Add a findObject function to CardTable for assertions in writer barri…

…ers (facebook#1513)

Summary: Pull Request resolved: facebook#1513

Differential Revision: D62169632

include/hermes/VM/AlignedHeapSegment.h

@@ -755,6 +755,16 @@ class FixedSizeHeapSegment : public AlignedHeapSegment {
   static void checkUnwritten(char *start, char *end);
 #endif
 
+#ifdef HERMES_SLOW_DEBUG
+  /// Find the object containing \p loc.
+  static GCCell *findObjectContaining(const void *loc) {
+    auto *lowLim = static_cast<char *>(storageStart(loc));
+    auto *hiLim = lowLim + kSize;
+    return contents(lowLim)->boundaryTable_.findObjectContaining(
+        lowLim, hiLim, loc);
+  }
+#endif
+
  private:
   FixedSizeHeapSegment(StorageProvider *provider, void *lowLim);
 };

include/hermes/VM/CardBoundaryTable.h

@@ -109,6 +109,12 @@ class CardBoundaryTable {
   ///
   /// \pre start is card-aligned.
   void verifyBoundaries(char *start, char *level) const;
+
+  /// Find the object that owns the memory at \p loc.
+  GCCell *findObjectContaining(
+      const char *lowLim,
+      const char *hiLim,
+      const void *loc) const;
 #endif // HERMES_SLOW_DEBUG
 
 #ifndef UNIT_TEST

lib/VM/gcs/CardBoundaryTable.cpp

@@ -110,6 +110,16 @@ void CardBoundaryTable::verifyBoundaries(char *start, char *level) const {
         "Card object boundary is broken: first obj doesn't extend into card");
   }
 }
+
+GCCell *CardBoundaryTable::findObjectContaining(
+    const char *lowLim,
+    const char *hiLim,
+    const void *loc) const {
+  GCCell *obj = firstObjForCard(lowLim, hiLim, addressToIndex(loc));
+  while (obj->nextCell() < loc)
+    obj = obj->nextCell();
+  return obj;
+}
 #endif // HERMES_SLOW_DEBUG
 
 } // namespace vm