[Haskell-cafe] Code runs 7x FASTER with profiling

Niklas Hambüchen mail at nh2.me
Sat Dec 23 01:57:16 UTC 2017

Has anybody filed a bug about this yet?
I think we really should.

I recently observed a similar behaviour in code of a client.
It looked like

main = do
  l <- getSomeList
  let m = buildExpensiveHashMap l
  for_ l $ \element -> do
    print (... lookup l m ...)

And it inlined the `buildExpensiveHashMap`, making the whole thing 100x
slower. A bang on `m` fixed it to keep it outside the loop, but I found
it weird that GHC considered that expression cheap enough to inline.


On 09/12/2017 00:59, Brandon Allbery wrote:
> On Fri, Dec 8, 2017 at 9:18 AM, Neil Mayhew
> <neil_mayhew at users.sourceforge.net
> <mailto:neil_mayhew at users.sourceforge.net>> wrote:
>     Good point. Having great optimization isn’t a justification for
>     complete mental laziness on the part of the programmer! However, I
>     did find the behaviour in this case surprising and unintuitive,
>     given ghc’s usual ability to optimize things, and having it change
>     on me when I enabled profiling left me wondering where to go next.
>     The code I presented here is considerably simplified from the
>     original program, and represents a lot of work already expended
>     trying to get to the root of the problem.
> This is actually why I (and likely SPJ) am inclined to consider it a
> bug; while it might be arguable as Sven does, the counterintuitive
> effect when you turn on profiling suggests that it's not intended or
> expected. (Although maybe it should be; the intuition is "profiling
> disables optimization" but what it really does is a bit more subtle than
> that overly simplistic notion.)
> -- 
> brandon s allbery kf8nh                               sine nomine associates
> allbery.b at gmail.com <mailto:allbery.b at gmail.com>                        
>          ballbery at sinenomine.net <mailto:ballbery at sinenomine.net>
> unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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