1 files changed, 14 insertions, 0 deletions
diff --git a/includes/rts/storage/ClosureMacros.h b/includes/rts/storage/ClosureMacros.h
index a3873cc49d..2af50863d0 100644
--- a/includes/rts/storage/ClosureMacros.h
+++ b/includes/rts/storage/ClosureMacros.h
@@ -107,6 +107,20 @@ 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;