libstdc++: Implement C++26 <debugging> features [PR119670]

This implements P2546R5 (Debugging Support), including the P2810R4
(is_debugger_present is_replaceable) changes, allowing
std::is_debugger_present to be replaced by the program.

It would be good to provide a macOS definition of is_debugger_present as
per https://developer.apple.com/library/archive/qa/qa1361/_index.html
but that isn't included in this change.

The src/c++26/debugging.cc file defines a global volatile int which can
be set by debuggers to indicate when they are attached and detached from
a running process. This allows std::is_debugger_present() to give a
reliable answer, and additionally allows a debugger to choose how
std::breakpoint() should behave. Setting the global to a positive value
will cause std::breakpoint() to use that value as an argument to
std::raise, so debuggers that prefer SIGABRT for breakpoints can select
that. By default std::breakpoint() will use a platform-specific action
such as the INT3 instruction on x86, or GCC's __builtin_trap().

On Linux the std::is_debugger_present() function checks whether the
process is being traced by a process named "gdb", "gdbserver" or
"lldb-server", to try to avoid interpreting other tracing processes
(such as strace) as a debugger. There have been comments suggesting this
isn't desirable and that std::is_debugger_present() should just return
true for any tracing process (which is the case for non-Linux targets
that support the ptrace system call).

libstdc++-v3/ChangeLog:

	PR libstdc++/119670
	* acinclude.m4 (GLIBCXX_CHECK_DEBUGGING): Check for facilities
	needed by <debugging>.
	* config.h.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Use GLIBCXX_CHECK_DEBUGGING.
	* include/Makefile.am: Add new header.
	* include/Makefile.in: Regenerate.
	* include/bits/version.def (debugging): Add.
	* include/bits/version.h: Regenerate.
	* include/precompiled/stdc++.h: Add new header.
	* src/c++26/Makefile.am: Add new file.
	* src/c++26/Makefile.in: Regenerate.
	* include/std/debugging: New file.
	* src/c++26/debugging.cc: New file.
	* testsuite/19_diagnostics/debugging/breakpoint.cc: New test.
	* testsuite/19_diagnostics/debugging/breakpoint_if_debugging.cc:
	New test.
	* testsuite/19_diagnostics/debugging/is_debugger_present.cc: New
	test.
	* testsuite/19_diagnostics/debugging/is_debugger_present-2.cc:
	New test.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>

17 files changed, 464 insertions, 3 deletions

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 027735e..eb2d262 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -5769,6 +5769,42 @@ AC_LANG_SAVE
   AC_LANG_RESTORE
 ])
 
+dnl
+dnl Check whether the dependencies for std::is_debugger_present are available.
+dnl
+dnl Defines:
+dnl   _GLIBCXX_USE_PTRACE if ptrace(int, pid_t, int, int) is in <sys/ptrace.h>.
+dnl   _GLIBCXX_USE_PROC_SELF_STATUS if /proc/self/status should be used.
+dnl
+AC_DEFUN([GLIBCXX_CHECK_DEBUGGING], [
+  AC_LANG_SAVE
+  AC_LANG_CPLUSPLUS
+
+  AC_CHECK_HEADERS([sys/ptrace.h debugapi.h])
+
+  case "$target_os" in
+    linux*)
+      AC_DEFINE([_GLIBCXX_USE_PROC_SELF_STATUS],1,
+		[Define if /proc/self/status should be used for <debugging>.])
+      ;;
+  esac
+
+  AC_MSG_CHECKING([whether ptrace(int, pid_t, int, int) is in <sys/ptrace.h>])
+  AC_TRY_COMPILE([
+  #include <sys/ptrace.h>
+  ],[
+    int i = ptrace(PTRACE_TRACEME, (pid_t)0, 1, 0);
+  ], [ac_ptrace=yes], [ac_ptrace=no])
+  AC_MSG_RESULT($ac_ptrace)
+  if test "$ac_ptrace" = yes; then
+    AC_DEFINE_UNQUOTED(_GLIBCXX_USE_PTRACE, 1,
+      [Define if ptrace should be used for std::is_debugger_present.])
+  fi
+
+  AC_LANG_RESTORE
+])
+
+
 # Macros from the top-level gcc directory.
 m4_include([../config/gc++filt.m4])
 m4_include([../config/tls.m4])
diff --git a/libstdc++-v3/config.h.in b/libstdc++-v3/config.h.in
index ffacdab..818117a 100644
--- a/libstdc++-v3/config.h.in
+++ b/libstdc++-v3/config.h.in
@@ -70,6 +70,9 @@
 /* Define to 1 if you have the `cosl' function. */
 #undef HAVE_COSL
 
