[Haskell-cafe] Can't prevent memoizing in simple code

[Yves Parès]

Thanks ^^

My other solution was a dirty trick:

Changing the second (l 1) by a (l (div 2 2)), which would only be good
until GHC knows how to statically analyse it (2-1 wasn't working for
instance).

I also noticed (while profiling to confirm that this was the source of the
memory leak) that adding manually cost centres forces the re-evaluation:

main = do
  print $ suminit2 ({-# SCC "list1" #-} l 1) 1000000 0
  print $ fst $ suminit ({-# SCC "list2" #-} l 1) 1000000 0
  where
    l n = enumFrom n

CAF:main5  Main                     97           0    0.0    0.0    26.6
50.0
  main      Main                    118           0    0.0    0.0    26.6
50.0
   list2    Main                    119           0    0.0    0.0    26.6  *50.0
*
    main.l  Main                    120           1   26.6   50.0    26.6
50.0
[...]
CAF:main8  Main                     95           0    0.0    0.0    30.4
50.0
  main      Main                    110           0    0.0    0.0    30.4
50.0
   list1    Main                    111           0    0.0    0.0    30.4
*50.0*
    main.l  Main                    112           1   30.4   50.0    30.4
50.0

We see here that allocations are shared between list1 and list2 (I expected
list1 to get 100% and list2 0%, due to sharing).
Strange...

2012/5/16 Anthony Cowley <acowley at gmail.com>

> On May 16, 2012, at 12:08 PM, Yves Parès wrote:
>
> The buffer http://hpaste.org/68595 presents a simple code I tried to
> profile.
> I spotted what I strongly think to be an abusive memoization. The problem
> is that I don't see how to (simply) get rid of it.
> Compiled with -O2, it consumes 130MB of memory, however lines A and B
> executed separately consume each only 1MB.
>
> The infinite list (l 1), whatever I do, keeps being shared between lines A
> and B.
> I tried to wrap it in a function, as you can see, I also tried to make it
> explicitely polymorphic (bypassing monomorphic restriction), nothing solves
> it, GHC is just to good at memoizing.
>
>
> Adding a {-# NOINLINE l #-} annotation helps here. Syntactically, it must
> be located somewhere a type signature for l would also be valid.
>
> Anthony
>
>
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