nonmoving: Use correct info table pointer accessor

Previously we used INFO_PTR_TO_STRUCT instead of
THUNK_INFO_PTR_TO_STRUCT when looking at a thunk. These two happen to be
equivalent on 64-bit architectures due to alignment considerations
however they are different on 32-bit platforms. This lead to #17487.

To fix this we also employ a small optimization: there is only one thunk
of type WHITEHOLE (namely stg_WHITEHOLE_info). Consequently, we can just
use a plain pointer comparison instead of testing against info->type.

2 files changed, 7 insertions, 19 deletions

diff --git a/includes/rts/storage/ClosureMacros.h b/includes/rts/storage/ClosureMacros.h
index b5ae2dafc6..1b3628f2b9 100644
--- a/includes/rts/storage/ClosureMacros.h
+++ b/includes/rts/storage/ClosureMacros.h
@@ -107,20 +107,6 @@ INLINE_HEADER const StgConInfoTable *get_con_itbl(const StgClosure *c)
    return CON_INFO_PTR_TO_STRUCT((c)->header.info);
 }
 
-/* Used when we expect another thread to be mutating the info table pointer of
- * a closure (e.g. when busy-waiting on a WHITEHOLE).
- */
-INLINE_HEADER const StgInfoTable *get_volatile_itbl(StgClosure *c) {
-    // The volatile here is import to ensure that the compiler does not
-    // optimise away multiple loads, e.g. in a busy-wait loop. Note that
-    // we can't use VOLATILE_LOAD here as the casts result in strict aliasing
-    // rule violations and this header may be compiled outside of the RTS
-    // (where we use -fno-strict-aliasing).
-    StgInfoTable * *volatile p = (StgInfoTable * *volatile) &c->header.info;
-    return INFO_PTR_TO_STRUCT(*p);
-}
-
-
 INLINE_HEADER StgHalfWord GET_TAG(const StgClosure *con)
 {
     return get_itbl(con)->srt;
diff --git a/rts/sm/NonMovingMark.c b/rts/sm/NonMovingMark.c
index b7ab772bdd..e236056092 100644
--- a/rts/sm/NonMovingMark.c
+++ b/rts/sm/NonMovingMark.c
@@ -567,9 +567,11 @@ inline void updateRemembSetPushThunk(Capability *cap, StgThunk *thunk)
 {
     const StgInfoTable *info;
     do {
-        info = get_volatile_itbl((StgClosure *) thunk);
-    } while (info->type == WHITEHOLE);
-    updateRemembSetPushThunkEager(cap, (StgThunkInfoTable *) info, thunk);
+        info = *(StgInfoTable* volatile*) &thunk->header.info;
+    } while (info == &stg_WHITEHOLE_info);
+
+    const StgThunkInfoTable *thunk_info = THUNK_INFO_PTR_TO_STRUCT(info);
+    updateRemembSetPushThunkEager(cap, thunk_info, thunk);
 }
 
 /* Push the free variables of a thunk to the update remembered set.
@@ -1229,7 +1231,7 @@ mark_closure (MarkQueue *queue, const StgClosure *p0, StgClosure **origin)
             goto done;
 
         case WHITEHOLE:
-            while (get_volatile_itbl(p)->type == WHITEHOLE);
+            while (*(StgInfoTable* volatile*) &p->header.info == &stg_WHITEHOLE_info);
                 // busy_wait_nop(); // FIXME
             goto try_again;
 
@@ -1588,7 +1590,7 @@ mark_closure (MarkQueue *queue, const StgClosure *p0, StgClosure **origin)
     }
 
     case WHITEHOLE:
-        while (get_volatile_itbl(p)->type == WHITEHOLE);
+        while (*(StgInfoTable* volatile*) &p->header.info == &stg_WHITEHOLE_info);
         goto try_again;
 
     case COMPACT_NFDATA: