diff --git a/include/llvm/Support/Valgrind.h b/include/llvm/Support/Valgrind.h
index 7ae40af754f..cebf75c49c1 100644
--- a/include/llvm/Support/Valgrind.h
+++ b/include/llvm/Support/Valgrind.h
@@ -24,12 +24,10 @@
 // tsan (Thread Sanitizer) is a valgrind-based tool that detects these exact
 // functions by name.
 extern "C" {
-LLVM_ATTRIBUTE_WEAK void AnnotateHappensAfter(const char *file, int line,
-                                              const volatile void *cv);
-LLVM_ATTRIBUTE_WEAK void AnnotateHappensBefore(const char *file, int line,
-                                               const volatile void *cv);
-LLVM_ATTRIBUTE_WEAK void AnnotateIgnoreWritesBegin(const char *file, int line);
-LLVM_ATTRIBUTE_WEAK void AnnotateIgnoreWritesEnd(const char *file, int line);
+void AnnotateHappensAfter(const char *file, int line, const volatile void *cv);
+void AnnotateHappensBefore(const char *file, int line, const volatile void *cv);
+void AnnotateIgnoreWritesBegin(const char *file, int line);
+void AnnotateIgnoreWritesEnd(const char *file, int line);
 }
 #endif
 
diff --git a/lib/Support/Valgrind.cpp b/lib/Support/Valgrind.cpp
index 2b250a35775..2c6d6aaadff 100644
--- a/lib/Support/Valgrind.cpp
+++ b/lib/Support/Valgrind.cpp
@@ -55,13 +55,21 @@ void llvm::sys::ValgrindDiscardTranslations(const void *Addr, size_t Len) {
 
 #if LLVM_ENABLE_THREADS != 0 && !defined(NDEBUG)
 // These functions require no implementation, tsan just looks at the arguments
-// they're called with.
+// they're called with. However, they are required to be weak as some other
+// application or library may already be providing these definitions for the
+// same reason we are.
 extern "C" {
+LLVM_ATTRIBUTE_WEAK void AnnotateHappensAfter(const char *file, int line,
+                                              const volatile void *cv);
+void AnnotateHappensAfter(const char *file, int line, const volatile void *cv) {
+}
+LLVM_ATTRIBUTE_WEAK void AnnotateHappensBefore(const char *file, int line,
+                                               const volatile void *cv);
 void AnnotateHappensBefore(const char *file, int line,
                            const volatile void *cv) {}
-void AnnotateHappensAfter(const char *file, int line,
-                          const volatile void *cv) {}
+LLVM_ATTRIBUTE_WEAK void AnnotateIgnoreWritesBegin(const char *file, int line);
 void AnnotateIgnoreWritesBegin(const char *file, int line) {}
+LLVM_ATTRIBUTE_WEAK void AnnotateIgnoreWritesEnd(const char *file, int line);
 void AnnotateIgnoreWritesEnd(const char *file, int line) {}
 }
 #endif