Index: llvm/include/llvm/WindowsManifest/WindowsManifestMerger.h
===================================================================
--- llvm/include/llvm/WindowsManifest/WindowsManifestMerger.h
+++ llvm/include/llvm/WindowsManifest/WindowsManifestMerger.h
@@ -37,14 +37,29 @@
 
 class MemoryBuffer;
 
+namespace windows_manifest {
+
 #if LLVM_LIBXML2_ENABLED
 typedef xmlDocPtr XMLDocumentImpl;
 typedef xmlNodePtr XMLNodeImpl;
+typedef xmlNsPtr XMLNsImpl;
+typedef xmlAttrPtr XMLAttrImpl;
 #else
 typedef void *XMLDocumentImpl;
 typedef void *XMLNodeImpl;
+typedef void *XMLNsImpl;
+typedef void *XMLAttrImpl;
 #endif
 
+static const std::vector<std::pair<std::string, std::string>>
+    MtNsHrefsPrefixes = {
+        {"urn:schemas-microsoft-com:asm.v1", "ms_asmv1"},
+        {"urn:schemas-microsoft-com:asm.v2", "ms_asmv2"},
+        {"urn:schemas-microsoft-com:asm.v3", "ms_asmv3"},
+        {"http://schemas.microsoft.com/SMI/2005/WindowsSettings",
+         "ms_windowsSettings"},
+        {"urn:schemas-microsoft-com:compatibility.v1", "ms_compatibilityv1"}};
+
 class WindowsManifestError : public ErrorInfo<WindowsManifestError, ECError> {
 public:
   static char ID;
@@ -76,5 +91,6 @@
   bool ParseErrorOccurred = false;
 };
 
+} // namespace windows_manifest
 } // namespace llvm
 #endif
Index: llvm/lib/WindowsManifest/WindowsManifestMerger.cpp
===================================================================
--- llvm/lib/WindowsManifest/WindowsManifestMerger.cpp
+++ llvm/lib/WindowsManifest/WindowsManifestMerger.cpp
@@ -20,8 +20,7 @@
 #define FROM_XML_CHAR(X) reinterpret_cast<const char *>(X)
 
 using namespace llvm;
-
-namespace llvm {
+using namespace windows_manifest;
 
 char WindowsManifestError::ID = 0;
 
@@ -31,6 +30,11 @@
 
 #if LLVM_LIBXML2_ENABLED
 static bool xmlStringsEqual(const unsigned char *A, const unsigned char *B) {
+  // Handle null pointers.  Comparison of 2 null pointers returns true because
+  // this indicates the prefix of a default namespace.
+  if (!(A && B))
+    return !A && !B;
+
   return strcmp(FROM_XML_CHAR(A), FROM_XML_CHAR(B)) == 0;
 }
 #endif
@@ -56,72 +60,485 @@
   return nullptr;
 }
 
