Fix #15038

We introduce a new Id for unused pointer values in unboxed sums that is
not CAFFY. Because the Id is not CAFFY it doesn't make non-CAFFY
definitions CAFFY, fixing #15038.

To make sure anything referenced by the new id will be retained we get a
stable pointer to in on RTS startup.

Test Plan: Passes validate

Reviewers: simonmar, simonpj, hvr, bgamari, erikd

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15038

Differential Revision: https://phabricator.haskell.org/D4680

1 files changed, 7 insertions, 0 deletions

diff --git a/rts/RtsStartup.c b/rts/RtsStartup.c
index e4ca6b906d..7eb98a8ba0 100644
--- a/rts/RtsStartup.c
+++ b/rts/RtsStartup.c
@@ -247,6 +247,10 @@ hs_init_ghc(int *argc, char **argv[], RtsConfig rts_config)
     /* Add some GC roots for things in the base package that the RTS
      * knows about.  We don't know whether these turn out to be CAFs
      * or refer to CAFs, but we have to assume that they might.
+     *
+     * Because these stable pointers will retain any CAF references in
+     * these closures `Id`s of these can be safely marked as non-CAFFY
+     * in the compiler.
      */
     getStablePtr((StgPtr)runIO_closure);
     getStablePtr((StgPtr)runNonIO_closure);
@@ -265,6 +269,9 @@ hs_init_ghc(int *argc, char **argv[], RtsConfig rts_config)
     getStablePtr((StgPtr)cannotCompactPinned_closure);
     getStablePtr((StgPtr)cannotCompactMutable_closure);
     getStablePtr((StgPtr)nestedAtomically_closure);
+    getStablePtr((StgPtr)absentSumFieldError_closure);
+        // `Id` for this closure is marked as non-CAFFY,
+        // see Note [aBSENT_SUM_FIELD_ERROR_ID] in MkCore.
 
     getStablePtr((StgPtr)runSparks_closure);
     getStablePtr((StgPtr)ensureIOManagerIsRunning_closure);