[Runtime] Memset Extradata fully before copy from pattern.

The third argument of memset needs a size in *bytes* not words.

Author: varungandhi-apple

include/swift/ABI/Metadata.h

@@ -3224,6 +3224,8 @@ class TargetGenericMetadataPatternTrailingObjects :
   /// metadata header (e.g. after the last members declared in StructMetadata).
   /// In class metadata, this section is relative to the end of the entire
   /// class metadata.
+  ///
+  /// See also: [pre-5.2-extra-data-zeroing]
   const GenericMetadataPartialPattern *getExtraDataPattern() const {
     assert(asSelf()->hasExtraDataPattern());
     return this->template getTrailingObjects<GenericMetadataPartialPattern>();

lib/IRGen/GenMeta.cpp

@@ -2225,6 +2225,7 @@ namespace {
     void layout() {
       asImpl().layoutHeader();
 
+      // See also: [pre-5.2-extra-data-zeroing]
       if (asImpl().hasExtraDataPattern()) {
         asImpl().addExtraDataPattern();
       }

stdlib/public/runtime/Metadata.cpp

@@ -404,7 +404,10 @@ initializeClassMetadataFromPattern(ClassMetadata *metadata,
     auto extraDataPattern = pattern->getExtraDataPattern();
 
     // Zero memory up to the offset.
-    memset(metadataExtraData, 0, size_t(extraDataPattern->OffsetInWords));
+    // [pre-5.2-extra-data-zeroing] Before Swift 5.2, the runtime did not
+    // correctly zero the zero-prefix of the extra-data pattern.
+    memset(metadataExtraData, 0,
+           size_t(extraDataPattern->OffsetInWords) * sizeof(void *));
 
     // Copy the pattern into the rest of the extra data.
     copyMetadataPattern(metadataExtraData, extraDataPattern);