-const unsigned char *getAttribute(XMLNodeImpl Node,
-                                  const unsigned char *AttributeName) {
+XMLAttrImpl getAttribute(XMLNodeImpl Node, const unsigned char *AttributeName) {
 #if LLVM_LIBXML2_ENABLED
   for (xmlAttrPtr Attribute = Node->properties; Attribute != nullptr;
        Attribute = Attribute->next) {
     if (xmlStringsEqual(Attribute->name, AttributeName))
-      return Attribute->children->content;
+      return Attribute;
+  }
+#endif
+  return nullptr;
+}
+
+// Check if namespace specified by HRef1 overrides that of HRef2.
+bool namespaceOverrides(const unsigned char *HRef1,
+                        const unsigned char *HRef2) {
+  auto HRef1Position = std::find_if(
+      MtNsHrefsPrefixes.begin(), MtNsHrefsPrefixes.end(),
+      [HRef1](const std::pair<std::string, std::string> &element) {
+        return xmlStringsEqual(HRef1, TO_XML_CHAR(element.first.c_str()));
+      });
+  if (HRef1Position == MtNsHrefsPrefixes.end())
+    return false;
+  auto HRef2Position = std::find_if(
+      MtNsHrefsPrefixes.begin(), MtNsHrefsPrefixes.end(),
+      [HRef2](const std::pair<std::string, std::string> &element) {
+        return xmlStringsEqual(HRef2, TO_XML_CHAR(element.first.c_str()));
+      });
+  if (HRef2Position == MtNsHrefsPrefixes.end())
+    return true;
+  int HRef1Priority = std::distance(HRef1Position, MtNsHrefsPrefixes.end());
+  int HRef2Priority = std::distance(HRef2Position, MtNsHrefsPrefixes.end());
+  if (HRef1Priority > HRef2Priority)
+    return true;
+  return false;
+}
+
+// Search for prefix-defined namespace specified by HRef, starting on Node and
+// continuing recurisvely upwards.  Returns the namespace or nullptr if not
+// found.
+XMLNsImpl search(const unsigned char *HRef, XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  for (XMLNsImpl Def = Node->nsDef; Def; Def = Def->next) {
+    if (Def->prefix && xmlStringsEqual(Def->href, HRef))
+      return Def;
+  }
+  if (Node->parent) {
+    return search(HRef, Node->parent);
+  }
+#endif
+  return nullptr;
+}
+
+// Return the prefix that corresponds to the HRef.  If HRef is not a recognized
+// URI, then just return the HRef itself to use as the prefix.
+const unsigned char *getPrefixForHref(const unsigned char *HRef) {
+  for (auto &Ns : MtNsHrefsPrefixes) {
+    if (xmlStringsEqual(HRef, TO_XML_CHAR(Ns.first.c_str()))) {
+      return TO_XML_CHAR(Ns.second.c_str());
+    }
   }
+  return HRef;
+}
+
+// Search for prefix-defined namespace specified by HRef, starting on Node and
+// continuing recurisvely upwards.  If it is found, then return it.  If it is
+// not found, then prefix-define that namespace on the node and return a
+// reference to it.
+Expected<XMLNsImpl> searchOrDefine(const unsigned char *HRef,
+                                   XMLNodeImpl Node) {
+  XMLNsImpl Def = search(HRef, Node);
+  if (Def)
+    return Def;
+  Def = xmlNewNs(Node, HRef, getPrefixForHref(HRef));
+  if (!Def)
+    return make_error<WindowsManifestError>("failed to create new namespace");
+  return Def;
+}
+
+// Set the namespace of OrigionalAttribute on OriginalNode to be that of
+// AdditionalAttribute's.
+Error copyAttributeNamespace(XMLAttrImpl OriginalAttribute,
+                             XMLNodeImpl OriginalNode,
+                             XMLAttrImpl AdditionalAttribute) {
+#if LLVM_LIBXML2_ENABLED
+  Expected<XMLNsImpl> ExplicitOrError =
+      searchOrDefine(AdditionalAttribute->ns->href, OriginalNode);
+  if (!ExplicitOrError)
+    return ExplicitOrError.takeError();
+  OriginalAttribute->ns = std::move(ExplicitOrError.get());
 #endif
+  return Error::success();
+}
+
+// Return the corresponding namespace definition for the prefix, defined on the
+// given Node.  Returns nullptr if there is no such definition.
+XMLNsImpl getNamespaceWithPrefix(const unsigned char *Prefix,
+                                 XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  if (Node == nullptr)
+    return nullptr;
+  for (XMLNsImpl Def = Node->nsDef; Def; Def = Def->next) {
+    if (xmlStringsEqual(Def->prefix, Prefix))
+      return Def;
+  }
+#endif
+  return nullptr;
+}
+
+// Search for the closest inheritable default namespace, starting on (and
+// including) the Node and traveling upwards through parent nodes.  Returns
+// nullptr if there are no inheritable default namespaces.
+XMLNsImpl getClosestDefault(XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  if (XMLNsImpl Ret = getNamespaceWithPrefix(nullptr, Node))
+    return Ret;
+  if (Node->parent == nullptr)
+    return nullptr;
+  return getClosestDefault(Node->parent);
+#else
   return nullptr;
+#endif
 }
 
+// Merge the attributes of AdditionalNode into OriginalNode.  If attributes
+// with identical types are present, they are not duplicated but rather if
+// their values are not consistent and error is thrown.  In addition, the
+// higher priority namespace is used for each attribute, EXCEPT in the case
+// of merging two default namespaces and the lower priority namespace
+// definition occurs closer than the higher priority one.
 Error mergeAttributes(XMLNodeImpl OriginalNode, XMLNodeImpl AdditionalNode) {
 #if LLVM_LIBXML2_ENABLED
-  for (xmlAttrPtr Attribute = AdditionalNode->properties; Attribute != nullptr;
+  XMLNsImpl ClosestDefault = getClosestDefault(OriginalNode);
+  for (xmlAttrPtr Attribute = AdditionalNode->properties; Attribute;
        Attribute = Attribute->next) {
-    if (const unsigned char *OriginalValue =
+    if (xmlAttrPtr OriginalAttribute =
             getAttribute(OriginalNode, Attribute->name)) {
-      // Attributes of the same name must also have the same value.  Otherwise
-      // an error is thrown.
-      if (!xmlStringsEqual(OriginalValue, Attribute->children->content))
+      if (!xmlStringsEqual(OriginalAttribute->children->content,
+                           Attribute->children->content))
         return make_error<WindowsManifestError>(
             Twine("conflicting attributes for ") +
             FROM_XML_CHAR(OriginalNode->name));
+      if (!Attribute->ns) {
+        continue;
+      }
+      if (!OriginalAttribute->ns) {
+        if (auto E = copyAttributeNamespace(OriginalAttribute, OriginalNode,
+                                            Attribute))
+          return E;
+        // In this case, the original attribute has a higher priority namespace
+        // than the incomiing attribute, however the namespace definition of
+        // the lower priority namespace occurs first traveling upwards in the
+        // tree.  Therefore the lower priority namespace is applied.
+      } else if (namespaceOverrides(OriginalAttribute->ns->href,
+                                    Attribute->ns->href)) {
+        if (!OriginalAttribute->ns->prefix && !Attribute->ns->prefix &&
+            ClosestDefault &&
+            xmlStringsEqual(Attribute->ns->href, ClosestDefault->href)) {
+          if (auto E = copyAttributeNamespace(OriginalAttribute, OriginalNode,
+                                              Attribute))
+            return E;
+        }
+        // This covers the case where the incoming attribute has the higher
+        // priority.  The higher priority namespace is applied in all cases
+        // EXCEPT when both of the namespaces are default inherited, and the
+        // closest inherited default is the lower priority one.
+      } else if (Attribute->ns->prefix || OriginalAttribute->ns->prefix ||
+                 (ClosestDefault &&
+                  !xmlStringsEqual(OriginalAttribute->ns->href,
+                                   ClosestDefault->href))) {
+        if (auto E = copyAttributeNamespace(OriginalAttribute, OriginalNode,
+                                            Attribute))
+          return E;
+      }
+      // If the incoming attribute is not already found on the node, append it
+      // to the end of the properties list.  Also explicitly apply its
+      // namespace as a prefix because it might be contained in a separate
+      // namespace that doesn't use the attribute.
     } else {
-      char *NameCopy = strdup(FROM_XML_CHAR(Attribute->name));
-      char *ContentCopy = strdup(FROM_XML_CHAR(Attribute->children->content));
-      xmlNewProp(OriginalNode, TO_XML_CHAR(NameCopy), TO_XML_CHAR(ContentCopy));
+      XMLAttrImpl NewProp = xmlNewProp(OriginalNode, Attribute->name,
+                                       Attribute->children->content);
+      Expected<XMLNsImpl> ExplicitOrError =
+          searchOrDefine(Attribute->ns->href, OriginalNode);
+      if (!ExplicitOrError)
+        return ExplicitOrError.takeError();
+      NewProp->ns = std::move(ExplicitOrError.get());
     }
   }
 #endif
   return Error::success();
 }
 
+// Given two nodes, return the one with the higher priority namespace.
+XMLNodeImpl getDominantNode(XMLNodeImpl Node1, XMLNodeImpl Node2) {
+#if LLVM_LIBXML2_ENABLED
+  if (!Node1 || !Node1->ns)
+    return Node2;
+  else if (!Node2 || !Node2->ns)
+    return Node1;
+  else if (namespaceOverrides(Node1->ns->href, Node2->ns->href))
+    return Node1;
+  else
+    return Node2;
+#else
+  return nullptr;
+#endif
+}
+
+// Checks if this Node's namespace is inherited or one it defined itself.
+bool hasInheritedNs(XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  if (Node->ns && Node->ns != getNamespaceWithPrefix(Node->ns->prefix, Node))
+    return true;
+  else
+#endif
+    return false;
+}
+
+// Check if this Node's namespace is a default namespace that it inherited, as
+// opposed to defining itself.
+bool hasInheritedDefaultNs(XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  if (hasInheritedNs(Node) && Node->ns->prefix == nullptr)
+    return true;
+  else
+#endif
+    return false;
+}
+
+// Check if this Node's namespace is a default namespace it defined itself.
+bool hasDefinedDefaultNamespace(XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  if (Node->ns && (Node->ns == getNamespaceWithPrefix(nullptr, Node)))
+    return true;
+  else
+#endif
+    return false;
+}
+
+// For the given explicit prefix-definition of a namespace, travel downwards
+// from a node recursively, and for every implicit, inherited default usage of
+// that namespace replace it with that explicit prefix use.  This is important
+// when namespace overriding occurs when merging, so that elements unique to a
+// namespace will still stay in that namespace.
+void explicateNamespace(XMLNsImpl PrefixDef, XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  // If a node as its own default namespace definition it clearly cannot have
+  // inherited the given default namespace, and neither will any of its
+  // children.
+  if (hasDefinedDefaultNamespace(Node))
+    return;
+  if (Node->ns && xmlStringsEqual(Node->ns->href, PrefixDef->href) &&
+      hasInheritedDefaultNs(Node))
+    Node->ns = PrefixDef;
+  for (xmlAttrPtr Attribute = Node->properties; Attribute;
+       Attribute = Attribute->next) {
+    if (Attribute->ns &&
+        xmlStringsEqual(Attribute->ns->href, PrefixDef->href)) {
+      Attribute->ns = PrefixDef;
+    }
+  }
+  for (XMLNodeImpl Child = Node->children; Child; Child = Child->next) {
+    explicateNamespace(PrefixDef, Child);
+  }
+#endif
+}
+
+// Perform the namespace merge between two nodes.
+Error mergeNamespaces(XMLNodeImpl OriginalNode, XMLNodeImpl AdditionalNode) {
+#if LLVM_LIBXML2_ENABLED
+  // Save the original default namespace definition in case the incoming node
+  // overrides it.
+  const unsigned char *OriginalDefinedDefaultHref = nullptr;
+  if (XMLNsImpl OriginalDefinedDefaultNs =
+          getNamespaceWithPrefix(nullptr, OriginalNode)) {
+    OriginalDefinedDefaultHref = xmlStrdup(OriginalDefinedDefaultNs->href);
+  }
+  const unsigned char *NewDefinedDefaultHref = nullptr;
+  // Copy all namespace definitions.  There can only be one default namespace
+  // definition per node, so the higher priority one takes precedence in the
+  // case of collision.
+  for (XMLNsImpl Def = AdditionalNode->nsDef; Def; Def = Def->next) {
+    if (XMLNsImpl OriginalNsDef =
+            getNamespaceWithPrefix(Def->prefix, OriginalNode)) {
+      if (!Def->prefix) {
+        if (namespaceOverrides(Def->href, OriginalNsDef->href)) {
+          NewDefinedDefaultHref = TO_XML_CHAR(strdup(FROM_XML_CHAR(Def->href)));
+        }
+      } else if (!xmlStringsEqual(OriginalNsDef->href, Def->href)) {
+        return make_error<WindowsManifestError>(
+            Twine("conflicting namespace definitions for ") +
+            FROM_XML_CHAR(Def->prefix));
+      }
+    } else {
+      XMLNsImpl NewDef = xmlCopyNamespace(Def);
+      NewDef->next = OriginalNode->nsDef;
+      OriginalNode->nsDef = NewDef;
+    }
+  }
+
+  // Check whether the original node or the incoming node has the higher
+  // priority namespace.  Depending on which one is dominant, we will have
+  // to recursively apply namespace changes down to children of the original
+  // node.
+  XMLNodeImpl DominantNode = getDominantNode(OriginalNode, AdditionalNode);
+  XMLNodeImpl NonDominantNode =
+      DominantNode == OriginalNode ? AdditionalNode : OriginalNode;
+  if (DominantNode == OriginalNode) {
+    if (OriginalDefinedDefaultHref) {
+      XMLNsImpl NonDominantDefinedDefault =
+          getNamespaceWithPrefix(nullptr, NonDominantNode);
+      // In this case, both the nodes defined a default namespace.  However
+      // the lower priority node ended up having a higher priority default
+      // definition.  This can occur if the higher priority node is prefix
+      // namespace defined.  In this case we have to define an explicit
+      // prefix for the overridden definition and apply it to all children
+      // who relied on that definition.
+      if (NonDominantDefinedDefault &&
+          namespaceOverrides(NonDominantDefinedDefault->href,
+                             OriginalDefinedDefaultHref)) {
+        Expected<XMLNsImpl> EC =
+            searchOrDefine(OriginalDefinedDefaultHref, DominantNode);
+        if (!EC)
+          return EC.takeError();
+        XMLNsImpl PrefixDominantDefinedDefault = std::move(EC.get());
+        explicateNamespace(PrefixDominantDefinedDefault, DominantNode);
+      }
+      // In this case the node with a higher priority namespace did not have a
+      // default namespace definition, but the lower priority node did.  In this
+      // case the new default namespace definition is copied.  A side effect of
+      // this is that all children will suddenly find themselves in a different
+      // default namespace.  To maintain correctness we need to ensure that all
