[libc++][hardening] Allow setting the assertion semantic via CMake.

libcxx/CMakeLists.txt

@@ -66,6 +66,19 @@ if (NOT "${LIBCXX_HARDENING_MODE}" IN_LIST LIBCXX_SUPPORTED_HARDENING_MODES)
   message(FATAL_ERROR
     "Unsupported hardening mode: '${LIBCXX_HARDENING_MODE}'. Supported values are ${LIBCXX_SUPPORTED_HARDENING_MODES}.")
 endif()
+set(LIBCXX_SUPPORTED_ASSERTION_SEMANTICS ignore observe quick_enforce enforce)
+set(LIBCXX_ASSERTION_SEMANTIC "" CACHE STRING
+  "Specify the default assertion semantic to use. This semantic will be used
+  inside the compiled library and will be the default when compiling user code.
+  If not defined, the library will select the assertion semantic based on the hardening
+  mode in effect. Note that users can override this setting in their own code.
+  This does not affect the ABI. Supported values are
+  ${LIBCXX_SUPPORTED_ASSERTION_SEMANTICS}.")
+
+if (NOT "${LIBCXX_ASSERTION_SEMANTIC}" STREQUAL "" AND NOT "${LIBCXX_ASSERTION_SEMANTIC}" IN_LIST LIBCXX_SUPPORTED_ASSERTION_SEMANTICS)
+  message(FATAL_ERROR
+    "Unsupported assertion semantic: '${LIBCXX_ASSERTION_SEMANTIC}'. Supported values are ${LIBCXX_SUPPORTED_ASSERTION_SEMANTICS}.")
+endif()
 set(LIBCXX_ASSERTION_HANDLER_FILE
   "vendor/llvm/default_assertion_handler.in"
   CACHE STRING
@@ -763,6 +776,15 @@ elseif (LIBCXX_HARDENING_MODE STREQUAL "extensive")
 elseif (LIBCXX_HARDENING_MODE STREQUAL "debug")
   config_define(8 _LIBCPP_HARDENING_MODE_DEFAULT)
 endif()
+if (LIBCXX_ASSERTION_SEMANTIC STREQUAL "ignore")
+  config_define(2 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+elseif (LIBCXX_ASSERTION_SEMANTIC STREQUAL "observe")
+  config_define(4 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+elseif (LIBCXX_ASSERTION_SEMANTIC STREQUAL "quick_enforce")
+  config_define(8 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+elseif (LIBCXX_ASSERTION_SEMANTIC STREQUAL "enforce")
+  config_define(16 _LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+endif()
 
 if (LIBCXX_PSTL_BACKEND STREQUAL "serial")
   config_define(1 _LIBCPP_PSTL_BACKEND_SERIAL)

libcxx/include/__config_site.in

@@ -40,6 +40,7 @@
 
 // Hardening.
 #cmakedefine _LIBCPP_HARDENING_MODE_DEFAULT @_LIBCPP_HARDENING_MODE_DEFAULT@
+#cmakedefine _LIBCPP_ASSERTION_SEMANTIC_DEFAULT @_LIBCPP_ASSERTION_SEMANTIC_DEFAULT@
 
 // __USE_MINGW_ANSI_STDIO gets redefined on MinGW
 #ifdef __clang__

libcxx/include/__configuration/hardening.h

@@ -156,26 +156,47 @@ _LIBCPP_HARDENING_MODE_DEBUG
 #  define _LIBCPP_ASSERTION_SEMANTIC_ENFORCE       (1 << 4)
 // clang-format on
 
-// Allow users to define an arbitrary assertion semantic; otherwise, use the default mapping from modes to semantics.
-// The default is for production-capable modes to use `quick-enforce` (i.e., trap) and for the `debug` mode to use
-// `enforce` (i.e., log and abort).
-#ifndef _LIBCPP_ASSERTION_SEMANTIC
-
-#  if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
-#    define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
-#  else
-#    define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE
-#  endif
-
-#else
-
+// If the user or the vendor attempt to configure the assertion semantic, check that it is allowed in the current
+// environment.
+#if defined(_LIBCPP_ASSERTION_SEMANTIC) || defined(_LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
 #  if !_LIBCPP_HAS_EXPERIMENTAL_LIBRARY
 #    error "Assertion semantics are an experimental feature."
 #  endif
 #  if defined(_LIBCPP_CXX03_LANG)
 #    error "Assertion semantics are not available in the C++03 mode."
 #  endif
+#endif // defined(_LIBCPP_ASSERTION_SEMANTIC) || defined(_LIBCPP_ASSERTION_SEMANTIC_DEFAULT)
+
+// There are 2 ways to configure the assertion semantic, listed in order of precedence:
+// 1. The `_LIBCPP_ASSERTION_SEMANTIC` macro, defined directly by the user.
+// 2. The `LIBCXX_ASSERTION_SEMANTIC` CMake variable, set by the vendor.
+// If neither `_LIBCPP_ASSERTION_SEMANTIC` nor `LIBCXX_ASSERTION_SEMANTIC` are defined, the default mapping is used
+// which determines the semantic based on the hardening mode in effect: production-capable modes map to `quick-enforce`
+// (i.e., trap) and the `debug` mode maps to `enforce` (i.e., log and abort).
+#ifndef _LIBCPP_ASSERTION_SEMANTIC // User-provided semantic takes top priority -- don't override if set.
+
+#  ifdef _LIBCPP_ASSERTION_SEMANTIC_DEFAULT // Vendor-provided semantic takes second priority.
+#    define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_DEFAULT
+#  else // Fallback: use the default mapping.
+#    if _LIBCPP_HARDENING_MODE == _LIBCPP_HARDENING_MODE_DEBUG
+#      define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+#    else
+#      define _LIBCPP_ASSERTION_SEMANTIC _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE
+#    endif
+#  endif //    ifdef _LIBCPP_ASSERTION_SEMANTIC_DEFAULT
 
 #endif // _LIBCPP_ASSERTION_SEMANTIC
 
+// Finally, validate the selected semantic (in case the user tries setting it to an incorrect value):
+#if _LIBCPP_ASSERTION_SEMANTIC != _LIBCPP_ASSERTION_SEMANTIC_IGNORE &&                                                 \
+    _LIBCPP_ASSERTION_SEMANTIC != _LIBCPP_ASSERTION_SEMANTIC_OBSERVE &&                                                \
+    _LIBCPP_ASSERTION_SEMANTIC != _LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE &&                                          \
+    _LIBCPP_ASSERTION_SEMANTIC != _LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+#  error _LIBCPP_ASSERTION_SEMANTIC must be set to one of the following values: \
+_LIBCPP_ASSERTION_SEMANTIC_IGNORE, \
+_LIBCPP_ASSERTION_SEMANTIC_OBSERVE, \
+_LIBCPP_ASSERTION_SEMANTIC_QUICK_ENFORCE, \
+_LIBCPP_ASSERTION_SEMANTIC_ENFORCE
+#endif
+
 #endif // _LIBCPP___CONFIGURATION_HARDENING_H