diff --git a/llvm/include/llvm/Support/Compiler.h b/llvm/include/llvm/Support/Compiler.h
--- a/llvm/include/llvm/Support/Compiler.h
+++ b/llvm/include/llvm/Support/Compiler.h
@@ -321,12 +321,14 @@
 /// LLVM_BUILTIN_UNREACHABLE - On compilers which support it, expands
 /// to an expression which states that it is undefined behavior for the
 /// compiler to reach this point.  Otherwise is not defined.
+///
+/// '#else' is intentionally left out so that other macro logic (e.g.,
+/// LLVM_ASSUME_ALIGNED and llvm_unreachable()) can detect whether
+/// LLVM_BUILTIN_UNREACHABLE has a definition.
 #if __has_builtin(__builtin_unreachable) || defined(__GNUC__)
 # define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
 #elif defined(_MSC_VER)
 # define LLVM_BUILTIN_UNREACHABLE __assume(false)
-#else
-# define LLVM_BUILTIN_UNREACHABLE
 #endif
 
 /// LLVM_BUILTIN_TRAP - On compilers which support it, expands to an expression