+      // children now explicitly refer to the namespace that they had previously
+      // implicitly inherited.
+    } else if (getNamespaceWithPrefix(nullptr, NonDominantNode)) {
+      if (DominantNode->parent) {
+        XMLNsImpl ClosestDefault = getClosestDefault(DominantNode->parent);
+        Expected<XMLNsImpl> EC =
+            searchOrDefine(ClosestDefault->href, DominantNode);
+        if (!EC)
+          return EC.takeError();
+        XMLNsImpl ExplicitDefault = std::move(EC.get());
+        explicateNamespace(ExplicitDefault, DominantNode);
+      }
+    }
+  } else {
+    // Covers case where the incoming node has a default namespace definition
+    // that overrides the original node's namespace.  This always leads to
+    // the original node receiving that new default namespace.
+    if (hasDefinedDefaultNamespace(DominantNode)) {
+      NonDominantNode->ns = getNamespaceWithPrefix(nullptr, NonDominantNode);
+    } else {
+      // This covers the case where the incoming node either has a prefix
+      // namespace, or an inherited default namespace.  Since the namespace
+      // may not yet be defined in the original tree we do a searchOrDefine
+      // for it, and then set the namespace equal to it.
+      Expected<XMLNsImpl> EC =
+          searchOrDefine(DominantNode->ns->href, NonDominantNode);
+      if (!EC)
+        return EC.takeError();
+      XMLNsImpl Explicit = std::move(EC.get());
+      NonDominantNode->ns = Explicit;
+    }
+    // This covers cases where the incoming dominant node HAS a default
+    // namespace definition, but MIGHT NOT NECESSARILY be in that namespace.
+    if (XMLNsImpl DominantDefaultDefined =
+            getNamespaceWithPrefix(nullptr, DominantNode)) {
+      if (OriginalDefinedDefaultHref) {
+        if (namespaceOverrides(DominantDefaultDefined->href,
+                               OriginalDefinedDefaultHref)) {
+          // In this case, the incoming node's default definition overrides
+          // the original default definition, all children who relied on that
+          // definition must be updated accordingly.
+          Expected<XMLNsImpl> EC =
+              searchOrDefine(OriginalDefinedDefaultHref, NonDominantNode);
+          if (!EC)
+            return EC.takeError();
+          XMLNsImpl ExplicitDefault = std::move(EC.get());
+          explicateNamespace(ExplicitDefault, NonDominantNode);
+        }
+      } else {
+        // The original did not define a default definition, however the new
+        // default definition still applies to all children, so they must be
+        // updated to explicitly refer to the namespace they had previously
+        // been inheriting implicitly.
+        XMLNsImpl ClosestDefault = getClosestDefault(NonDominantNode);
+        Expected<XMLNsImpl> EC =
+            searchOrDefine(ClosestDefault->href, NonDominantNode);
+        if (!EC)
+          return EC.takeError();
+        XMLNsImpl ExplicitDefault = std::move(EC.get());
+        explicateNamespace(ExplicitDefault, NonDominantNode);
+      }
+    }
+  }
+  if (NewDefinedDefaultHref) {
+    XMLNsImpl OriginalNsDef = getNamespaceWithPrefix(nullptr, OriginalNode);
+    xmlFree(const_cast<unsigned char *>(OriginalNsDef->href));
+    OriginalNsDef->href = NewDefinedDefaultHref;
+  }
+  xmlFree(const_cast<unsigned char *>(OriginalDefinedDefaultHref));
+#endif
+  return Error::success();
+}
+
+bool isRecognizedNamespace(const unsigned char *NsHref) {
+  for (auto &Ns : MtNsHrefsPrefixes) {
+    if (xmlStringsEqual(NsHref, TO_XML_CHAR(Ns.first.c_str())))
+      return true;
+  }
+  return false;
+}
+
+bool hasRecognizedNamespace(XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  return isRecognizedNamespace(Node->ns->href);
+#else
+  return false;
+#endif
+}
+
+// Ensure a node's inherited namespace is actually defined in the tree it
+// resides in.
+Error reconcileNamespaces(XMLNodeImpl Node) {
+#if LLVM_LIBXML2_ENABLED
+  if (Node == nullptr)
+    return Error::success();
+  if (hasInheritedNs(Node)) {
+    Expected<XMLNsImpl> ExplicitOrError = searchOrDefine(Node->ns->href, Node);
+    if (!ExplicitOrError)
+      return ExplicitOrError.takeError();
+    XMLNsImpl Explicit = std::move(ExplicitOrError.get());
+    Node->ns = Explicit;
+  }
+  for (XMLNodeImpl Child = Node->children; Child; Child = Child->next) {
+    if (auto E = reconcileNamespaces(Child))
+      return E;
+  }
+#endif
+  return Error::success();
+}
+
+// Recursively merge the two given manifest trees, depending on which elements
+// are of a mergeable type, and choose namespaces according to which have
+// higher priority.
 Error treeMerge(XMLNodeImpl OriginalRoot, XMLNodeImpl AdditionalRoot) {
 #if LLVM_LIBXML2_ENABLED
+  if (auto E = mergeAttributes(OriginalRoot, AdditionalRoot))
+    return E;
+  if (auto E = mergeNamespaces(OriginalRoot, AdditionalRoot))
+    return E;
   XMLNodeImpl AdditionalFirstChild = AdditionalRoot->children;
   xmlNode StoreNext;
   for (XMLNodeImpl Child = AdditionalFirstChild; Child; Child = Child->next) {
     XMLNodeImpl OriginalChildWithName;
     if (!isMergeableElement(Child->name) ||
         !(OriginalChildWithName =
-              getChildWithName(OriginalRoot, Child->name))) {
+              getChildWithName(OriginalRoot, Child->name)) ||
+        !hasRecognizedNamespace(Child)) {
       StoreNext.next = Child->next;
       xmlUnlinkNode(Child);
       if (!xmlAddChild(OriginalRoot, Child))
         return make_error<WindowsManifestError>(Twine("could not merge ") +
                                                 FROM_XML_CHAR(Child->name));
+      if (auto E = reconcileNamespaces(Child))
+        return E;
       Child = &StoreNext;
     } else if (auto E = treeMerge(OriginalChildWithName, Child)) {
       return E;
     }
   }
-  if (auto E = mergeAttributes(OriginalRoot, AdditionalRoot))
-    return E;
 #endif
   return Error::success();
 }
 
-void stripCommentsAndText(XMLNodeImpl Root) {
+void stripComments(XMLNodeImpl Root) {
 #if LLVM_LIBXML2_ENABLED
   xmlNode StoreNext;
   for (XMLNodeImpl Child = Root->children; Child; Child = Child->next) {
-    if (!xmlStringsEqual(Child->name, TO_XML_CHAR("text")) &&
-        !xmlStringsEqual(Child->name, TO_XML_CHAR("comment"))) {
-      stripCommentsAndText(Child);
+    if (!xmlStringsEqual(Child->name, TO_XML_CHAR("comment"))) {
+      stripComments(Child);
     } else {
       StoreNext.next = Child->next;
       XMLNodeImpl Remove = Child;
@@ -140,6 +557,22 @@
 #endif
 }
 
+// The libxml2 assumes that attributes do not inherit default namespaces,
+// whereas the original  mt.exe does make this assumption.  This function
+// reconciles this by setting all attributes to have the inherited default
+// namespace.
+void setAttributeNamespaces(XMLNodeImpl Node) {
+  for (XMLAttrImpl Attribute = Node->properties; Attribute;
+       Attribute = Attribute->next) {
+    if (!Attribute->ns) {
+      Attribute->ns = getClosestDefault(Node);
+    }
+  }
+  for (XMLNodeImpl Child = Node->children; Child; Child = Child->next) {
+    setAttributeNamespaces(Child);
+  }
+}
+
 Error WindowsManifestMerger::merge(const MemoryBuffer &Manifest) {
 #if LLVM_LIBXML2_ENABLED
   if (Manifest.getBufferSize() == 0)
@@ -153,13 +586,15 @@
   if (auto E = getParseError())
     return E;
   XMLNodeImpl AdditionalRoot = xmlDocGetRootElement(ManifestXML);
-  stripCommentsAndText(AdditionalRoot);
+  stripComments(AdditionalRoot);
+  setAttributeNamespaces(AdditionalRoot);
   if (CombinedDoc == nullptr) {
     CombinedDoc = ManifestXML;
   } else {
     XMLNodeImpl CombinedRoot = xmlDocGetRootElement(CombinedDoc);
     if (xmlStringsEqual(CombinedRoot->name, AdditionalRoot->name) &&
-        isMergeableElement(AdditionalRoot->name)) {
+        isMergeableElement(AdditionalRoot->name) &&
+        hasRecognizedNamespace(AdditionalRoot)) {
       if (auto E = treeMerge(CombinedRoot, AdditionalRoot)) {
         return E;
       }
@@ -172,15 +607,70 @@
   return Error::success();
 }
 
