Index: llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
===================================================================
--- llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
+++ llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyld.cpp
@@ -168,6 +168,7 @@
     uint32_t Flags = I->getFlags();
 
     bool IsCommon = Flags & SymbolRef::SF_Common;
+    bool IsWeak = Flags & SymbolRef::SF_Weak;
     if (IsCommon)
       CommonSymbols.push_back(*I);
     else {
@@ -188,6 +189,11 @@
           continue;
         StringRef SectionData;
         Check(SI->getContents(SectionData));
+        // TODO: It make make sense to delay emitting the section for weak
+        // symbols until they are actually required, but that's not possible
+        // currently, because we only know whether we will need the symbol
+        // in resolveRelocations, which happens after we have already finalized
+        // the Load.
         bool IsCode = SI->isText();
         unsigned SectionID =
             findOrEmitSection(Obj, *SI, IsCode, LocalSections);
@@ -198,7 +204,11 @@
         SymbolInfo::Visibility Vis =
           (Flags & SymbolRef::SF_Exported) ?
             SymbolInfo::Default : SymbolInfo::Hidden;
-        GlobalSymbolTable[Name] = SymbolInfo(SectionID, SectOffset, Vis);
+        if (!IsWeak) {
+          GlobalSymbolTable[Name] = SymbolInfo(SectionID, SectOffset, Vis);
+        } else {
+          WeakSymbolTable[Name] = SymbolInfo(SectionID, SectOffset, Vis);
+        }
       }
     }
   }
@@ -768,6 +778,21 @@
                SymInfo.getOffset();
       }
 
+      // If we didn't find the symbol yet, and it is present in the weak symbol
+      // table, the definition from this object file needs to be used, so emit
+      // it now
+      if (!Addr) {
+        RTDyldSymbolTable::const_iterator Loc = WeakSymbolTable.find(Name);
+        if (Loc != WeakSymbolTable.end()) {
+          SymbolInfo SymInfo = Loc->second;
+          Addr = getSectionLoadAddress(SymInfo.getSectionID()) + SymInfo.getOffset();
+          // Since the weak symbol is now, materialized, add it to the
+          // GlobalSymbolTable. If somebody later asks the ExecutionEngine
+          // for the address of this symbol that's where it'll look
+          GlobalSymbolTable[Name] = SymInfo;
+        }
+      }
+
       // FIXME: Implement error handling that doesn't kill the host program!
       if (!Addr)
         report_fatal_error("Program used external function '" + Name +
@@ -784,6 +809,7 @@
 
     ExternalSymbolRelocations.erase(i);
   }
+  WeakSymbolTable.clear();
 }
 
 //===----------------------------------------------------------------------===//
Index: llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.cpp
===================================================================
--- llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.cpp
+++ llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldELF.cpp
@@ -947,6 +947,7 @@
       break;
     }
     case SymbolRef::ST_Data:
+    case SymbolRef::ST_Function:
     case SymbolRef::ST_Unknown: {
       Value.SymbolName = TargetName.data();
       Value.Addend = Addend;
Index: llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldImpl.h
===================================================================
--- llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldImpl.h
+++ llvm/trunk/lib/ExecutionEngine/RuntimeDyld/RuntimeDyldImpl.h
@@ -203,6 +203,9 @@
   // A global symbol table for symbols from all loaded modules.
   RTDyldSymbolTable GlobalSymbolTable;
 
+  // Like the global symbol table but for weak symbols
+  RTDyldSymbolTable WeakSymbolTable;
+
   // Keep a map of common symbols to their info pairs
   typedef std::vector<SymbolRef> CommonSymbolList;
 
Index: llvm/trunk/unittests/ExecutionEngine/MCJIT/CMakeLists.txt
===================================================================
--- llvm/trunk/unittests/ExecutionEngine/MCJIT/CMakeLists.txt
+++ llvm/trunk/unittests/ExecutionEngine/MCJIT/CMakeLists.txt
@@ -1,5 +1,6 @@
 set(LLVM_LINK_COMPONENTS
   Analysis
+  AsmParser
   Core
   ExecutionEngine
   IPO
Index: llvm/trunk/unittests/ExecutionEngine/MCJIT/MCJITTest.cpp
===================================================================
--- llvm/trunk/unittests/ExecutionEngine/MCJIT/MCJITTest.cpp
+++ llvm/trunk/unittests/ExecutionEngine/MCJIT/MCJITTest.cpp
@@ -13,6 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ExecutionEngine/MCJIT.h"
+#include "llvm/Support/DynamicLibrary.h"
 #include "MCJITTestBase.h"
 #include "gtest/gtest.h"
 
