[Runtime] Fixed zeroing of extra data padding for values.

See also bcc309e .

Author: nate-chandler

include/swift/ABI/Metadata.h

@@ -3373,6 +3373,7 @@ class TargetGenericMetadataPatternTrailingObjects :
   /// class metadata.
   ///
   /// See also: [pre-5.2-extra-data-zeroing]
+  /// See also: [pre-5.3-extra-data-zeroing]
   const GenericMetadataPartialPattern *getExtraDataPattern() const {
     assert(asSelf()->hasExtraDataPattern());
     return this->template getTrailingObjects<GenericMetadataPartialPattern>();

lib/IRGen/GenMeta.cpp

@@ -2317,6 +2317,7 @@ namespace {
       asImpl().layoutHeader();
 
       // See also: [pre-5.2-extra-data-zeroing]
+      // See also: [pre-5.3-extra-data-zeroing]
       if (asImpl().hasExtraDataPattern()) {
         asImpl().addExtraDataPattern();
       }

stdlib/public/runtime/Metadata.cpp

@@ -617,7 +617,10 @@ initializeValueMetadataFromPattern(ValueMetadata *metadata,
     auto extraDataPattern = pattern->getExtraDataPattern();
 
     // Zero memory up to the offset.
-    memset(metadataExtraData, 0, size_t(extraDataPattern->OffsetInWords));
+    // [pre-5.3-extra-data-zeroing] Before Swift 5.3, 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);