+/* Define to 1 if you have the <debugapi.h> header file. */
+#undef HAVE_DEBUGAPI_H
+
 /* Define to 1 if you have the declaration of `strnlen', and to 0 if you
    don't. */
 #undef HAVE_DECL_STRNLEN
@@ -435,6 +438,9 @@
 /* Define to 1 if you have the <sys/param.h> header file. */
 #undef HAVE_SYS_PARAM_H
 
+/* Define to 1 if you have the <sys/ptrace.h> header file. */
+#undef HAVE_SYS_PTRACE_H
+
 /* Define to 1 if you have the <sys/resource.h> header file. */
 #undef HAVE_SYS_RESOURCE_H
 
@@ -846,6 +852,9 @@
 /* Define if nl_langinfo_l should be used for std::text_encoding. */
 #undef _GLIBCXX_USE_NL_LANGINFO_L
 
+/* Define if /proc/self/status should be used for <debugging>. */
+#undef _GLIBCXX_USE_PROC_SELF_STATUS
+
 /* Define if pthreads_num_processors_np is available in <pthread.h>. */
 #undef _GLIBCXX_USE_PTHREADS_NUM_PROCESSORS_NP
 
@@ -862,6 +871,9 @@
 /* Define if POSIX read/write locks are available in <gthr.h>. */
 #undef _GLIBCXX_USE_PTHREAD_RWLOCK_T
 
+/* Define if ptrace should be used for std::is_debugger_present. */
+#undef _GLIBCXX_USE_PTRACE
+
 /* Define if /dev/random and /dev/urandom are available for the random_device
    of TR1 (Chapter 5.1). */
 #undef _GLIBCXX_USE_RANDOM_TR1
diff --git a/libstdc++-v3/configure b/libstdc++-v3/configure
index d57cfd5..c0eaeb9 100755
--- a/libstdc++-v3/configure
+++ b/libstdc++-v3/configure
@@ -54501,6 +54501,79 @@ ac_compiler_gnu=$ac_cv_c_compiler_gnu
 
 
 
+# For std::is_debugger_present
+
+
+  ac_ext=cpp
+ac_cpp='$CXXCPP $CPPFLAGS'
+ac_compile='$CXX -c $CXXFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CXX -o conftest$ac_exeext $CXXFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
+
+
+  for ac_header in sys/ptrace.h debugapi.h
+do :
+  as_ac_Header=`$as_echo "ac_cv_header_$ac_header" | $as_tr_sh`
+ac_fn_cxx_check_header_mongrel "$LINENO" "$ac_header" "$as_ac_Header" "$ac_includes_default"
+if eval test \"x\$"$as_ac_Header"\" = x"yes"; then :
+  cat >>confdefs.h <<_ACEOF
+#define `$as_echo "HAVE_$ac_header" | $as_tr_cpp` 1
+_ACEOF
+
+fi
+
+done
+
+
+  case "$target_os" in
+    linux*)
+
+$as_echo "#define _GLIBCXX_USE_PROC_SELF_STATUS 1" >>confdefs.h
+
+      ;;
+  esac
+
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ptrace(int, pid_t, int, int) is in <sys/ptrace.h>" >&5
+$as_echo_n "checking whether ptrace(int, pid_t, int, int) is in <sys/ptrace.h>... " >&6; }
+  cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+  #include <sys/ptrace.h>
+
+int
+main ()
+{
+
+    int i = ptrace(PTRACE_TRACEME, (pid_t)0, 1, 0);
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_cxx_try_compile "$LINENO"; then :
+  ac_ptrace=yes
+else
+  ac_ptrace=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $ac_ptrace" >&5
+$as_echo "$ac_ptrace" >&6; }
+  if test "$ac_ptrace" = yes; then
+
+cat >>confdefs.h <<_ACEOF
+#define _GLIBCXX_USE_PTRACE 1
+_ACEOF
+
+  fi
+
+  ac_ext=c
+ac_cpp='$CPP $CPPFLAGS'
+ac_compile='$CC -c $CFLAGS $CPPFLAGS conftest.$ac_ext >&5'
+ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $LIBS >&5'
+ac_compiler_gnu=$ac_cv_c_compiler_gnu
+
+
+
 # Define documentation rules conditionally.
 
