Index: llvm/include/llvm/Support/WindowsManifestMerger.h
===================================================================
--- llvm/include/llvm/Support/WindowsManifestMerger.h
+++ llvm/include/llvm/Support/WindowsManifestMerger.h
@@ -35,6 +35,15 @@
 
 namespace llvm {
 
+static const char *const MT_Strings[]{
+    "application",         "assembly",  "assemblyIdentity",
+    "compatibility",       "noInherit", "requestedExecutionLevel",
+    "requestedPrivileges", "security",  "trustInfo",
+};
+
+static const std::vector<std::string> MTMergeableElements(MT_Strings,
+                                                          std::end(MT_Strings));
+
 class MemoryBuffer;
 
 #if LLVM_LIBXML2_ENABLED
@@ -49,6 +58,7 @@
 public:
   static char ID;
   WindowsManifestError(const Twine &Msg);
+  WindowsManifestError(const Twine &Prefix, const Twine &Message);
   void log(raw_ostream &OS) const override;
 
 private:
@@ -57,6 +67,10 @@
 
 class WindowsManifestMerger {
 public:
+#if LLVM_LIBXML2_ENABLED
+  ~WindowsManifestMerger();
+#endif
+
   Error merge(const MemoryBuffer &Manifest);
 
   // Returns vector containing merged xml manifest, or uninitialized vector for
@@ -68,7 +82,7 @@
   Error getParseError();
 
 #if LLVM_LIBXML2_ENABLED
-  XMLNodeImpl CombinedRoot = nullptr;
+  XMLDocumentImpl CombinedDoc = nullptr;
 #endif
   bool ParseErrorOccurred = false;
 };
Index: llvm/lib/Support/WindowsManifestMerger.cpp
===================================================================
--- llvm/lib/Support/WindowsManifestMerger.cpp
+++ llvm/lib/Support/WindowsManifestMerger.cpp
@@ -16,24 +16,139 @@
 
 #include <stdarg.h>
 
+using namespace llvm;
+
 namespace llvm {
 
 char WindowsManifestError::ID = 0;
 
 WindowsManifestError::WindowsManifestError(const Twine &Msg) : Msg(Msg.str()) {}
 
+WindowsManifestError::WindowsManifestError(const Twine &Prefix,
+                                           const Twine &Message)
+    : Msg((Prefix.concat(Message)).str()) {}
+
 void WindowsManifestError::log(raw_ostream &OS) const { OS << Msg; }
 
