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HashMap has thunks for internal nodes after construction using strict operations #232

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mantasg opened this issue May 6, 2019 · 0 comments · Fixed by #233
Closed

HashMap has thunks for internal nodes after construction using strict operations #232

mantasg opened this issue May 6, 2019 · 0 comments · Fixed by #233

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@mantasg
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mantasg commented May 6, 2019

I've been trying to check strictness of my model using ppHeapTree function from (slightly altered) ghc-heap-view package and discovered that there seem to be some thunks in HashMap following a series of insert using foldl'. However if I union it with singleton it comes out without thunks.

I've create a little project to demonstrate this behaviour: https://github.com/mantasg/hashmap-test. The version of ghc-heap-view I used doesn't work with GHC 8.6 therefore I used stack lts-11.20 with GHC 8.2. Running it requires stack and can be invoked by:

cd hashmap-test && stack build hashmap-test && stack exec hashmap-test

HeapTree for test1 = foldl' (\m v -> HM.insert v v m) HM.empty [0..5]

HeapTree for test1
"BitmapIndexed ([ (_thunk (I# 0) (I# 0) 0), (_thunk (I# 1) (I# 1) 1), (Leaf (I# 2) (I# 2) 2), (Leaf (I# 3) (I# 3) 3), (Leaf (I# 4) (I# 4) 4), (Leaf (I# 5) (I# 5) 5) ]) 63"

HeapTree for test2 = foldl' (\m v -> HM.union (HM.singleton v v) m) HM.empty [0..5]

HeapTree for test2
"BitmapIndexed ([ (Leaf (I# 0) (I# 0) 0), (Leaf (I# 1) (I# 1) 1), (Leaf (I# 2) (I# 2) 2), (Leaf (I# 3) (I# 3) 3), (Leaf (I# 4) (I# 4) 4), (Leaf (I# 5) (I# 5) 5) ]) 63"
treeowl added a commit to treeowl/unordered-containers that referenced this issue May 6, 2019
@treeowl
Make two strict in its key arguments
5cbf8c5 
`two` wasn't strict in its key arguments. We thought this was okay,
because its key arguments are always in WHNF and it's marked
`INLINE`. But `two` is defined as a *recursive* `go` function
(I haven't looked into why), which can't be inlined. I believe
that's the reason GHC doesn't *realize* that the keys are in WHNF.
Anyway, the end result was that `two` would defer the creation of
the `Leaf` values stored in the array, producing very silly thunks.

Fixes haskell-unordered-containers#232
@treeowl treeowl mentioned this issue May 6, 2019
Merged
treeowl added a commit that referenced this issue May 6, 2019
@treeowl
Make two strict in its key arguments
ec84ad5 
`two` wasn't strict in its key arguments. We thought this was okay,
because its key arguments are always in WHNF and it's marked
`INLINE`. But `two` is defined as a *recursive* `go` function
(I haven't looked into why), which can't be inlined. I believe
that's the reason GHC doesn't *realize* that the keys are in WHNF.
Anyway, the end result was that `two` would defer the creation of
the `Leaf` values stored in the array, producing very silly thunks.

Fixes #232
@treeowl treeowl mentioned this issue May 7, 2019
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Make two strict in its key arguments treeowl/unordered-containers
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@mantasg