 # See if makeinfo has been installed and is modern enough
diff --git a/libstdc++-v3/configure.ac b/libstdc++-v3/configure.ac
index a6c01b2..0bf2191 100644
--- a/libstdc++-v3/configure.ac
+++ b/libstdc++-v3/configure.ac
@@ -587,6 +587,9 @@ GLIBCXX_CHECK_FILEBUF_NATIVE_HANDLES
 # For std::text_encoding
 GLIBCXX_CHECK_TEXT_ENCODING
 
+# For std::is_debugger_present
+GLIBCXX_CHECK_DEBUGGING
+
 # Define documentation rules conditionally.
 
 # See if makeinfo has been installed and is modern enough
diff --git a/libstdc++-v3/include/Makefile.am b/libstdc++-v3/include/Makefile.am
index 6f248fe..6f59469 100644
--- a/libstdc++-v3/include/Makefile.am
+++ b/libstdc++-v3/include/Makefile.am
@@ -68,6 +68,7 @@ std_headers = \
 	${std_srcdir}/codecvt \
 	${std_srcdir}/complex \
 	${std_srcdir}/condition_variable \
+	${std_srcdir}/debugging \
 	${std_srcdir}/deque \
 	${std_srcdir}/execution \
 	${std_srcdir}/filesystem \
diff --git a/libstdc++-v3/include/Makefile.in b/libstdc++-v3/include/Makefile.in
index 014466f..4b5917e 100644
--- a/libstdc++-v3/include/Makefile.in
+++ b/libstdc++-v3/include/Makefile.in
@@ -424,6 +424,7 @@ std_freestanding = \
 @GLIBCXX_HOSTED_TRUE@	${std_srcdir}/codecvt \
 @GLIBCXX_HOSTED_TRUE@	${std_srcdir}/complex \
 @GLIBCXX_HOSTED_TRUE@	${std_srcdir}/condition_variable \
+@GLIBCXX_HOSTED_TRUE@	${std_srcdir}/debugging \
 @GLIBCXX_HOSTED_TRUE@	${std_srcdir}/deque \
 @GLIBCXX_HOSTED_TRUE@	${std_srcdir}/execution \
 @GLIBCXX_HOSTED_TRUE@	${std_srcdir}/filesystem \
diff --git a/libstdc++-v3/include/bits/version.def b/libstdc++-v3/include/bits/version.def
index 9d5122f..84c755d 100644
--- a/libstdc++-v3/include/bits/version.def
+++ b/libstdc++-v3/include/bits/version.def
@@ -1907,6 +1907,14 @@ ftms = {
 };
 
 ftms = {
+  name = debugging;
+  values = {
+    v = 202403;
+    cxxmin = 26;
+  };
+};
+
+ftms = {
   name = fstream_native_handle;
   values = {
     v = 202306;
diff --git a/libstdc++-v3/include/bits/version.h b/libstdc++-v3/include/bits/version.h
index 6feeaa4..410e320 100644
--- a/libstdc++-v3/include/bits/version.h
+++ b/libstdc++-v3/include/bits/version.h
@@ -2134,6 +2134,16 @@
 #endif /* !defined(__cpp_lib_constexpr_new) && defined(__glibcxx_want_constexpr_new) */
 #undef __glibcxx_want_constexpr_new
 
