Index: lib/Driver/ToolChains.cpp
===================================================================
--- lib/Driver/ToolChains.cpp
+++ lib/Driver/ToolChains.cpp
@@ -368,47 +368,34 @@
 
   const SanitizerArgs &Sanitize = getSanitizerArgs();
 
-  // Add Ubsan runtime library, if required.
-  if (Sanitize.needsUbsanRt()) {
-    // FIXME: Move this check to SanitizerArgs::filterUnsupportedKinds.
-    if (isTargetIOSBased()) {
-      getDriver().Diag(diag::err_drv_clang_unsupported_per_platform)
-        << "-fsanitize=undefined";
-    } else {
-      assert(isTargetMacOS() && "unexpected non OS X target");
-      AddLinkRuntimeLib(Args, CmdArgs, "libclang_rt.ubsan_osx.a", true);
-
-      // The Ubsan runtime library requires C++.
-      AddCXXStdlibLibArgs(Args, CmdArgs);
-    }
-  }
-
-  // Add ASAN runtime library, if required. Dynamic libraries and bundles
-  // should not be linked with the runtime library.
-  if (Sanitize.needsAsanRt()) {
-    // FIXME: Move this check to SanitizerArgs::filterUnsupportedKinds.
-    if (isTargetIPhoneOS()) {
-      getDriver().Diag(diag::err_drv_clang_unsupported_per_platform)
-        << "-fsanitize=address";
+  // Add ASan/UBSan runtime libraries, if required.
+  if (Sanitize.needsAsanRt() || Sanitize.needsUbsanRt()) {
+    // For now we can't use ASan and UBSan together on Mac (PR21112).
+    if (Sanitize.needsAsanRt() && Sanitize.needsUbsanRt()) {
+      getDriver().Diag(diag::err_drv_argument_not_allowed_with)
+          << "-fsanitize=address"
+          << "-fsanitize=undefined";
+    } else if (!isTargetMacOS() && !isTargetIOSSimulator()) {
+      // FIXME: Move this check to SanitizerArgs::filterUnsupportedKinds.
+      StringRef Arg = Sanitize.needsAsanRt() ? "-fsanitize=address"
+                                             : "-fsanitize=undefined";
+      getDriver().Diag(diag::err_drv_clang_unsupported_per_platform) << Arg;
     } else {
       if (!Args.hasArg(options::OPT_dynamiclib) &&
           !Args.hasArg(options::OPT_bundle)) {
-        // The ASAN runtime library requires C++.
+        // Sanitizer runtime libraries requires C++.
         AddCXXStdlibLibArgs(Args, CmdArgs);
+        // Add explicit dependcy on -lc++abi, as -lc++ doesn't re-export
+        // all RTTI-related symbols that UBSan uses.
+        if (Sanitize.needsUbsanRt())
+          CmdArgs.push_back("-lc++abi");
       }
-      if (isTargetMacOS()) {
-        AddLinkRuntimeLib(Args, CmdArgs,
-                          "libclang_rt.asan_osx_dynamic.dylib",
-                          /*AlwaysLink*/ true, /*IsEmbedded*/ false,
-                          /*AddRPath*/ true);
-      } else {
-        if (isTargetIOSSimulator()) {
-          AddLinkRuntimeLib(Args, CmdArgs,
-                            "libclang_rt.asan_iossim_dynamic.dylib",
-                            /*AlwaysLink*/ true, /*IsEmbedded*/ false,
-                            /*AddRPath*/ true);
-        }
-      }
+      StringRef OS = isTargetMacOS() ? "osx" : "iossim";
+      StringRef Sanitizer = Sanitize.needsAsanRt() ? "asan" : "ubsan";
+      AddLinkRuntimeLib(Args, CmdArgs, (Twine("libclang_rt.") + Sanitizer +
+                                        "_" + OS + "_dynamic.dylib").str(),
+                        /*AlwaysLink*/ true, /*IsEmbedded*/ false,
+                        /*AddRPath*/ true);
     }
   }
 
Index: runtime/compiler-rt/Makefile
===================================================================
--- runtime/compiler-rt/Makefile
+++ runtime/compiler-rt/Makefile
@@ -79,7 +79,7 @@
 	eprintf.a 10.4.a osx.a cc_kext.a \
 	asan_osx_dynamic.dylib \
 	profile_osx.a \
-	ubsan_osx.a
+	ubsan_osx_dynamic.dylib
 
 IOS_SDK := $(shell xcrun --show-sdk-path -sdk iphoneos 2> /dev/null)
 IOSSIM_SDK := $(shell xcrun --show-sdk-path -sdk iphonesimulator 2> /dev/null)
@@ -93,7 +93,8 @@
 endif
 
 ifneq ($(IOSSIM_SDK),)
-RuntimeLibrary.darwin.Configs += asan_iossim_dynamic.dylib
+RuntimeLibrary.darwin.Configs += asan_iossim_dynamic.dylib \
+                                 ubsan_iossim_dynamic.dylib
 endif
 
 RuntimeLibrary.macho_embedded.Configs := \
Index: test/Driver/darwin-sanitizer-ld.c
===================================================================
--- test/Driver/darwin-sanitizer-ld.c
+++ test/Driver/darwin-sanitizer-ld.c
@@ -35,8 +35,10 @@
 // RUN:   | FileCheck --check-prefix=CHECK-UBSAN %s
 
 // CHECK-UBSAN: "{{.*}}ld{{(.exe)?}}"
-// CHECK-UBSAN: libclang_rt.ubsan_osx.a"
 // CHECK-UBSAN: stdc++
+// CHECK-UBSAN: libclang_rt.ubsan_osx_dynamic.dylib"
+// CHECK-UBSAN: "-rpath" "@executable_path"
+// CHECK-UBSAN: "-rpath" "{{.*}}lib{{.*}}darwin"
 
 // RUN: %clang -no-canonical-prefixes -### -target x86_64-darwin \
 // RUN:   -fsanitize=bounds -fsanitize-undefined-trap-on-error \
@@ -52,7 +54,9 @@
 
 // CHECK-DYN-UBSAN: "{{.*}}ld{{(.exe)?}}"
 // CHECK-DYN-UBSAN: "-dylib"
-// CHECK-DYN-UBSAN: libclang_rt.ubsan_osx.a
+// CHECK-DYN-UBSAN: libclang_rt.ubsan_osx_dynamic.dylib"
+// CHECK-DYN-UBSAN: "-rpath" "@executable_path"
+// CHECK-DYN-UBSAN: "-rpath" "{{.*}}lib{{.*}}darwin"
 
 // RUN: %clang -no-canonical-prefixes -### -target x86_64-darwin \
 // RUN:   -fsanitize=bounds -fsanitize-undefined-trap-on-error \
@@ -60,4 +64,10 @@
 // RUN:   | FileCheck --check-prefix=CHECK-DYN-BOUNDS %s
 
 // CHECK-DYN-BOUNDS: "{{.*}}ld{{(.exe)?}}"
-// CHECK-DYN-BOUNDS-NOT: libclang_rt.ubsan_osx.a
+// CHECK-DYN-BOUNDS-NOT: ubsan_osx
+
+// RUN: %clang -no-canonical-prefixes -### -target x86_64-darwin \
+// RUN:   -fsanitize=address,undefined %s -o %t.o 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-ASAN-UBSAN %s
+
+// CHECK-ASAN-UBSAN: error: invalid argument '-fsanitize=address' not allowed with '-fsanitize=undefined'