Index: include/llvm/Linker/Linker.h
===================================================================
--- include/llvm/Linker/Linker.h
+++ include/llvm/Linker/Linker.h
@@ -68,8 +68,10 @@
   void deleteModule();
 
   /// \brief Link \p Src into the composite. The source is destroyed.
+  /// Passing OverrideSymbols as true will have symbols from Src
+  /// shadow those in the Dest.
   /// Returns true on error.
-  bool linkInModule(Module *Src);
+  bool linkInModule(Module *Src, bool OverrideSymbols = false);
 
   /// \brief Set the composite to the passed-in module.
   void setModule(Module *Dst);
Index: lib/Linker/LinkModules.cpp
===================================================================
--- lib/Linker/LinkModules.cpp
+++ lib/Linker/LinkModules.cpp
@@ -424,12 +424,17 @@
 
   DiagnosticHandlerFunction DiagnosticHandler;
 
+  /// For symbol clashes, prefer those from Src.
+  bool OverrideFromSrc;
+
 public:
   ModuleLinker(Module *dstM, Linker::IdentifiedStructTypeSet &Set, Module *srcM,
-               DiagnosticHandlerFunction DiagnosticHandler)
+               DiagnosticHandlerFunction DiagnosticHandler,
+               bool Override = false)
       : DstM(dstM), SrcM(srcM), TypeMap(Set),
         ValMaterializer(TypeMap, DstM, LazilyLinkGlobalValues),
-        DiagnosticHandler(DiagnosticHandler) {}
+        DiagnosticHandler(DiagnosticHandler),
+        OverrideFromSrc(Override) {}
 
   bool run();
 
@@ -794,6 +799,12 @@
   assert(!Dest.hasExternalWeakLinkage());
   assert(Dest.hasExternalLinkage() && Src.hasExternalLinkage() &&
          "Unexpected linkage type!");
+
+  if (OverrideFromSrc) {
+    LinkFromSrc = true;
+    return false;
+  }
+
   return emitError("Linking globals named '" + Src.getName() +
                    "': symbol multiply defined!");
 }
@@ -1738,9 +1749,9 @@
   Composite = nullptr;
 }
 
-bool Linker::linkInModule(Module *Src) {
+bool Linker::linkInModule(Module *Src, bool OverrideSymbols) {
   ModuleLinker TheLinker(Composite, IdentifiedStructTypes, Src,
-                         DiagnosticHandler);
+                         DiagnosticHandler, OverrideSymbols);
   bool RetCode = TheLinker.run();
   Composite->dropTriviallyDeadConstantArrays();
   return RetCode;
Index: test/tools/llvm-link/Inputs/foo.ll
===================================================================
--- /dev/null
+++ test/tools/llvm-link/Inputs/foo.ll
@@ -0,0 +1,4 @@
+define i32 @foo(i32 %i) {
+entry:
+  ret i32 4
+}
Index: test/tools/llvm-link/hello.ll
===================================================================
--- /dev/null
+++ test/tools/llvm-link/hello.ll
@@ -0,0 +1,18 @@
+; RUN: llvm-link %s -override %p/Inputs/foo.ll -S | FileCheck %s
+
+
+; CHECK-LABEL: define i32 @foo
+; CHECK-NEXT: entry:
+; CHECK-NEXT: ret i32 4
+define i32 @foo(i32 %i) {
+entry:
+  %add = add nsw i32 %i, %i
+  ret i32 %add
+}
+
+; Function Attrs: nounwind ssp uwtable
+define i32 @main(i32 %argc, i8** %argv) {
+entry:
+  %a = call i32 @foo(i32 2)
+  ret i32 %a
+}
Index: test/tools/llvm-link/lit.local.cfg
===================================================================
--- /dev/null
+++ test/tools/llvm-link/lit.local.cfg
@@ -0,0 +1 @@
+config.suffixes = ['.ll']
Index: tools/llvm-link/llvm-link.cpp
===================================================================
--- tools/llvm-link/llvm-link.cpp
+++ tools/llvm-link/llvm-link.cpp
@@ -38,6 +38,11 @@
 InputFilenames(cl::Positional, cl::OneOrMore,
                cl::desc("<input bitcode files>"));
 
+static cl::list<std::string>
+OverridingInputs("override", cl::ZeroOrMore, cl::value_desc("filename"),
+                 cl::desc("input bitcode file which can "
+                          "override previously defined symbol"));
+
 static cl::opt<std::string>
 OutputFilename("o", cl::desc("Override output filename"), cl::init("-"),
                cl::value_desc("filename"));
@@ -119,24 +124,29 @@
   auto Composite = make_unique<Module>("llvm-link", Context);
   Linker L(Composite.get(), diagnosticHandler);
 
-  for (unsigned i = 0; i < InputFilenames.size(); ++i) {
-    std::unique_ptr<Module> M = loadFile(argv[0], InputFilenames[i], Context);
-    if (!M.get()) {
-      errs() << argv[0] << ": error loading file '" <<InputFilenames[i]<< "'\n";
-      return 1;
-    }
+  auto linkFiles = [&](const cl::list<std::string>& Files, bool Override) {
+    for (auto File : Files) {
+      std::unique_ptr<Module> M = loadFile(argv[0], File, Context);
+      if (!M.get()) {
+        errs() << argv[0] << ": error loading file '" << File << "'\n";
+        exit(1);
+      }
 
-    if (verifyModule(*M, &errs())) {
-      errs() << argv[0] << ": " << InputFilenames[i]
-             << ": error: input module is broken!\n";
-      return 1;
-    }
+      if (verifyModule(*M, &errs())) {
+        errs() << argv[0] << ": " << File
+               << ": error: input module is broken!\n";
+        exit(1);
+      }
 
-    if (Verbose) errs() << "Linking in '" << InputFilenames[i] << "'\n";
+      if (Verbose) errs() << "Linking in '" << File << "'\n";
 
-    if (L.linkInModule(M.get()))
-      return 1;
-  }
+      if (L.linkInModule(M.get(), Override))
+        exit(1);
+    }
+  };
+
+  linkFiles(InputFilenames, false);
+  linkFiles(OverridingInputs, true);
 
   if (DumpAsm) errs() << "Here's the assembly:\n" << *Composite;