+#if !defined(__cpp_lib_debugging)
+# if (__cplusplus >  202302L)
+#  define __glibcxx_debugging 202403L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_debugging)
+#   define __cpp_lib_debugging 202403L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_debugging) && defined(__glibcxx_want_debugging) */
+#undef __glibcxx_want_debugging
+
 #if !defined(__cpp_lib_fstream_native_handle)
 # if (__cplusplus >  202302L) && _GLIBCXX_HOSTED
 #  define __glibcxx_fstream_native_handle 202306L
diff --git a/libstdc++-v3/include/precompiled/stdc++.h b/libstdc++-v3/include/precompiled/stdc++.h
index 733a5e5..636632a 100644
--- a/libstdc++-v3/include/precompiled/stdc++.h
+++ b/libstdc++-v3/include/precompiled/stdc++.h
@@ -237,6 +237,7 @@
 #endif
 
 #if __cplusplus > 202302L
+#include <debugging>
 #include <inplace_vector>
 #include <text_encoding>
 #include <stdbit.h>
diff --git a/libstdc++-v3/include/std/debugging b/libstdc++-v3/include/std/debugging
new file mode 100644
index 0000000..4cf7e4a
--- /dev/null
+++ b/libstdc++-v3/include/std/debugging
@@ -0,0 +1,77 @@
+// Debugging support -*- C++ -*-
+
+// Copyright The GNU Toolchain Authors.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+/** @file include/debugging
+ *  This is a Standard C++ Library header.
+ */
+
+#ifndef _GLIBCXX_DEBUGGING
+#define _GLIBCXX_DEBUGGING 1
+
+#define __glibcxx_want_debugging
+#include <bits/version.h>
+
+#if __cpp_lib_debugging // C++ >= 26
+namespace std _GLIBCXX_VISIBILITY(default)
+{
+// N.B. _GLIBCXX_BEGIN_NAMESPACE_VERSION is not used here.
+
+/** Try to determine if the program is running under control of a debugger.
+ *
+ * On GNU/Linux systems this function will only return true if the program
+ * is being traced by another program which is known to be a debugger.
+ * This is determined by checking the command name of the tracing program
+ * against a list of known debuggers, such as "gdb".
+ *
+ * On other POSIX-based systems, this function will return true if the
+ * program is being traced by any other process, which means it can return
+ * true for non-debugger utilities that use the ptrace system call.
+ *
+ * @since C++26
+ */
+bool
+is_debugger_present() noexcept;
+
+/** Stop the program with a breakpoint or debug trap.
+ *
+ * The details of how a breakpoint is implemented are platform-specific.
+ * Some systems provide a special instruction, such as `int3` in x86.
+ * When no more appropriate mechanism is available, this will stop the
+ * program using `__builtin_trap()`. It might not be possible for the
+ * program to continue after such a breakpoint.
+ *
+ * @since C++26
+ */
+void
+breakpoint() noexcept;
+
+/** Stop the program if it is running under control of a debugger.
+ *
+ * @since C++26
+ */
+void
+breakpoint_if_debugging() noexcept;
+
+} // namespace std
+#endif
+#endif // _GLIBCXX_DEBUGGING
diff --git a/libstdc++-v3/src/c++26/Makefile.am b/libstdc++-v3/src/c++26/Makefile.am
index 5defa4a..4123b7d 100644
--- a/libstdc++-v3/src/c++26/Makefile.am
+++ b/libstdc++-v3/src/c++26/Makefile.am
@@ -35,7 +35,9 @@ else
 inst_sources =
 endif
 
