[Git][ghc/ghc][wip/nounpack-z-encs-faststring] Never UNPACK `FastMutInt` for counting z-encoded `FastString`s

[Hannes Siebenhandl (@fendor)]

Hannes Siebenhandl pushed to branch wip/nounpack-z-encs-faststring at Glasgow Haskell Compiler / GHC


Commits:
ce050190 by Fendor at 2024-04-04T14:47:20+02:00
Never UNPACK `FastMutInt` for counting z-encoded `FastString`s

In `FastStringTable`, we count the number of z-encoded FastStrings
that exist in a GHC session.
We used to UNPACK the counters to not waste memory, but live retainer
analysis showed that we allocate a lot of `FastMutInt`s, retained by
`mkFastZString`.

We lazily compute the `FastZString`, only incrementing the counter when the `FastZString` is
forced.
The function `mkFastStringWith` calls `mkZFastString` and boxes the
`FastMutInt`, leading to the following core:

    mkFastStringWith
      = \ mk_fs _  ->
             = case stringTable of
                { FastStringTable _ n_zencs segments# _ ->
                    ...
                         case ((mk_fs (I# ...) (FastMutInt n_zencs))
                            `cast` <Co:2> :: ...)
                            ...

Marking this field as `NOUNPACK` avoids this reboxing, eliminating the
allocation of a fresh `FastMutInt` on every `FastString` allocation.

- - - - -


1 changed file:

- compiler/GHC/Data/FastString.hs


Changes:

=====================================
compiler/GHC/Data/FastString.hs
=====================================
@@ -304,9 +304,18 @@ and updates to multiple buckets with low synchronization overhead.
 See Note [Updating the FastString table] on how it's updated.
 -}
 data FastStringTable = FastStringTable
-  {-# UNPACK #-} !FastMutInt -- the unique ID counter shared with all buckets
-  {-# UNPACK #-} !FastMutInt -- number of computed z-encodings for all buckets
-  (Array# (IORef FastStringTableSegment)) -- concurrent segments
+  {-# UNPACK #-} !FastMutInt
+  -- ^ The unique ID counter shared with all buckets
+  --
+  -- We unpack the 'FastMutInt' counter as it is always consumed strictly.
+  {-# NOUNPACK #-} !FastMutInt
+  -- ^ Number of computed z-encodings for all buckets.
+  --
+  -- We mark this as 'NOUNPACK' as this 'FastMutInt' is retained by a thunk
+  -- in 'mkFastStringWith' and needs to be boxed any way.
+  -- If this is unpacked, then we box this single 'FastMutInt' once for each
+  -- allocated FastString.
+  (Array# (IORef FastStringTableSegment)) -- ^  concurrent segments
 
 data FastStringTableSegment = FastStringTableSegment
   {-# UNPACK #-} !(MVar ())  -- the lock for write in each segment



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/ce0501906e8dc90d485b6576a09920b29810b132

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/ce0501906e8dc90d485b6576a09920b29810b132
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