codeGen: Ensure that static datacon apps are included in SRTswip/T20959

When generating an SRT for a recursive group, GHC.Cmm.Info.Build.oneSRT
filters out recursive references, as described in Note [recursive SRTs].
However, doing so for static functions would be unsound, for the reason
described in Note [Invalid optimisation: shortcutting].

However, the same argument applies to static data constructor
applications, as we discovered in #20959. Fix this by ensuring that
static data constructor applications are included in recursive SRTs.

The approach here is not entirely satisfactory, but it is a starting
point.

Fixes #20959.

1 files changed, 6 insertions, 0 deletions

diff --git a/rts/sm/Storage.h b/rts/sm/Storage.h
index 00f2943a51..0fecc50208 100644
--- a/rts/sm/Storage.h
+++ b/rts/sm/Storage.h
@@ -147,6 +147,12 @@ void move_STACK (StgStack *src, StgStack *dest);
   bits = link_field & 3;
   if ((bits | prev_static_flag) != 3) { ... }
 
+  However, this mechanism for tracking liveness has an important implication:
+  once a static object becomes unreachable it must never become reachable again.
+  One would think that this can by definition never happen but in the past SRT
+  generation bugs have caused precisely this behavior with disasterous results.
+  See Note [No static object resurrection] in GHC.Cmm.Info.Build for details.
+
   -------------------------------------------------------------------------- */
 
 #define STATIC_BITS 3