Index: lib/fuzzer/CMakeLists.txt
===================================================================
--- lib/fuzzer/CMakeLists.txt
+++ lib/fuzzer/CMakeLists.txt
@@ -3,8 +3,8 @@
   FuzzerDataFlowTrace.cpp
   FuzzerDriver.cpp
   FuzzerExtFunctionsDlsym.cpp
-  FuzzerExtFunctionsWeakAlias.cpp
   FuzzerExtFunctionsWeak.cpp
+  FuzzerExtFunctionsWindows.cpp
   FuzzerExtraCounters.cpp
   FuzzerIO.cpp
   FuzzerIOPosix.cpp
Index: lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp
===================================================================
--- lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp
+++ lib/fuzzer/FuzzerExtFunctionsWeakAlias.cpp
@@ -1,56 +0,0 @@
-//===- FuzzerExtFunctionsWeakAlias.cpp - Interface to external functions --===//
-//
-//                     The LLVM Compiler Infrastructure
-//
-// This file is distributed under the University of Illinois Open Source
-// License. See LICENSE.TXT for details.
-//
-//===----------------------------------------------------------------------===//
-// Implementation using weak aliases. Works for Windows.
-//===----------------------------------------------------------------------===//
-#include "FuzzerDefs.h"
-#if LIBFUZZER_WINDOWS
-
-#include "FuzzerExtFunctions.h"
-#include "FuzzerIO.h"
-
-using namespace fuzzer;
-
-extern "C" {
-// Declare these symbols as weak to allow them to be optionally defined.
-#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
-  RETURN_TYPE NAME##Def FUNC_SIG {                                             \
-    Printf("ERROR: Function \"%s\" not defined.\n", #NAME);                    \
-    exit(1);                                                                   \
-  }                                                                            \
-  RETURN_TYPE NAME FUNC_SIG __attribute__((weak, alias(#NAME "Def")));
-
-#include "FuzzerExtFunctions.def"
-
-#undef EXT_FUNC
-}
-
-template <typename T>
-static T *GetFnPtr(T *Fun, T *FunDef, const char *FnName, bool WarnIfMissing) {
-  if (Fun == FunDef) {
-    if (WarnIfMissing)
-      Printf("WARNING: Failed to find function \"%s\".\n", FnName);
-    return nullptr;
-  }
-  return Fun;
-}
-
-namespace fuzzer {
-
-ExternalFunctions::ExternalFunctions() {
-#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)                            \
-  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME##Def, #NAME, WARN);
-
-#include "FuzzerExtFunctions.def"
-
-#undef EXT_FUNC
-}
-
-} // namespace fuzzer
-
-#endif // LIBFUZZER_WINDOWS
Index: lib/fuzzer/FuzzerExtFunctionsWindows.cpp
===================================================================
--- lib/fuzzer/FuzzerExtFunctionsWindows.cpp
+++ lib/fuzzer/FuzzerExtFunctionsWindows.cpp
@@ -0,0 +1,83 @@
+//=== FuzzerExtWindows.cpp - Interface to external functions --------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+// Implementation of FuzzerExtFunctions for Windows. Uses alternatename when
+// compiled with MSVC. Uses weak aliases when compiled with clang. Unfortunately
+// the method each compiler supports is not supported by the other.
+//===----------------------------------------------------------------------===//
+#include "FuzzerDefs.h"
+#if LIBFUZZER_WINDOWS
+
+#include "FuzzerExtFunctions.h"
+#include "FuzzerIO.h"
+
+using namespace fuzzer;
+
+// Intermediate macro to ensure the parameter is expanded before stringified.
+#define STRINGIFY_(A) #A
+#define STRINGIFY(A) STRINGIFY_(A)
+
+#if LIBFUZZER_MSVC
+// Copied from compiler-rt/lib/sanitizer_common/sanitizer_win_defs.h
+#if defined(_M_IX86) || defined(__i386__)
+#define WIN_SYM_PREFIX "_"
+#else
+#define WIN_SYM_PREFIX
+#endif
+
+// Declare external functions as having alternativenames, so that we can
+// determine if they are not defined.
+#define EXTERNAL_FUNC(Name, Default)                                   \
+  __pragma(comment(linker, "/alternatename:" WIN_SYM_PREFIX STRINGIFY( \
+                               Name) "=" WIN_SYM_PREFIX STRINGIFY(Default)))
+#else
+// Declare external functions as weak to allow them to default to a specified
+// function if not defined explicitly. We must use weak symbols because clang's
+// support for alternatename is not 100%, see
+// https://bugs.llvm.org/show_bug.cgi?id=40218 for more details.
+#define EXTERNAL_FUNC(Name, Default) \
+  __attribute__((weak, alias(STRINGIFY(Default))))
+#endif  // LIBFUZZER_MSVC
+
+extern "C" {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN)         \
+  RETURN_TYPE NAME##Def FUNC_SIG {                          \
+    Printf("ERROR: Function \"%s\" not defined.\n", #NAME); \
+    exit(1);                                                \
+  }                                                         \
+  EXTERNAL_FUNC(NAME, NAME##Def) RETURN_TYPE NAME FUNC_SIG;
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+template <typename T>
+static T *GetFnPtr(T *Fun, T *FunDef, const char *FnName, bool WarnIfMissing) {
+  if (Fun == FunDef) {
+    if (WarnIfMissing)
+      Printf("WARNING: Failed to find function \"%s\".\n", FnName);
+    return nullptr;
+  }
+  return Fun;
+}
+
+namespace fuzzer {
+
+ExternalFunctions::ExternalFunctions() {
+#define EXT_FUNC(NAME, RETURN_TYPE, FUNC_SIG, WARN) \
+  this->NAME = GetFnPtr<decltype(::NAME)>(::NAME, ::NAME##Def, #NAME, WARN);
+
+#include "FuzzerExtFunctions.def"
+
+#undef EXT_FUNC
+}
+
+}  // namespace fuzzer
+
+#endif // LIBFUZZER_WINDOWS