-/// \macro LLVM_IS_UNALIGNED_ACCESS_FAST
-/// \brief Is unaligned memory access fast on the host machine.
-///
-/// Don't specialize on alignment for platforms where unaligned memory accesses
-/// generates the same code as aligned memory accesses for common types.
-#if defined(_M_AMD64) || defined(_M_IX86) || defined(__amd64) || \
- defined(__amd64__) || defined(__x86_64) || defined(__x86_64__) || \
- defined(_X86_) || defined(__i386) || defined(__i386__)
-# define LLVM_IS_UNALIGNED_ACCESS_FAST 1
+/// \macro LLVM_THREAD_SANITIZER_BUILD
+/// \brief Whether LLVM itself is built with ThreadSanitizer instrumentation.
+#if __has_feature(thread_sanitizer) || defined(__SANITIZE_THREAD__)
+# define LLVM_THREAD_SANITIZER_BUILD 1
+#else
+# define LLVM_THREAD_SANITIZER_BUILD 0
+#endif
+
+#if LLVM_THREAD_SANITIZER_BUILD
+// Thread Sanitizer is a tool that finds races in code.
+// See http://code.google.com/p/data-race-test/wiki/DynamicAnnotations .
+// tsan detects these exact functions by name.
+extern "C" {
+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);
+}
+
+// This marker is used to define a happens-before arc. The race detector will
+// infer an arc from the begin to the end when they share the same pointer
+// argument.
+# define TsanHappensBefore(cv) AnnotateHappensBefore(__FILE__, __LINE__, cv)
+
+// This marker defines the destination of a happens-before arc.
+# define TsanHappensAfter(cv) AnnotateHappensAfter(__FILE__, __LINE__, cv)
+
+// Ignore any races on writes between here and the next TsanIgnoreWritesEnd.
+# define TsanIgnoreWritesBegin() AnnotateIgnoreWritesBegin(__FILE__, __LINE__)
+
+// Resume checking for racy writes.
+# define TsanIgnoreWritesEnd() AnnotateIgnoreWritesEnd(__FILE__, __LINE__)