[PATCH] libstdc++: Implement std::unreachable() for C++23 (P0627R6)

[Jonathan Wakely <jwakely@redhat.com>]

This is a tiny C++23 feature that I plan to add for GCC 12. Does anybody
have any comments on the choices below in terms of when to make reaching
std::unreachable do an assert/trap/unreachable?

My thoughts on avoiding the overhead in the _GLIBCXX_ASSERTIONS case is
that this differs from e.g. assertions in operator[] where we want to
catch accidental bad indices. A std::unreachable() call should not be
reached accidentally. I hope it will only be used for conditions that
really are unreachable, and probably quite often where performance
matters. If using std::unreachable() increased code size significantly
then it would make it much less useful for guiding optimizations.


-- >8 --

This defines std::unreachable as an assertion for debug mode, a trap
when _GLIBCXX_ASSERTIONS is defined, and __builtin_unreachable()
otherwise.

The reason for only using __builtin_trap() in the second case is to
avoid the overhead of setting up a call to __glibcxx_assert_fail that
should never happen.

While thinking about what the debug assertion failure should print, I
noticed that the __glibcxx_assert_fail function doesn't check for null
pointers. This adds a check so we don't try to print them if null.

libstdc++-v3/ChangeLog:

	* include/std/utility (unreachable): Define for C++23.
	* include/std/version (__cpp_lib_unreachable): Define.
	* src/c++11/debug.cc (__glibcxx_assert_fail): Check for valid
	arguments.
	* testsuite/20_util/unreachable/1.cc: New test.
	* testsuite/20_util/unreachable/version.cc: New test.
---
 libstdc++-v3/include/std/utility                | 15 +++++++++++++++
 libstdc++-v3/include/std/version                |  1 +
 libstdc++-v3/src/c++11/debug.cc                 |  5 +++--
 libstdc++-v3/testsuite/20_util/unreachable/1.cc | 17 +++++++++++++++++
 .../testsuite/20_util/unreachable/version.cc    | 10 ++++++++++
 5 files changed, 46 insertions(+), 2 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/20_util/unreachable/1.cc
 create mode 100644 libstdc++-v3/testsuite/20_util/unreachable/version.cc

diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index 0d7f8954c5a..e5b5212381d 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -186,6 +186,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     constexpr underlying_type_t<_Tp>
     to_underlying(_Tp __value) noexcept
     { return static_cast<underlying_type_t<_Tp>>(__value); }
+
+#define __cpp_lib_unreachable 202202L
+  [[noreturn,__gnu__::__always_inline__]]
+  void
+  unreachable()
+  {
+#ifdef _GLIBCXX_DEBUG
+    std::__glibcxx_assert_fail("<utility>", 0, "std::unreachable()",
+			       "inconceivable!");
+#elif defined _GLIBCXX_ASSERTIONS
+    __builtin_trap();
+#else
+    __builtin_unreachable();
+#endif
+  }
 #endif // C++23
 #endif // C++20
 #endif // C++17
diff --git a/libstdc++-v3/include/std/version b/libstdc++-v3/include/std/version
index 44b8a9f88b5..51f2110b68e 100644
--- a/libstdc++-v3/include/std/version
+++ b/libstdc++-v3/include/std/version
@@ -326,6 +326,7 @@
 # define __cpp_lib_string_resize_and_overwrite 202110L
 #endif
 #define __cpp_lib_to_underlying 202102L
+#define __cpp_lib_unreachable 202202L
 #endif
 #endif // C++2b
 #endif // C++20
diff --git a/libstdc++-v3/src/c++11/debug.cc b/libstdc++-v3/src/c++11/debug.cc
index 98fe2dcc153..ff3723eb93b 100644
--- a/libstdc++-v3/src/c++11/debug.cc
+++ b/libstdc++-v3/src/c++11/debug.cc
@@ -52,8 +52,9 @@ namespace std
   __glibcxx_assert_fail(const char* file, int line,
 			const char* function, const char* condition) noexcept
   {
-    fprintf(stderr, "%s:%d: %s: Assertion '%s' failed.\n",
-		      file, line, function, condition);
+    if (file && function && condition)
+      fprintf(stderr, "%s:%d: %s: Assertion '%s' failed.\n",
+	      file, line, function, condition);
     abort();
   }
 }
diff --git a/libstdc++-v3/testsuite/20_util/unreachable/1.cc b/libstdc++-v3/testsuite/20_util/unreachable/1.cc
new file mode 100644
index 00000000000..0c463d52a48
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/unreachable/1.cc
@@ -0,0 +1,17 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do compile { target c++23 } }
+
+#include <utility>
+
+#ifndef __cpp_lib_unreachable
+# error "Feature-test macro for unreachable missing in <utility>"
+#elif __cpp_lib_unreachable != 202202L
+# error "Feature-test macro for unreachable has wrong value in <utility>"
+#endif
+
+bool test01(int i)
+{
+  if (i == 4)
+    return true;
+  std::unreachable();
+} // { dg-bogus "control reaches end of non-void function" }
diff --git a/libstdc++-v3/testsuite/20_util/unreachable/version.cc b/libstdc++-v3/testsuite/20_util/unreachable/version.cc
new file mode 100644
index 00000000000..c7795900c30
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/unreachable/version.cc
@@ -0,0 +1,10 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do preprocess { target c++23 } }
+
+#include <version>
+
+#ifndef __cpp_lib_unreachable
+# error "Feature-test macro for unreachable missing in <version>"
+#elif __cpp_lib_unreachable != 202202L
+# error "Feature-test macro for unreachable has wrong value in <version>"
+#endif
-- 
2.34.1