+// The merging process may create too many prefix defined namespaces.  This
+// function removes all unnecessary ones from the tree.
+void checkAndStripPrefixes(XMLNodeImpl Node,
+                           std::vector<XMLNsImpl> &RequiredPrefixes) {
+#if LLVM_LIBXML2_ENABLED
+  for (XMLNodeImpl Child = Node->children; Child; Child = Child->next) {
+    checkAndStripPrefixes(Child, RequiredPrefixes);
+  }
+  if (Node->ns && Node->ns->prefix != nullptr) {
+    XMLNsImpl ClosestDefault = getClosestDefault(Node);
+    if (ClosestDefault &&
+        xmlStringsEqual(ClosestDefault->href, Node->ns->href)) {
+      Node->ns = ClosestDefault;
+    } else if (std::find(RequiredPrefixes.begin(), RequiredPrefixes.end(),
+                         Node->ns) == RequiredPrefixes.end()) {
+      RequiredPrefixes.push_back(Node->ns);
+    }
+  }
+  for (XMLAttrImpl Attribute = Node->properties; Attribute;
+       Attribute = Attribute->next) {
+    if (Attribute->ns && Attribute->ns->prefix != nullptr) {
+      XMLNsImpl ClosestDefault = getClosestDefault(Node);
+      if (ClosestDefault &&
+          xmlStringsEqual(ClosestDefault->href, Attribute->ns->href)) {
+        Attribute->ns = ClosestDefault;
+      } else if (std::find(RequiredPrefixes.begin(), RequiredPrefixes.end(),
+                           Node->ns) == RequiredPrefixes.end()) {
+        RequiredPrefixes.push_back(Attribute->ns);
+      }
+    }
+  }
+  XMLNsImpl Prev;
+  xmlNs Temp;
+  for (XMLNsImpl Def = Node->nsDef; Def; Def = Def->next) {
+    if (Def->prefix &&
+        (std::find(RequiredPrefixes.begin(), RequiredPrefixes.end(), Def) ==
+         RequiredPrefixes.end())) {
+      if (Def == Node->nsDef)
+        Node->nsDef = Def->next;
+      else
+        Prev->next = Def->next;
+      Temp.next = Def->next;
+      xmlFreeNs(Def);
+      Def = &Temp;
+    } else {
+      Prev = Def;
+    }
+  }
+#endif
+}
+
 std::unique_ptr<MemoryBuffer> WindowsManifestMerger::getMergedManifest() {
 #if LLVM_LIBXML2_ENABLED
   unsigned char *XmlBuff;
   int BufferSize = 0;
   if (CombinedDoc) {
+    XMLNodeImpl CombinedRoot = xmlDocGetRootElement(CombinedDoc);
+    std::vector<XMLNsImpl> RequiredPrefixes;
+    checkAndStripPrefixes(CombinedRoot, RequiredPrefixes);
     std::unique_ptr<xmlDoc> OutputDoc(xmlNewDoc((const unsigned char *)"1.0"));
-    xmlDocSetRootElement(OutputDoc.get(), xmlDocGetRootElement(CombinedDoc));
+    xmlDocSetRootElement(OutputDoc.get(), CombinedRoot);
     xmlKeepBlanksDefault(0);
-    xmlDocDumpFormatMemory(OutputDoc.get(), &XmlBuff, &BufferSize, 1);
+    xmlDocDumpFormatMemoryEnc(OutputDoc.get(), &XmlBuff, &BufferSize, "UTF-8",
+                              1);
   }
   if (BufferSize == 0)
     return nullptr;
@@ -201,5 +691,3 @@
     return Error::success();
   return make_error<WindowsManifestError>("invalid xml document");
 }
-
-} // namespace llvm
Index: llvm/test/tools/llvm-mt/Inputs/assembly_identity.manifest
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/Inputs/assembly_identity.manifest
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<assembly xmlns="urn:schemas-microsoft-com:asm.v2" manifestVersion="1.0">
+  <dependency>
+    <dependentAssembly>
+      <assemblyIdentity foo="bar" name="identity1" version="1.2.3.4"></assemblyIdentity>
+    </dependentAssembly>
+  </dependency>
+</assembly>
Index: llvm/test/tools/llvm-mt/Inputs/compatibility.manifest
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/Inputs/compatibility.manifest
@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<assembly manifestVersion="1.0" xmlns="urn:schemas-microsoft-com:asm.v1">
+  <application>
+    <compatibility xmlns="urn:schemas-microsoft-com:compatibility.v1">
+      <supportedOS Id="FooOS"/>
+      <supportedOS Id="BarOS"/>
+    </compatibility>
+  </application>
+</assembly>
Index: llvm/test/tools/llvm-mt/Inputs/test_manifest.manifest
===================================================================
--- llvm/test/tools/llvm-mt/Inputs/test_manifest.manifest
+++ llvm/test/tools/llvm-mt/Inputs/test_manifest.manifest
@@ -1,4 +1,4 @@
