Don't track free variables in STG syntax by default

Summary:
Currently, `CoreToStg` annotates `StgRhsClosure`s with their set of non-global
free variables.  This free variable information is only needed in the final
code generation step (i.e. `StgCmm.codeGen`), which leads to transformations
such as `StgCse` and `StgUnarise` having to maintain this information.

This is tiresome and unnecessary, so this patch introduces a trees-to-grow-like
approach that only introduces the free variable set into the syntax tree in the
code gen pass, along with a free variable analysis on STG terms to generate
that information.

Fixes #15754.

Reviewers: simonpj, osa1, bgamari, simonmar

Reviewed By: osa1

Subscribers: rwbarton, carter

GHC Trac Issues: #15754

Differential Revision: https://phabricator.haskell.org/D5324

1 files changed, 6 insertions, 1 deletions

diff --git a/compiler/utils/UniqDSet.hs b/compiler/utils/UniqDSet.hs
index 92d924ee37..c2ace5787f 100644
--- a/compiler/utils/UniqDSet.hs
+++ b/compiler/utils/UniqDSet.hs
@@ -33,7 +33,8 @@ module UniqDSet (
         isEmptyUniqDSet,
         lookupUniqDSet,
         uniqDSetToList,
-        partitionUniqDSet
+        partitionUniqDSet,
+        mapUniqDSet
     ) where
 
 import GhcPrelude
@@ -121,6 +122,10 @@ uniqDSetToList = eltsUDFM . getUniqDSet
 partitionUniqDSet :: (a -> Bool) -> UniqDSet a -> (UniqDSet a, UniqDSet a)
 partitionUniqDSet p = coerce . partitionUDFM p . getUniqDSet
 
+-- See Note [UniqSet invariant] in UniqSet.hs
+mapUniqDSet :: Uniquable b => (a -> b) -> UniqDSet a -> UniqDSet b
+mapUniqDSet f = mkUniqDSet . map f . uniqDSetToList
+
 -- Two 'UniqDSet's are considered equal if they contain the same
 -- uniques.
 instance Eq (UniqDSet a) where