[Git][ghc/ghc][master] Fix sharing of 'IfaceTyConInfo' during core to iface type translation

[Marge Bot (@marge-bot)]

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


Commits:
73be65ab by Fendor at 2024-03-19T01:42:53-04:00
Fix sharing of 'IfaceTyConInfo' during core to iface type translation

During heap analysis, we noticed that during generation of
'mi_extra_decls' we have lots of duplicates for the instances:

* `IfaceTyConInfo NotPromoted IfaceNormalTyCon`
* `IfaceTyConInfo IsPromoted IfaceNormalTyCon`

which should be shared instead of duplicated. This duplication increased
the number of live bytes by around 200MB while loading the agda codebase
into GHCi.

These instances are created during `CoreToIface` translation, in
particular `toIfaceTyCon`.

The generated core looks like:

    toIfaceTyCon
      = \ tc_sjJw ->
          case $wtoIfaceTyCon tc_sjJw of
          { (# ww_sjJz, ww1_sjNL, ww2_sjNM #) ->
          IfaceTyCon ww_sjJz (IfaceTyConInfo ww1_sjNL ww2_sjNM)
          }

whichs removes causes the sharing to work propery.

Adding explicit sharing, with NOINLINE annotations, changes the core to:

    toIfaceTyCon
      = \ tc_sjJq ->
          case $wtoIfaceTyCon tc_sjJq of { (# ww_sjNB, ww1_sjNC #) ->
          IfaceTyCon ww_sjNB ww1_sjNC
          }

which looks much more like sharing is happening.
We confirmed via ghc-debug that all duplications were eliminated and the
number of live bytes are noticeably reduced.

- - - - -


1 changed file:

- compiler/GHC/Iface/Type.hs


Changes:

=====================================
compiler/GHC/Iface/Type.hs
=====================================
@@ -361,12 +361,51 @@ data IfaceTyConInfo   -- Used only to guide pretty-printing
                    , ifaceTyConSort       :: IfaceTyConSort }
     deriving (Eq)
 
--- This smart constructor allows sharing of the two most common
--- cases. See #19194
+-- | This smart constructor allows sharing of the two most common
+-- cases. See Note [Sharing IfaceTyConInfo]
 mkIfaceTyConInfo :: PromotionFlag -> IfaceTyConSort -> IfaceTyConInfo
-mkIfaceTyConInfo IsPromoted  IfaceNormalTyCon = IfaceTyConInfo IsPromoted  IfaceNormalTyCon
-mkIfaceTyConInfo NotPromoted IfaceNormalTyCon = IfaceTyConInfo NotPromoted IfaceNormalTyCon
-mkIfaceTyConInfo prom        sort             = IfaceTyConInfo prom        sort
+mkIfaceTyConInfo IsPromoted  IfaceNormalTyCon = promotedNormalTyConInfo
+mkIfaceTyConInfo NotPromoted IfaceNormalTyCon = notPromotedNormalTyConInfo
+mkIfaceTyConInfo prom        sort             = IfaceTyConInfo prom sort
+
+{-# NOINLINE promotedNormalTyConInfo #-}
+-- | See Note [Sharing IfaceTyConInfo]
+promotedNormalTyConInfo :: IfaceTyConInfo
+promotedNormalTyConInfo = IfaceTyConInfo IsPromoted IfaceNormalTyCon
+
+{-# NOINLINE notPromotedNormalTyConInfo #-}
+-- | See Note [Sharing IfaceTyConInfo]
+notPromotedNormalTyConInfo :: IfaceTyConInfo
+notPromotedNormalTyConInfo = IfaceTyConInfo NotPromoted IfaceNormalTyCon
+
+{-
+Note [Sharing IfaceTyConInfo]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+'IfaceTyConInfo' occurs an awful lot in 'ModIface', see #19194 for an example.
+But almost all of them are
+
+   IfaceTyConInfo IsPromoted IfaceNormalTyCon
+   IfaceTyConInfo NotPromoted IfaceNormalTyCon.
+
+The smart constructor `mkIfaceTyConInfo` arranges to share these instances,
+thus:
+
+  promotedNormalTyConInfo    = IfaceTyConInfo IsPromoted  IfaceNormalTyCon
+  notPromotedNormalTyConInfo = IfaceTyConInfo NotPromoted IfaceNormalTyCon
+
+  mkIfaceTyConInfo IsPromoted  IfaceNormalTyCon = promotedNormalTyConInfo
+  mkIfaceTyConInfo NotPromoted IfaceNormalTyCon = notPromotedNormalTyConInfo
+  mkIfaceTyConInfo prom        sort             = IfaceTyConInfo prom sort
+
+But ALAS, the (nested) CPR transform can lose this sharing, completely
+negating the effect of `mkIfaceTyConInfo`: see #24530 and #19326.
+
+Sticking-plaster solution: add a NOINLINE pragma to those top-level constants.
+When we fix the CPR bug we can remove the NOINLINE pragmas.
+
+This one change leads to an 15% reduction in residency for GHC when embedding
+'mi_extra_decls': see !12222.
+-}
 
 data IfaceMCoercion
   = IfaceMRefl



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/73be65abafcd4db8e3913415ec866f6e89881f22

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/commit/73be65abafcd4db8e3913415ec866f6e89881f22
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