+bool isMergeableElement(const unsigned char *ElementName) {
+  bool result =
+      std::find(MTMergeableElements.begin(), MTMergeableElements.end(),
+                std::string(reinterpret_cast<const char *>(ElementName))) !=
+      MTMergeableElements.end();
+  return result;
+}
+
+XMLNodeImpl hasChildWithName(XMLNodeImpl Parent,
+                             const unsigned char *ElementName) {
+#if LLVM_LIBXML2_ENABLED
+  for (XMLNodeImpl Child = Parent->children; Child; Child = Child->next)
+    if (strcmp(reinterpret_cast<const char *>(Child->name),
+               reinterpret_cast<const char *>(ElementName)) == 0) {
+      return Child;
+    }
+#endif
+  return nullptr;
+}
+
+const unsigned char *hasAttribute(XMLNodeImpl Node,
+                                  const unsigned char *AttributeName) {
+#if LLVM_LIBXML2_ENABLED
+  for (xmlAttrPtr Attribute = Node->properties; Attribute != nullptr;
+       Attribute = Attribute->next) {
+    if (strcmp(reinterpret_cast<const char *>(Attribute->name),
+               reinterpret_cast<const char *>(AttributeName)) == 0)
+      return Attribute->children->content;
+  }
+#endif
+  return nullptr;
+}
+
+Error mergeAttributes(XMLNodeImpl OriginalNode, XMLNodeImpl AdditionalNode) {
+#if LLVM_LIBXML2_ENABLED
+  for (xmlAttrPtr Attribute = AdditionalNode->properties; Attribute != nullptr;
+       Attribute = Attribute->next) {
+    if (const unsigned char *OriginalValue =
+            hasAttribute(OriginalNode, Attribute->name)) {
+      if (strcmp(reinterpret_cast<const char *>(OriginalValue),
+                 reinterpret_cast<const char *>(
+                     Attribute->children->content)) != 0)
+        return make_error<WindowsManifestError>(
+            "conflicting attributes for ",
+            reinterpret_cast<const char *>(OriginalNode->name));
+    } else {
+      char *NameCopy = strdup(reinterpret_cast<const char *>(Attribute->name));
+      char *ContentCopy =
+          strdup(reinterpret_cast<const char *>(Attribute->children->content));
+      xmlNewProp(OriginalNode,
+                 reinterpret_cast<const unsigned char *>(NameCopy),
+                 reinterpret_cast<const unsigned char *>(ContentCopy));
+    }
+  }
+#endif
+  return Error::success();
+}
+
+Error treeMerge(XMLNodeImpl OriginalRoot, XMLNodeImpl AdditionalRoot) {
+#if LLVM_LIBXML2_ENABLED
+  XMLNodeImpl AdditionalFirstChild = AdditionalRoot->children;
+  if (AdditionalFirstChild != nullptr) {
+    for (XMLNodeImpl Child = AdditionalFirstChild; Child; Child = Child->next) {
+      XMLNodeImpl OriginalChildWithName;
+      if ((strcmp(reinterpret_cast<const char *>(Child->name), "text") != 0) &&
+          (strcmp(reinterpret_cast<const char *>(Child->name), "comment") !=
+           0)) {
+        if (!(OriginalChildWithName =
+                  hasChildWithName(OriginalRoot, Child->name)) ||
+            !isMergeableElement(Child->name)) {
+          XMLNodeImpl NewChild = xmlCopyNode(Child, 1);
+          if (NewChild->ns)
+            xmlFreeNs(NewChild->ns); // xmlCopyNode explicitly defines default
+                                     // namespace, undo this here.
+          xmlAddChild(OriginalRoot, NewChild);
+        } else {
+          if (auto E = treeMerge(OriginalChildWithName, Child))
+            return E;
+        }
+      }
+    }
+  }
+  if (auto E = mergeAttributes(OriginalRoot, AdditionalRoot))
+    return E;
+  xmlTextMerge(OriginalRoot, AdditionalRoot);
+#endif
+  return Error::success();
+}
+
+#if LLVM_LIBXML2_ENABLED
+WindowsManifestMerger::~WindowsManifestMerger() { xmlFreeDoc(CombinedDoc); }
+#endif
+
 Error WindowsManifestMerger::merge(const MemoryBuffer &Manifest) {
 #if LLVM_LIBXML2_ENABLED
   xmlSetGenericErrorFunc((void *)this, WindowsManifestMerger::errorCallback);
   XMLDocumentImpl ManifestXML =
       xmlReadMemory(Manifest.getBufferStart(), Manifest.getBufferSize(),
-                    "manifest.xml", nullptr, 0);
+                    "manifest.xml", nullptr, XML_PARSE_NOBLANKS);
   xmlSetGenericErrorFunc(nullptr, nullptr);
   if (auto E = getParseError())
     return E;
-  CombinedRoot = xmlDocGetRootElement(ManifestXML);
+  if (CombinedDoc == nullptr)
+    CombinedDoc = ManifestXML;
+  else {
+    XMLNodeImpl CombinedRoot = xmlDocGetRootElement(CombinedDoc);
+    XMLNodeImpl AdditionalRoot = xmlDocGetRootElement(ManifestXML);
+    if (strcmp(reinterpret_cast<const char *>(CombinedRoot->name),
+               reinterpret_cast<const char *>(AdditionalRoot->name)) == 0 &&
+        isMergeableElement(AdditionalRoot->name)) {
+      if (auto E = treeMerge(CombinedRoot, AdditionalRoot)) {
+        xmlFreeDoc(ManifestXML);
+        return E;
+      }
+    } else {
+      XMLNodeImpl NewChild = xmlCopyNode(AdditionalRoot, 1);
+      xmlAddChild(CombinedRoot, NewChild);
+    }
+  }
 #endif
   return Error::success();
 }
@@ -42,10 +157,13 @@
 #if LLVM_LIBXML2_ENABLED
   unsigned char *XmlBuff;
   int BufferSize = 0;
-  if (CombinedRoot) {