@@ -199,4 +200,45 @@
   EXPECT_EQ(A, B) << "Repeat calls to getPointerToFunction fail.";
 }
 
+// Test weak symbol linking when the weak symbol is present in a shared
+// library
+TEST_F(MCJITTest, weak_symbol_present) {
+  SKIP_UNSUPPORTED_PLATFORM;
+
+  int FakeWeakSymbol;
+  llvm::sys::DynamicLibrary::AddSymbol("FakeWeakSymbol", &FakeWeakSymbol);
+  createJITFromAssembly(
+  "$FakeWeakSymbol = comdat any\n"
+  "@FakeWeakSymbol = linkonce_odr global i32 42, comdat, align 4\n"
+  "define i32 @weak_test(i32* %arg) {\n"
+  "    %r = icmp eq i32* %arg, @FakeWeakSymbol\n"
+  "    %ret = zext i1 %r to i32\n"
+  "    ret i32 %ret\n"
+  "  }");
+
+  uint64_t Addr = TheJIT->getFunctionAddress("weak_test");;
+  EXPECT_TRUE(Addr != 0);
+  int32_t(*FuncPtr)(int32_t *) = (int32_t(*)(int32_t *))Addr;
+  EXPECT_EQ(FuncPtr(&FakeWeakSymbol),1);
+  EXPECT_TRUE(TheJIT->getGlobalValueAddress("FakeWeakSymbol") == 0);
+}
+
+// Test weak symbol linking when the weak symbol is not present in a
+// shared library
+TEST_F(MCJITTest, weak_symbol_absent) {
+  SKIP_UNSUPPORTED_PLATFORM;
+
+  SMDiagnostic Error;
+  createJITFromAssembly(
+  "  $FakeWeakSymbol2 = comdat any\n"
+  "  @FakeWeakSymbol2 = linkonce_odr global i32 42, comdat, align 4\n"
+  "  define i32* @get_weak() {\n"
+  "    ret i32* @FakeWeakSymbol2\n"
+  "  }\n");
+  void*(*FuncPtr)() =
+    (void*(*)(void))TheJIT->getFunctionAddress("get_weak");
+  EXPECT_EQ(FuncPtr(),(void*)TheJIT->getGlobalValueAddress("FakeWeakSymbol2"));
+}
+
+
 }
Index: llvm/trunk/unittests/ExecutionEngine/MCJIT/MCJITTestBase.h
===================================================================
--- llvm/trunk/unittests/ExecutionEngine/MCJIT/MCJITTestBase.h
+++ llvm/trunk/unittests/ExecutionEngine/MCJIT/MCJITTestBase.h
@@ -18,6 +18,7 @@
 #define LLVM_UNITTESTS_EXECUTIONENGINE_MCJIT_MCJITTESTBASE_H
 
 #include "MCJITTestAPICommon.h"
+#include "llvm/AsmParser/Parser.h"
 #include "llvm/Config/config.h"
 #include "llvm/ExecutionEngine/ExecutionEngine.h"
 #include "llvm/ExecutionEngine/SectionMemoryManager.h"
@@ -27,6 +28,8 @@
 #include "llvm/IR/Module.h"
 #include "llvm/IR/TypeBuilder.h"
 #include "llvm/Support/CodeGen.h"
+#include "llvm/Support/SourceMgr.h"
+#include "llvm/Support/raw_ostream.h"
 
 namespace llvm {
 
@@ -338,6 +341,21 @@
     assert(TheJIT.get() != NULL && "error creating MCJIT with EngineBuilder");
   }
 
+  void createJITFromAssembly(const char *Test) {
+    SMDiagnostic Error;
+    M = parseAssemblyString(Test, Error, Context);
+
+    std::string errMsg;
+    raw_string_ostream os(errMsg);
+    Error.print("", os);
+
+    // A failure here means that the test itself is buggy.
+    if (!M)
+      report_fatal_error(os.str().c_str());
+
+    createJIT(std::move(M));
+  }
+
   CodeGenOpt::Level OptLevel;
   Reloc::Model RelocModel;
   CodeModel::Model CodeModel;
Index: llvm/trunk/unittests/ExecutionEngine/MCJIT/Makefile
===================================================================
--- llvm/trunk/unittests/ExecutionEngine/MCJIT/Makefile
+++ llvm/trunk/unittests/ExecutionEngine/MCJIT/Makefile
@@ -9,7 +9,7 @@
 
 LEVEL = ../../..
 TESTNAME = MCJIT
-LINK_COMPONENTS := core ipo mcjit native support
+LINK_COMPONENTS := core asmparser ipo mcjit native support
 
 include $(LEVEL)/Makefile.config
 include $(LLVM_SRC_ROOT)/unittests/Makefile.unittest