-sources = text_encoding.cc
+sources = \
+	  debugging.cc \
+	  text_encoding.cc
 
 vpath % $(top_srcdir)/src/c++26
 
diff --git a/libstdc++-v3/src/c++26/Makefile.in b/libstdc++-v3/src/c++26/Makefile.in
index 77e73b2..1c317d6 100644
--- a/libstdc++-v3/src/c++26/Makefile.in
+++ b/libstdc++-v3/src/c++26/Makefile.in
@@ -121,7 +121,7 @@ CONFIG_CLEAN_FILES =
 CONFIG_CLEAN_VPATH_FILES =
 LTLIBRARIES = $(noinst_LTLIBRARIES)
 libc__26convenience_la_LIBADD =
-am__objects_1 = text_encoding.lo
+am__objects_1 = debugging.lo text_encoding.lo
 am__objects_2 =
 @GLIBCXX_HOSTED_TRUE@am_libc__26convenience_la_OBJECTS =  \
 @GLIBCXX_HOSTED_TRUE@	$(am__objects_1) $(am__objects_2)
@@ -430,7 +430,10 @@ headers =
 
 # XTEMPLATE_FLAGS = -fno-implicit-templates
 @ENABLE_EXTERN_TEMPLATE_TRUE@inst_sources = 
-sources = text_encoding.cc
+sources = \
+	  debugging.cc \
+	  text_encoding.cc
+
 @GLIBCXX_HOSTED_FALSE@libc__26convenience_la_SOURCES = 
 @GLIBCXX_HOSTED_TRUE@libc__26convenience_la_SOURCES = $(sources)  $(inst_sources)
 
