext-core) but there’s a wonderful plugin package called dump-core that produces HTML output with colouring and interactivity. You just install it from Hackage and use the extra options it provides.

It seems to me that gcc’s compile-time evaluation of this loop is a special-case that matches the kind of thing that often crops up in C. I assume it’s not capable of doing that for every expression that could be evaluated at compile time, so a more complicated and realistic example would probably defeat it. After all, ghc could in theory evaluate any pure value (CAF) at compile time if it chose to, but that’s usually not what you want.

Also, it’s worth noting that due to Haskell’s lazy evaluation, a pure value (CAF) will never be evaluated more than once at runtime, which isn’t something you get with C.

​

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