Add a cmake flag to turn llvm_unreachable() into builtin_trap() when assertions are disabled

This re-lands 6316129 after fixing the condition logic.

The new flag seems to not be working yet on Windows, where the builtin
trap isn't "no return".

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D121750

Author: joker-eph

llvm/CMakeLists.txt

@@ -465,6 +465,7 @@ option(LLVM_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
 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 "Optimize llvm_unreachable() as undefined behavior (default), guaranteed trap when OFF" ON)
 
 if( NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG" )
   option(LLVM_ENABLE_ASSERTIONS "Enable assertions" OFF)

llvm/docs/CMake.rst

@@ -300,6 +300,12 @@ enabled sub-projects. Nearly all of these variable names begin with
   enabled or not.  A version of LLVM built with ABI breaking checks
   is not ABI compatible with a version built without it.
 
+**LLVM_UNREACHABLE_OPTIMIZE**:BOOL
+  This flag controls the behavior of `llvm_unreachable()` in release build
+  (when assertions are disabled in general). When ON (default) then
+  `llvm_unreachable()` is considered "undefined behavior" and optimized as
+  such. When OFF it is instead replaced with a guaranteed "trap".
+
 **LLVM_APPEND_VC_REV**:BOOL
   Embed version control revision info (Git revision id).
   The version info is provided by the ``LLVM_REVISION`` macro in

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

llvm/include/llvm/Support/ErrorHandling.h

@@ -124,15 +124,23 @@ llvm_unreachable_internal(const char *msg = nullptr, const char *file = nullptr,
 
 /// 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 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.
 #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