diff --git a/libstdc++-v3/src/c++26/debugging.cc b/libstdc++-v3/src/c++26/debugging.cc
new file mode 100644
index 0000000..72e305d
--- /dev/null
+++ b/libstdc++-v3/src/c++26/debugging.cc
@@ -0,0 +1,175 @@
+// Implementation of <debugging> -*- C++ -*-
+
+// Copyright The GNU Toolchain Authors.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// Under Section 7 of GPL version 3, you are granted additional
+// permissions described in the GCC Runtime Library Exception, version
+// 3.1, as published by the Free Software Foundation.
+
+// You should have received a copy of the GNU General Public License and
+// a copy of the GCC Runtime Library Exception along with this program;
+// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
+// <http://www.gnu.org/licenses/>.
+
+#include <debugging>
+
+#if __cpp_lib_debugging
+
+#include <csignal> // std::raise
+
+#if _GLIBCXX_USE_PROC_SELF_STATUS
+# include <fstream>
+# include <string>
+#endif
+
+#if _GLIBCXX_HAVE_SYS_PTRACE_H
+# include <sys/ptrace.h>
+# include <errno.h>
+#endif
+
+#if _GLIBCXX_HAVE_DEBUGAPI_H
+# include <debugapi.h>
+#endif
+
+#ifdef _GLIBCXX_HAVE_SYS_SDT_H
+# include <sys/sdt.h>
+/* We only want to use stap probes starting with v3.  Earlier versions
+   added too much startup cost.  */
+# if defined (STAP_PROBE) && _SDT_NOTE_TYPE >= 3
+#  define PROBE(name) STAP_PROBE(libstdcxx, name)
+# endif
+#endif
+
+#ifndef PROBE
+# define PROBE(name)
+#endif
+
+namespace __gnu_cxx
+{
+  // This should be changed to non-zero by debuggers when they attach
+  // and back to zero when they detach.
+  // If the value is positive, std::breakpoint() will use it as the argument
+  // to std::raise, so it should be a valid signal number, e.g. SIGABRT or
+  // SIGTRAP.
+  // If the value is negative, std::breakpoint() will use a target-specific
+  // trap, e.g. asm("int3") or __builtin_trap().
+  volatile int debugger_signal_for_breakpoint = 0;
+}
+
+_GLIBCXX_WEAK_DEFINITION
+bool
+std::is_debugger_present() noexcept
+{
+  PROBE(std::is_debugger_present);
+
+  if (__gnu_cxx::debugger_signal_for_breakpoint != 0)
+    return true;
+
+#if _GLIBCXX_HOSTED
+# if _GLIBCXX_USE_PROC_SELF_STATUS
+  const string_view prefix = "TracerPid:\t"; // populated since Linux 2.6.0
+  ifstream in("/proc/self/status");
+  string line;
+  while (std::getline(in, line))
+    {
+      if (!line.starts_with(prefix))
+	continue;
+
+      string_view tracer = line;
+      tracer.remove_prefix(prefix.size());
+      if (tracer.size() == 1 && tracer[0] == '0') [[likely]]
+	return false; // Not being traced.
+
+      in.close();
+      string_view cmd;
+      string proc_dir = "/proc/" + string(tracer) + '/';
+      in.open(proc_dir + "comm"); // since Linux 2.6.33
+      if (std::getline(in, line)) [[likely]]
+	cmd = line;
+      else
+	{
+	  in.close();
+	  in.open(proc_dir + "cmdline");
+	  if (std::getline(in, line))
+	    cmd = line.c_str(); // Only up to first '\0'
+	  else
+	    return false;
+	}
+
+      for (auto i : {"gdb", "gdbserver", "lldb-server"}) // known debuggers
+	if (cmd.ends_with(i))
+	  return true;
+
+      // We found the TracerPid line, no need to do any more work.
+      return false;
+    }
+# elif _GLIBCXX_USE_PTRACE
+  if (::ptrace(PTRACE_TRACEME, 0, 1, 0) == -1)
+    return errno == EPERM;
+# endif
+# if _GLIBCXX_HAVE_DEBUGAPI_H && defined(_WIN32) && !defined(__CYGWIN__)
+  return IsDebuggerPresent();
+# endif
+#endif // HOSTED
+  return false;
+}
+
+void
+std::breakpoint() noexcept
+{
+  PROBE(std::breakpoint);
+
+  if (__gnu_cxx::debugger_signal_for_breakpoint > 0)
+    std::raise(__gnu_cxx::debugger_signal_for_breakpoint);
+
+#if _GLIBCXX_HAVE_DEBUGAPI_H && defined(_WIN32) && !defined(__CYGWIN__)
+  DebugBreak();
+#elif __has_builtin(__builtin_debugtrap)
+  __builtin_debugtrap(); // Clang
+#elif defined(__i386__) || defined(__x86_64__)
+  // nop is for GDB, see https://sourceware.org/bugzilla/show_bug.cgi?id=31194
