diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -465,6 +465,7 @@
 option(LLVM_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
 
 option(LLVM_ENABLE_DUMP "Enable dump functions even when assertions are disabled" OFF)
+option(LLVM_UNREACHABLE_OPTIMIZE "Whether llvm_unreachable() should be considered undefined behavior (default) or instead trap when assertions are disabled" ON)
 
 if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" OFF)
diff --git a/llvm/include/llvm/Config/llvm-config.h.cmake b/llvm/include/llvm/Config/llvm-config.h.cmake
--- a/llvm/include/llvm/Config/llvm-config.h.cmake
+++ b/llvm/include/llvm/Config/llvm-config.h.cmake
@@ -112,4 +112,8 @@
 /* Define if building LLVM with LLVM_FORCE_USE_OLD_TOOLCHAIN_LIBS */
 #cmakedefine LLVM_FORCE_USE_OLD_TOOLCHAIN ${LLVM_FORCE_USE_OLD_TOOLCHAIN}
 
+/* Define if llvm_unreachable should be optimized with undefined behavior
+ * in non assert builds */
+#cmakedefine01 LLVM_UNREACHABLE_OPTIMIZE
+
 #endif
diff --git a/llvm/include/llvm/Support/ErrorHandling.h b/llvm/include/llvm/Support/ErrorHandling.h
--- a/llvm/include/llvm/Support/ErrorHandling.h
+++ b/llvm/include/llvm/Support/ErrorHandling.h
@@ -124,15 +124,23 @@
 
 /// Marks that the current location is not supposed to be reachable.
 /// In !NDEBUG builds, prints the message and location info to stderr.
-/// In NDEBUG builds, becomes an optimizer hint that the current location
-/// is not supposed to be reachable.  On compilers that don't support
-/// such hints, prints a reduced message instead and aborts the program.
+/// In NDEBUG builds, the behavior is controlled by the CMake flag
+///   -DLLVM_UNREACHABLE_OPTIMIZE
+/// * When "ON" (default) llvm_unreachable() becomes an optimizer hint
+///   that the current location is not supposed to be reachable: the hint
+///   turns such code path into undefined behavior.  On compilers that don't
+///   support such hints, prints a reduced message instead and aborts the
+///   program.
+/// * When "OFF", a builtin_trap is emitted instead of involving undefined
+///   behavior.
 ///
 /// Use this instead of assert(0).  It conveys intent more clearly and
 /// allows compilers to omit some unnecessary code.
 #ifndef NDEBUG
 #define llvm_unreachable(msg) \
   ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
+#elif LLVM_UNREACHABLE_OPTIMIZE
+#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP
 #elif defined(LLVM_BUILTIN_UNREACHABLE)
 #define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
 #else