-<?xml version="1.0"?>
+<?xml version="1.0" encoding="UTF-8"?>
 <assembly xmlns="urn:schemas-microsoft-com:asm.v1">
   <trustInfo>
     <security>
Index: llvm/test/tools/llvm-mt/Inputs/trust_and_identity.manifest
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/Inputs/trust_and_identity.manifest
@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+
+  <!--random comment -->
+
+  <dependency>
+    <dependentAssembly>
+      <assemblyIdentity bar="foo" name="my assembly" version="5"></assemblyIdentity>
+    </dependentAssembly>
+  </dependency>
+
+  <trustInfo>
+    <security>
+      <requestedPrivileges>
+        <requestedExecutionLevel level="trust1" access="false"/>
+      </requestedPrivileges>
+    </security>
+  </trustInfo>
+
+</assembly>
\ No newline at end of file
Index: llvm/test/tools/llvm-mt/Inputs/trust_info.manifest
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/Inputs/trust_info.manifest
@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+<trustInfo xmlns="urn:schemas-microsoft-com:asm.v3">
+  <security>
+    <requestedPrivileges>
+      <requestedExecutionLevel level="trust1"></requestedExecutionLevel>
+    </requestedPrivileges>
+  </security>
+</trustInfo></assembly>
Index: llvm/test/tools/llvm-mt/Inputs/windows_settings.manifest
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/Inputs/windows_settings.manifest
@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
+<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+ <asmv3:application xmlns:asmv3="urn:schemas-microsoft-com:asm.v3">
+    <asmv3:windowsSettings xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">
+      <dpiAware>true/pm</dpiAware>
+    </asmv3:windowsSettings>
+  </asmv3:application>
+</assembly>
Index: llvm/test/tools/llvm-mt/big_merge.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/big_merge.test
@@ -0,0 +1,39 @@
+REQUIRES: libxml2
+UNSUPPORTED: windows
+
+RUN: llvm-mt /manifest %p/Inputs/trust_info.manifest /manifest \
+RUN:   %p/Inputs/assembly_identity.manifest /manifest \
+RUN:   %p/Inputs/trust_and_identity.manifest /manifest \
+RUN:   %p/Inputs/compatibility.manifest /manifest \
+RUN:   %p/Inputs/windows_settings.manifest /out:%t
+RUN: FileCheck %s -input-file=%t
+
+CHECK:      <?xml version="1.0" encoding="UTF-8"?>
+CHECK-NEXT: <assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+CHECK-NEXT: <ms_asmv1:trustInfo xmlns="urn:schemas-microsoft-com:asm.v3" xmlns:ms_asmv1="urn:schemas-microsoft-com:asm.v1">
+CHECK-NEXT:   <ms_asmv1:security>
+CHECK-NEXT:     <ms_asmv1:requestedPrivileges>
+CHECK-NEXT:       <ms_asmv1:requestedExecutionLevel level="trust1" ms_asmv1:access="false"/>
+CHECK-NEXT:     </ms_asmv1:requestedPrivileges>
+CHECK-NEXT:   </ms_asmv1:security>
+CHECK-NEXT: </ms_asmv1:trustInfo>
+CHECK-NEXT: <ms_asmv2:dependency xmlns:ms_asmv2="urn:schemas-microsoft-com:asm.v2">
+CHECK-NEXT:   <ms_asmv2:dependentAssembly>
+CHECK-NEXT:     <ms_asmv2:assemblyIdentity foo="bar" name="identity1" version="1.2.3.4"/>
+CHECK-NEXT:   </ms_asmv2:dependentAssembly>
+CHECK-NEXT: </ms_asmv2:dependency>
+CHECK-NEXT: <dependency>
+CHECK-NEXT:   <dependentAssembly>
+CHECK-NEXT:     <assemblyIdentity bar="foo" name="my assembly" version="5"/>
+CHECK-NEXT:   </dependentAssembly>
+CHECK-NEXT: </dependency>
+CHECK-NEXT: <application xmlns:asmv3="urn:schemas-microsoft-com:asm.v3">
+CHECK-NEXT:   <compatibility xmlns="urn:schemas-microsoft-com:compatibility.v1">
+CHECK-NEXT:     <supportedOS Id="FooOS"/>
+CHECK-NEXT:     <supportedOS Id="BarOS"/>
+CHECK-NEXT:   </compatibility>
+CHECK-NEXT:   <asmv3:windowsSettings xmlns="http://schemas.microsoft.com/SMI/2005/WindowsSettings">
+CHECK-NEXT:     <dpiAware>true/pm</dpiAware>
+CHECK-NEXT:   </asmv3:windowsSettings>
+CHECK-NEXT: </application>
+CHECK-NEXT: </assembly>
Index: llvm/test/tools/llvm-mt/simple_merge.test
===================================================================
--- llvm/test/tools/llvm-mt/simple_merge.test
+++ llvm/test/tools/llvm-mt/simple_merge.test
@@ -5,7 +5,7 @@
 RUN:   %p/Inputs/additional.manifest /out:%t
 RUN: FileCheck %s -input-file=%t
 
