5 files changed, 64 insertions, 6 deletions

diff --git a/libstdc++-v3/include/bits/atomic_base.h b/libstdc++-v3/include/bits/atomic_base.h
index 7e4ad2b..ccea132 100644
--- a/libstdc++-v3/include/bits/atomic_base.h
+++ b/libstdc++-v3/include/bits/atomic_base.h
@@ -1535,7 +1535,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       }
 
       static consteval int
-      _S_required_aligment()
+      _S_required_alignment()
       {
 	if constexpr (is_floating_point_v<_Vt> || is_pointer_v<_Vt>)
 	  return __alignof__(_Vt);
@@ -1555,7 +1555,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       static_assert(is_always_lock_free || !is_volatile_v<_Tp>,
 	"atomic operations on volatile T must be lock-free");
 
-      static constexpr size_t required_alignment = _S_required_aligment();
+      static constexpr size_t required_alignment = _S_required_alignment();
 
       __atomic_ref_base() = delete;
       __atomic_ref_base& operator=(const __atomic_ref_base&) = delete;
diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index 1118da4..f60a518 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1951,6 +1951,15 @@ ftms = {
 };
 
 ftms = {
+  name = observable_checkpoint;
+  values = {
+    v = 202506;
+    cxxmin = 26;
+    extra_cond = "__has_builtin(__builtin_observable_checkpoint)";
+  };
+};
+
+ftms = {
   name = algorithm_default_value_type;
   values = {
     v = 202403;
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index c452bbe..cbd82ff 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -2181,6 +2181,16 @@
 #endif /* !defined(__cpp_lib_unreachable) */
 #undef __glibcxx_want_unreachable
 
+#if !defined(__cpp_lib_observable_checkpoint)
+# if (__cplusplus >  202302L) && (__has_builtin(__builtin_observable_checkpoint))
+#  define __glibcxx_observable_checkpoint 202506L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_observable_checkpoint)
+#   define __cpp_lib_observable_checkpoint 202506L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_observable_checkpoint) && defined(__glibcxx_want_observable_checkpoint) */
+#undef __glibcxx_want_observable_checkpoint
+
 #if !defined(__cpp_lib_algorithm_default_value_type)
 # if (__cplusplus >  202302L)
 #  define __glibcxx_algorithm_default_value_type 202403L
diff --git a/libstdc++-v3/include/std/stacktrace b/libstdc++-v3/include/std/stacktrace
index b9e260e..587a163 100644
--- a/libstdc++-v3/include/std/stacktrace
+++ b/libstdc++-v3/include/std/stacktrace
@@ -683,13 +683,32 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   inline ostream&
   operator<<(ostream& __os, const stacktrace_entry& __f)
   {
+    if (!__f) [[unlikely]]
+      return __os << "<unknown>";
+
     string __desc, __file;
     int __line;
-    if (__f._M_get_info(&__desc, &__file, &__line))
+    if (__f._M_get_info(&__desc, &__file, &__line)) [[likely]]
       {
-	__os.width(4);
-	__os << __desc << " at " << __file << ':' << __line;
+	__os << ' ';
+	if (__desc.empty()) [[unlikely]]
+	  __os << "<unknown>";
+	else
+	  __os << __desc;
+	if (!__file.empty()) [[likely]]
+	  __os << " at " << __file << ':' << __line;
       }
+
+    struct _Flag_guard // Set and restore hex format
+    {
+      _Flag_guard(ios& __s) : _M_ios(__s) { }
+      ~_Flag_guard() { _M_ios.setf(_M_f); }
+
+      ios& _M_ios;
+      ios::fmtflags _M_f = _M_ios.setf(ios::hex, ios::basefield);
+    };
+    _Flag_guard __g(__os);
+    __os << " [0x" << __f.native_handle() << ']';
     return __os;
   }
 
@@ -697,7 +716,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     inline ostream&
     operator<<(ostream& __os, const basic_stacktrace<_Allocator>& __st)
     {
-      for (stacktrace::size_type __i = 0; __i < __st.size(); ++__i)
+      using size_type = typename basic_stacktrace<_Allocator>::size_type;
+      for (size_type __i = 0, __size = __st.size(); __i < __size; ++__i)
 	{
 	  __os.width(4);
 	  __os << __i << "# " << __st[__i] << '\n';
diff --git a/libstdc++-v3/include/std/utility b/libstdc++-v3/include/std/utility
index 8a85ccf..3ae1852 100644
--- a/libstdc++-v3/include/std/utility
+++ b/libstdc++-v3/include/std/utility
@@ -98,6 +98,7 @@
 #define __glibcxx_want_tuple_element_t
 #define __glibcxx_want_tuples_by_type
 #define __glibcxx_want_unreachable
+#define __glibcxx_want_observable_checkpoint
 #define __glibcxx_want_tuple_like
 #define __glibcxx_want_constrained_equality
 #include <bits/version.h>
@@ -234,6 +235,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   }
 #endif
 
+
+#ifdef __cpp_lib_observable_checkpoint // C++ >= 26
+  /// Informs the compiler that prior actions are considered observable.
+  /**
+   * This may be used to limit the extent to which optimisations based on the
+   * assumed unreachability of undefined behaviour can propagate to earlier
+   * code.
+   *
+   * @since C++26
+   */
+  [[__gnu__::__always_inline__]]
+  inline void
+  observable_checkpoint() noexcept
+  {
+    return __builtin_observable_checkpoint();
+  }
+#endif
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace