Adds ABSL_UNREACHABLE(), a public symbol to replace ABSL_INTERNAL_UNREACHABLE

ABSL_UNREACHABLE() is an unreachable statement.  A program which reaches
one has undefined behavior, and the compiler may optimize accordingly.

The behavior is changed to abort the program in !NDEBUG or
ABSL_OPTION_HARDENED modes.

PiperOrigin-RevId: 496917150
Change-Id: If036b2d9567933fa266fbcd33f3f98c682ad7f41

absl/base/macros.h

@@ -103,17 +103,11 @@ ABSL_NAMESPACE_END
 // aborts the program in release mode (when NDEBUG is defined). The
 // implementation should abort the program as quickly as possible and ideally it
 // should not be possible to ignore the abort request.
-#if (ABSL_HAVE_BUILTIN(__builtin_trap) &&         \
-     ABSL_HAVE_BUILTIN(__builtin_unreachable)) || \
-    (defined(__GNUC__) && !defined(__clang__))
-#define ABSL_INTERNAL_HARDENING_ABORT() \
-  do {                                  \
-    __builtin_trap();                   \
-    __builtin_unreachable();            \
+#define ABSL_INTERNAL_HARDENING_ABORT()   \
+  do {                                    \
+    ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL(); \
+    ABSL_INTERNAL_UNREACHABLE_IMPL();     \
   } while (false)
-#else
-#define ABSL_INTERNAL_HARDENING_ABORT() abort()
-#endif
 
 // ABSL_HARDENING_ASSERT()
 //
@@ -144,15 +138,7 @@ ABSL_NAMESPACE_END
 #define ABSL_INTERNAL_RETHROW do {} while (false)
 #endif  // ABSL_HAVE_EXCEPTIONS
 
-// `ABSL_INTERNAL_UNREACHABLE` is an unreachable statement.  A program which
-// reaches one has undefined behavior, and the compiler may optimize
-// accordingly.
-#if defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
-#define ABSL_INTERNAL_UNREACHABLE __builtin_unreachable()
-#elif defined(_MSC_VER)
-#define ABSL_INTERNAL_UNREACHABLE __assume(0)
-#else
-#define ABSL_INTERNAL_UNREACHABLE
-#endif
+// TODO(b/261916195): Replace ABSL_INTERNAL_UNREACHABLE with ABSL_UNREACHABLE().
+#define ABSL_INTERNAL_UNREACHABLE ABSL_UNREACHABLE()
 
 #endif  // ABSL_BASE_MACROS_H_

absl/base/optimization.h

@@ -181,6 +181,52 @@
 #define ABSL_PREDICT_TRUE(x) (x)
 #endif
 
+// `ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL()` aborts the program in the fastest
+// possible way, with no attempt at logging. One use is to implement hardening
+// aborts with ABSL_OPTION_HARDENED.  Since this is an internal symbol, it
+// should not be used directly outside of Abseil.
+#if ABSL_HAVE_BUILTIN(__builtin_trap) || \
+    (defined(__GNUC__) && !defined(__clang__))
+#define ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL() __builtin_trap()
+#else
+#define ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL() abort()
+#endif
+
+// `ABSL_INTERNAL_UNREACHABLE_IMPL()` is the platform specific directive to
+// indicate that a statement is unreachable, and to allow the compiler to
+// optimize accordingly. Clients should use `ABSL_UNREACHABLE()`, which is
+// defined below.
+#if defined(__cpp_lib_unreachable) && __cpp_lib_unreachable >= 202202L
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() std::unreachable()
+#elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() __builtin_unreachable()
+#elif ABSL_HAVE_BUILTIN(__builtin_assume)
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() __builtin_assume(false)
+#elif defined(_MSC_VER)
+#define ABSL_INTERNAL_UNREACHABLE_IMPL() __assume(false)
+#else
+#define ABSL_INTERNAL_UNREACHABLE_IMPL()
+#endif
+
+// `ABSL_UNREACHABLE()` is an unreachable statement.  A program which reaches
+// one has undefined behavior, and the compiler may optimize accordingly.
+#if ABSL_OPTION_HARDENED == 1 && defined(NDEBUG)
+// Abort in hardened mode to avoid dangerous undefined behavior.
+#define ABSL_UNREACHABLE()                \
+  do {                                    \
+    ABSL_INTERNAL_IMMEDIATE_ABORT_IMPL(); \
+    ABSL_INTERNAL_UNREACHABLE_IMPL();     \
+  } while (false)
+#else
+// The assert only fires in debug mode to aid in debugging.
+// When NDEBUG is defined, reaching ABSL_UNREACHABLE() is undefined behavior.
+#define ABSL_UNREACHABLE()                       \
+  do {                                           \
+    assert(false && "ABSL_UNREACHABLE reached"); \
+    ABSL_INTERNAL_UNREACHABLE_IMPL();            \
+  } while (false)
+#endif
+
 // ABSL_ASSUME(cond)
 //
 // Informs the compiler that a condition is always true and that it can assume
@@ -209,18 +255,23 @@
 #define ABSL_ASSUME(cond) assert(cond)
 #elif ABSL_HAVE_BUILTIN(__builtin_assume)
 #define ABSL_ASSUME(cond) __builtin_assume(cond)
+#elif defined(_MSC_VER)
+#define ABSL_ASSUME(cond) __assume(cond)
+#elif defined(__cpp_lib_unreachable) && __cpp_lib_unreachable >= 202202L
+#define ABSL_ASSUME(cond)            \
+  do {                               \
+    if (!(cond)) std::unreachable(); \
+  } while (false)
 #elif defined(__GNUC__) || ABSL_HAVE_BUILTIN(__builtin_unreachable)
 #define ABSL_ASSUME(cond)                 \
   do {                                    \
     if (!(cond)) __builtin_unreachable(); \
-  } while (0)
-#elif defined(_MSC_VER)
-#define ABSL_ASSUME(cond) __assume(cond)
+  } while (false)
 #else
 #define ABSL_ASSUME(cond)               \
   do {                                  \
     static_cast<void>(false && (cond)); \
-  } while (0)
+  } while (false)
 #endif
 
 // ABSL_INTERNAL_UNIQUE_SMALL_NAME(cond)