-CHECK:      <?xml version="1.0"?>
+CHECK:      <?xml version="1.0" encoding="UTF-8"?>
 CHECK-NEXT: <assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
 CHECK-NEXT:   <trustInfo>
 CHECK-NEXT:     <security>
Index: llvm/test/tools/llvm-mt/single_file.test
===================================================================
--- llvm/test/tools/llvm-mt/single_file.test
+++ llvm/test/tools/llvm-mt/single_file.test
@@ -4,7 +4,7 @@
 RUN: llvm-mt /manifest %p/Inputs/test_manifest.manifest /out:%t
 RUN: FileCheck %s --input-file=%t
 
-CHECK:      <?xml version="1.0"?>
+CHECK:      <?xml version="1.0" encoding="UTF-8"?>
 CHECK-NEXT: <assembly xmlns="urn:schemas-microsoft-com:asm.v1">
 CHECK-NEXT:   <trustInfo>
 CHECK-NEXT:     <security>
Index: llvm/tools/llvm-mt/Opts.td
===================================================================
--- llvm/tools/llvm-mt/Opts.td
+++ llvm/tools/llvm-mt/Opts.td
@@ -1,7 +1,8 @@
 include "llvm/Option/OptParser.td"
 
+def supported : OptionGroup<"supported">;
 def unsupported : OptionGroup<"unsupported">;
-def manifest : Separate<["/", "-"], "manifest">, HelpText<"Used to specify each manifest that need to be processed">, MetaVarName<"manifest">;
+def manifest : Separate<["/", "-"], "manifest">, HelpText<"Used to specify each manifest that need to be processed">, MetaVarName<"manifest">, Group<supported>;
 def identity : Joined<["/", "-"], "identity:">, HelpText<"Not supported">, MetaVarName<"identity">, Group<unsupported>;
 def rgs : Joined<["/", "-"], "rgs:">, HelpText<"Not supported">, MetaVarName<"script">, Group<unsupported>;
 def tlb : Joined<["/", "-"], "tlb:">, HelpText<"Not supported">, MetaVarName<"file">, Group<unsupported>;
@@ -10,8 +11,8 @@
 def managed_assembly_name : Joined<["/", "-"], "managedassemblyname:">, HelpText<"Not supported">, MetaVarName<"assembly">, Group<unsupported>;
 def no_dependency : Flag<["/", "-"], "nodependency">, HelpText<"Not supported">, Group<unsupported>;
 def category : Flag<["/", "-"], "category">, HelpText<"Not supported">, Group<unsupported>;
-def no_logo : Flag<["/", "-"], "nologo">, HelpText<"No effect as this tool never writes copyright data.  Included for parity">;
-def out : Joined<["/", "-"], "out:">, HelpText<"Name of the output manifest.  If this is skipped and only one manifest is being operated upon by the tool, that manifest is modified in place">, MetaVarName<"manifest">;
+def no_logo : Flag<["/", "-"], "nologo">, HelpText<"No effect as this tool never writes copyright data.  Included for parity">, Group<supported>;
+def out : Joined<["/", "-"], "out:">, HelpText<"Name of the output manifest.  If this is skipped and only one manifest is being operated upon by the tool, that manifest is modified in place">, MetaVarName<"manifest">, Group<supported>;
 def input_resource : Joined<["/", "-"], "inputresource:">, HelpText<"Not supported">, MetaVarName<"file">, Group<unsupported>;
 def output_resource : Joined<["/", "-"], "outputresource:">, HelpText<"Not supported">, MetaVarName<"file">, Group<unsupported>;
 def output_resource_flag : Flag<["/", "-"], "outputresource">, Alias<output_resource>, HelpText<"Not supported">, Group<unsupported>;
@@ -24,6 +25,6 @@
 def check_for_duplicates : Flag<["/", "-"], "check_for_duplicates:">, HelpText<"Not supported">, Group<unsupported>;
 def make_cdfs : Flag<["/", "-"], "makecdfs:">, HelpText<"Not supported">, Group<unsupported>;
 def verbose : Flag<["/", "-"], "verbose">, HelpText<"Not supported">, Group<unsupported>;
-def help : Flag<["/", "-"], "?">;
-def help_long : Flag<["/", "-"], "help">, Alias<help>;
-def h : Flag<["/", "-"], "h">, Alias<help>;
+def help : Flag<["/", "-"], "?">, Group<supported>;
+def help_long : Flag<["/", "-"], "help">, Alias<help>, Group<supported>;
+def h : Flag<["/", "-"], "h">, Alias<help>, Group<supported>;
Index: llvm/tools/llvm-mt/llvm-mt.cpp
===================================================================
--- llvm/tools/llvm-mt/llvm-mt.cpp
+++ llvm/tools/llvm-mt/llvm-mt.cpp
@@ -102,6 +102,13 @@
   ArrayRef<const char *> ArgsArr = makeArrayRef(argv + 1, argc);
   opt::InputArgList InputArgs = T.ParseArgs(ArgsArr, MAI, MAC);
 
+  for (auto &Arg : InputArgs) {
+    if (!(Arg->getOption().matches(OPT_unsupported) ||
+          Arg->getOption().matches(OPT_supported))) {
+      reportError(Twine("invalid option ") + Arg->getSpelling());
+    }
+  }
+
   for (auto &Arg : InputArgs) {
     if (Arg->getOption().matches(OPT_unsupported)) {
       outs() << "llvm-mt: ignoring unsupported '" << Arg->getOption().getName()
@@ -129,7 +136,7 @@
     reportError("no output file specified");
   }
 
-  WindowsManifestMerger Merger;
+  windows_manifest::WindowsManifestMerger Merger;
 
   for (const auto &File : InputFiles) {
     ErrorOr<std::unique_ptr<MemoryBuffer>> ManifestOrErr =