/// 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, the behavior is controlled by the CMake flag
+/// In NDEBUG builds, if the platform does not support a builtin unreachable
+/// then we call an internal LLVM runtime function. Otherwise 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
/// * When "OFF", a builtin_trap is emitted instead of an
// optimizer hint or printing a reduced message.
///
-/// Use this instead of assert(0). It conveys intent more clearly and
-/// allows compilers to omit some unnecessary code.
+/// Use this instead of assert(0). It conveys intent more clearly, suppresses
+/// diagnostics for unreachable code paths, and allows compilers to omit
+/// 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)
+#elif !defined(LLVM_BUILTIN_UNREACHABLE)
+#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
+#elif LLVM_UNREACHABLE_OPTIMIZE
#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
#else
-#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()
+#define llvm_unreachable(msg) LLVM_BUILTIN_TRAP, LLVM_BUILTIN_UNREACHABLE
#endif
#endif