+  if (CombinedDoc) {
     std::unique_ptr<xmlDoc> OutputDoc(xmlNewDoc((const unsigned char *)"1.0"));
-    xmlDocSetRootElement(OutputDoc.get(), CombinedRoot);
-    xmlDocDumpMemory(OutputDoc.get(), &XmlBuff, &BufferSize);
+    xmlDocSetRootElement(OutputDoc.get(), xmlDocGetRootElement(CombinedDoc));
+    xmlKeepBlanksDefault(0);
+    errs() << "about to dump memory\n";
+    xmlDocDumpFormatMemory(OutputDoc.get(), &XmlBuff, &BufferSize, 1);
+    errs() << "dumped\n";
   }
   if (BufferSize == 0)
     return nullptr;
Index: llvm/test/tools/llvm-mt/Inputs/additional.manifest
===================================================================
--- llvm/test/tools/llvm-mt/Inputs/additional.manifest
+++ llvm/test/tools/llvm-mt/Inputs/additional.manifest
@@ -3,13 +3,21 @@
   <trustInfo>
     <security>
       <requestedPrivileges>
-         <requestedExecutionLevel level="3" uiAccess="1"/>
+         <requestedExecutionLevel level="3" admin="false"/>
       </requestedPrivileges>
     </security>
   </trustInfo>
   <dependency>
     <dependentAssembly>
-      <assemblyIdentity program="displayDriver"/>
+      <assemblyIdentity program="logDisplay"/>
     </dependentAssembly>
   </dependency>
+  <compatibility>
+    <dependency>
+      <assemblyIdentity program="compatibilityCheck2"/>
+    </dependency>
+    <application>
+      <supportedOS Id="BarOS"/>
+    </application>
+  </compatibility>
 </assembly>
Index: llvm/test/tools/llvm-mt/Inputs/conflicting.manifest
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/Inputs/conflicting.manifest
@@ -0,0 +1,10 @@
+<?xml version="1.0"?>
+<assembly xmlns="urn:schemas-microsoft-com:asm.v1">
+  <trustInfo>
+    <security>
+      <requestedPrivileges>
+        <requestedExecutionLevel level="2" uiAccess="1"/>
+      </requestedPrivileges>
+    </security>
+  </trustInfo>
+</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
@@ -1,9 +1,9 @@
 <?xml version="1.0"?>
-<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+<assembly xmlns="urn:schemas-microsoft-com:asm.v1">
   <trustInfo>
     <security>
       <requestedPrivileges>
-         <requestedExecutionLevel level="3" uiAccess="1"/>
+        <requestedExecutionLevel level="3" uiAccess="1"/>
       </requestedPrivileges>
     </security>
   </trustInfo>
@@ -12,4 +12,12 @@
       <assemblyIdentity program="displayDriver"/>
     </dependentAssembly>
   </dependency>
+  <compatibility>
+    <dependency>
+      <assemblyIdentity program="compatibilityCheck1"/>
+    </dependency>
+    <application>
+      <supportedOS Id="FooOS"/>
+    </application>
+  </compatibility>
 </assembly>
Index: llvm/test/tools/llvm-mt/conflicting.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/conflicting.test
@@ -0,0 +1,7 @@
+REQUIRES: libxml2
+UNSUPPORTED: windows
+
+RUN: not llvm-mt /manifest %p/Inputs/test_manifest.manifest /manifest \
+RUN:   %p/Inputs/conflicting.manifest /out:%t 2>&1 >/dev/null | FileCheck %s
+
+CHECK: llvm-mt error: conflicting attributes for requestedExecutionLevel
Index: llvm/test/tools/llvm-mt/simple_merge.test
===================================================================
--- /dev/null
+++ llvm/test/tools/llvm-mt/simple_merge.test
@@ -0,0 +1,39 @@
+REQUIRES: libxml2
+UNSUPPORTED: windows
+
+RUN: llvm-mt /manifest %p/Inputs/test_manifest.manifest /manifest \
+RUN:   %p/Inputs/additional.manifest /out:%t
+RUN: FileCheck %s -input-file=%t
+
+CHECK:      <?xml version="1.0"?>
+CHECK-NEXT: <assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
+CHECK-NEXT: <trustInfo>
+CHECK-NEXT: <security>
+CHECK-NEXT: <requestedPrivileges>
+CHECK-NEXT: <requestedExecutionLevel level="3" uiAccess="1" admin="false"/>
+CHECK-NEXT: </requestedPrivileges>
+CHECK-NEXT: </security>
+CHECK-NEXT: </trustInfo>
+CHECK-NEXT: <dependency>
+CHECK-NEXT: <dependentAssembly>
+CHECK-NEXT: <assemblyIdentity program="displayDriver"/>
+CHECK-NEXT: </dependentAssembly>
+CHECK-NEXT: </dependency>
+CHECK-NEXT: <compatibility>
+CHECK-NEXT: <dependency>
+CHECK-NEXT:	<assemblyIdentity program="compatibilityCheck1"/>
+CHECK-NEXT:	</dependency>
+CHECK-NEXT: <application>
+CHECK-NEXT: <supportedOS Id="FooOS"/>
+CHECK-NEXT: <supportedOS Id="BarOS"/>
+CHECK-NEXT: </application>
+CHECK-NEXT:	<dependency>
+CHECK-NEXT: <assemblyIdentity program="compatibilityCheck2"/>
+CHECK-NEXT: </dependency>
+CHECK-NEXT: </compatibility>
+CHECK-NEXT: <dependency>
+CHECK-NEXT: <dependentAssembly>
+CHECK-NEXT: <assemblyIdentity program="logDisplay"/>
+CHECK-NEXT: </dependentAssembly>
+CHECK-NEXT: </dependency>
+CHECK-NEXT: </assembly>
Index: llvm/tools/llvm-mt/llvm-mt.cpp
===================================================================
--- llvm/tools/llvm-mt/llvm-mt.cpp
+++ llvm/tools/llvm-mt/llvm-mt.cpp
@@ -95,7 +95,6 @@
   SpecificBumpPtrAllocator<char> ArgAllocator;
   ExitOnErr(errorCodeToError(sys::Process::GetArgumentVector(
       argv_buf, makeArrayRef(argv, argc), ArgAllocator)));
-
   llvm_shutdown_obj Y; // Call llvm_shutdown() on exit.
 
   CvtResOptTable T;