diff --git a/clang/include/clang/Basic/Module.h b/clang/include/clang/Basic/Module.h
--- a/clang/include/clang/Basic/Module.h
+++ b/clang/include/clang/Basic/Module.h
@@ -62,6 +62,15 @@
 
   explicit operator bool() const { return *this != BaseT({{0}}); }
 
+  /// Returns the value truncated to the size of an uint64_t.
+  uint64_t truncatedValue() const {
+    uint64_t Value = 0;
+    static_assert(sizeof(*this) >= sizeof(uint64_t), "No need to truncate.");
+    for (unsigned I = 0; I < sizeof(uint64_t); ++I)
+      Value |= static_cast<uint64_t>((*this)[I]) << (I * 8);
+    return Value;
+  }
+
   static ASTFileSignature create(StringRef Bytes) {
     return create(Bytes.bytes_begin(), Bytes.bytes_end());
   }
diff --git a/clang/lib/CodeGen/CGDebugInfo.cpp b/clang/lib/CodeGen/CGDebugInfo.cpp
--- a/clang/lib/CodeGen/CGDebugInfo.cpp
+++ b/clang/lib/CodeGen/CGDebugInfo.cpp
@@ -2545,12 +2545,11 @@
     // We use the lower 64 bits for debug info.
 
     uint64_t Signature = 0;
-    if (const auto &ModSig = Mod.getSignature()) {
-      for (unsigned I = 0; I != sizeof(Signature); ++I)
-        Signature |= (uint64_t)ModSig[I] << (I * 8);
-    } else {
+    if (const auto &ModSig = Mod.getSignature())
+      Signature = ModSig.truncatedValue();
+    else
       Signature = ~1ULL;
-    }
+
     llvm::DIBuilder DIB(CGM.getModule());
     SmallString<0> PCM;
     if (!llvm::sys::path::is_absolute(Mod.getASTFile()))
diff --git a/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp b/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
--- a/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
+++ b/clang/lib/CodeGen/ObjectFilePCHContainerOperations.cpp
@@ -250,10 +250,10 @@
     // PCH files don't have a signature field in the control block,
     // but LLVM detects DWO CUs by looking for a non-zero DWO id.
     // We use the lower 64 bits for debug info.
+
     uint64_t Signature =
-        Buffer->Signature
-            ? (uint64_t)Buffer->Signature[1] << 32 | Buffer->Signature[0]
-            : ~1ULL;
+        Buffer->Signature ? Buffer->Signature.truncatedValue() : ~1ULL;
+
     Builder->getModuleDebugInfo()->setDwoId(Signature);
 
     // Finalize the Builder.
diff --git a/clang/test/Modules/Inputs/DebugDwoId.h b/clang/test/Modules/Inputs/DebugDwoId.h
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/Inputs/DebugDwoId.h
@@ -0,0 +1,4 @@
+#ifndef DEBUG_DWO_ID_H
+#define DEBUG_DWO_ID_H
+struct Dummy {};
+#endif
diff --git a/clang/test/Modules/Inputs/module.map b/clang/test/Modules/Inputs/module.map
--- a/clang/test/Modules/Inputs/module.map
+++ b/clang/test/Modules/Inputs/module.map
@@ -357,6 +357,10 @@
   }
 }
 
+module DebugDwoId {
+  header "DebugDwoId.h"
+}
+
 module ImportNameInDir {
   header "ImportNameInDir.h"
   export *
diff --git a/clang/test/Modules/ModuleDebugInfoDwoId.cpp b/clang/test/Modules/ModuleDebugInfoDwoId.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/Modules/ModuleDebugInfoDwoId.cpp
@@ -0,0 +1,22 @@
+// Tests that dwoIds in modules match the dwoIDs in the main file.
+
+// RUN: rm -rf %t.cache
+// RUN: %clang_cc1 -triple %itanium_abi_triple -x objective-c++ -std=c++11 -debugger-tuning=lldb -debug-info-kind=limited -fmodules -fmodule-format=obj -dwarf-ext-refs -fimplicit-module-maps -fmodules-cache-path=%t.cache %s -I %S/Inputs -emit-llvm -o %t.ll -mllvm -debug-only=pchcontainer 2> %t.mod-out
+// RUN: cat %t.ll > %t.combined_output
+// RUN: cat %t.mod-out >> %t.combined_output
+// RUN: cat %t.combined_output | FileCheck %s
+// RUN: cat %t.ll | FileCheck --check-prefix=CHECK-REALIDS %s
+// RUN: cat %t.mod-out | FileCheck --check-prefix=CHECK-REALIDS %s
+
+@import DebugDwoId;
+
+Dummy d;
+
+// Find the emitted dwoID for DebugInfoId and compare it against the one in the PCM.
+// CHECK: DebugDwoId-{{[A-Z0-9]+}}.pcm
+// CHECK-SAME: dwoId: [[DWOID:[0-9]+]]
+// CHECK: dwoId: [[DWOID]]
+// CHECK-NEXT: !DIFile(filename: "DebugDwoId"
+
+// Make sure the dwo IDs are real IDs and not fallback values (~1ULL).
+// CHECK-REALIDS-NOT: dwoId: 18446744073709551615