[Git][ghc/ghc][master] Eliminate thunk in 'IfaceTyCon'

[Marge Bot (@marge-bot)]

Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC


Commits:
349ea330 by Fendor at 2024-03-19T14:50:00-04:00
Eliminate thunk in 'IfaceTyCon'

Heap analysis showed that `IfaceTyCon` retains a thunk to
`IfaceTyConInfo`, defeating the sharing of the most common instances of
`IfaceTyConInfo`.
We make sure the indirection is removed by adding bang patterns to
`IfaceTyCon`.

Experimental results on the agda code base, where the `mi_extra_decls`
were read from disk:

Before this change, we observe around 8654045 instances of:

`IfaceTyCon[Name,THUNK_1_0]`

But these thunks almost exclusively point to a shared value!

Forcing the thunk a little bit more, leads to `ghc-debug` reporting:

`IfaceTyCon[Name:Name,IfaceTyConInfo]`

and a noticeable reduction of live bytes (on agda ~10%).

- - - - -


1 changed file:

- compiler/GHC/Iface/Type.hs


Changes:

=====================================
compiler/GHC/Iface/Type.hs
=====================================
@@ -93,6 +93,7 @@ import {-# SOURCE #-} GHC.Tc.Utils.TcType ( isMetaTyVar, isTyConableTyVar )
 import Data.Maybe( isJust )
 import qualified Data.Semigroup as Semi
 import Control.DeepSeq
+import Control.Monad ((<$!>))
 
 {-
 ************************************************************************
@@ -244,7 +245,18 @@ instance Monoid IfaceAppArgs where
 -- We have to tag them in order to pretty print them
 -- properly.
 data IfaceTyCon = IfaceTyCon { ifaceTyConName :: IfExtName
-                             , ifaceTyConInfo :: IfaceTyConInfo }
+                             , ifaceTyConInfo :: !IfaceTyConInfo
+                             -- ^ We add a bang to this field as heap analysis
+                             -- showed that this constructor retains a thunk to
+                             -- a value that is usually shared.
+                             --
+                             -- See !12200 for how this bang saved ~10% residency
+                             -- when loading 'mi_extra_decls' on the agda
+                             -- code base.
+                             --
+                             -- See Note [Sharing IfaceTyConInfo] for why
+                             -- sharing is so important for 'IfaceTyConInfo'.
+                             }
     deriving (Eq)
 
 -- | The various types of TyCons which have special, built-in syntax.
@@ -2050,7 +2062,13 @@ instance Binary IfaceTyConSort where
 instance Binary IfaceTyConInfo where
    put_ bh (IfaceTyConInfo i s) = put_ bh i >> put_ bh s
 
-   get bh = mkIfaceTyConInfo <$> get bh <*> get bh
+   get bh = mkIfaceTyConInfo <$!> get bh <*> get bh
+    -- We want to make sure, when reading from disk, as the most common case
+    -- is supposed to be shared. Any thunk adds an additional indirection
+    -- making sharing less useful.
+    --
+    -- See !12200 for how this bang and the one in 'IfaceTyCon' reduces the
+    -- residency by ~10% when loading 'mi_extra_decls' from disk.
 
 instance Outputable IfaceTyLit where
   ppr = pprIfaceTyLit



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/349ea3302bd81daa896e0541b6b8e34a5da16b52

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/349ea3302bd81daa896e0541b6b8e34a5da16b52
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