+  __asm__ volatile ("int3\n\tnop");
+#elifdef __thumb__
+  __asm__ volatile (".inst 0xde01");
+#elifdef __aarch64__
+  __asm__ volatile (".inst 0xd4200000");
+#elifdef __arm__
+  __asm__ volatile (".inst 0xe7f001f0");
+#elifdef __riscv
+  /* section 2.8 in the RISC-V unprivileged ISA manual says for semi-hosted
+   * environments we want the sequence:
+   * slli x0, x0, 0x1f     # Entry NOP
+   * ebreak         # Break to debugger
+   * srai x0, x0, 7    # NOP encoding the semihosting call number 7
+   */
+  __asm__ volatile (".4byte 0x00100073");
+#elif defined __powerpc__ && ! defined _AIX
+  __asm__ volatile(".4byte 0x7d821008");
+#else
+  __builtin_trap();
+#endif
+} // If the debugger stops here, std::breakpoint() was called.
+
+// This is intentionally not defined inline. A non-inline definition allows
+// debuggers to insert a breakpoint on calls to the function, avoiding the
+// overhead of calling `is_debugger_present()`.
+void
+std::breakpoint_if_debugging() noexcept
+{
+  PROBE(std::breakpoint_if_debugging);
+
+  if (std::is_debugger_present()) [[unlikely]]
+    std::breakpoint();
+}
+
+#endif // __cpp_lib_debugging
diff --git a/libstdc++-v3/testsuite/19_diagnostics/debugging/breakpoint.cc b/libstdc++-v3/testsuite/19_diagnostics/debugging/breakpoint.cc
new file mode 100644
index 0000000..ad24177
--- /dev/null
+++ b/libstdc++-v3/testsuite/19_diagnostics/debugging/breakpoint.cc
@@ -0,0 +1,13 @@
+// { dg-do run { target c++26 xfail c++26 } }
+// { dg-options "-lstdc++exp" }
+// { dg-require-cpp-feature-test __cpp_lib_debugging }
+#include <debugging>
+#include <type_traits>
+
+static_assert( noexcept(std::breakpoint()) );
+static_assert( std::is_void_v<decltype(std::breakpoint())> );
+
+int main()
+{
+  std::breakpoint();
+}
diff --git a/libstdc++-v3/testsuite/19_diagnostics/debugging/breakpoint_if_debugging.cc b/libstdc++-v3/testsuite/19_diagnostics/debugging/breakpoint_if_debugging.cc
new file mode 100644
index 0000000..2646183
--- /dev/null
+++ b/libstdc++-v3/testsuite/19_diagnostics/debugging/breakpoint_if_debugging.cc
@@ -0,0 +1,13 @@
+// { dg-do run { target c++26 } }
+// { dg-options "-lstdc++exp" }
+// { dg-require-cpp-feature-test __cpp_lib_debugging }
+#include <debugging>
+#include <type_traits>
+
+static_assert( noexcept(std::breakpoint_if_debugging()) );
+static_assert( std::is_void_v<decltype(std::breakpoint_if_debugging())> );
+
+int main()
+{
+  std::breakpoint_if_debugging();
+}
diff --git a/libstdc++-v3/testsuite/19_diagnostics/debugging/is_debugger_present-2.cc b/libstdc++-v3/testsuite/19_diagnostics/debugging/is_debugger_present-2.cc
new file mode 100644
index 0000000..aa4690c
--- /dev/null
+++ b/libstdc++-v3/testsuite/19_diagnostics/debugging/is_debugger_present-2.cc
@@ -0,0 +1,19 @@
+// { dg-do run { target c++26 } }
+// { dg-options "-lstdc++exp" }
+// { dg-require-cpp-feature-test __cpp_lib_debugging }
+// { dg-xfail-run-if "no replaceable functions on AIX" { powerpc-ibm-aix* } }
+
+// P2810R4 is_debugger_present is_replaceable
+
+#include <debugging>
+#include <testsuite_hooks.h>
+
+bool called = false;
+
+bool std::is_debugger_present() noexcept { called = true; return true; }
+
+int main()
+{
+  VERIFY( std::is_debugger_present() );
+  VERIFY( called );
+}
diff --git a/libstdc++-v3/testsuite/19_diagnostics/debugging/is_debugger_present.cc b/libstdc++-v3/testsuite/19_diagnostics/debugging/is_debugger_present.cc
new file mode 100644
index 0000000..8dbfa69
--- /dev/null
+++ b/libstdc++-v3/testsuite/19_diagnostics/debugging/is_debugger_present.cc
@@ -0,0 +1,14 @@
+// { dg-do run { target c++26 } }
+// { dg-options "-lstdc++exp" }
+// { dg-require-cpp-feature-test __cpp_lib_debugging }
+#include <debugging>
+#include <type_traits>
+#include <testsuite_hooks.h>
+
+static_assert( noexcept(std::is_debugger_present()) );
+static_assert( std::is_same_v<decltype(std::is_debugger_present()), bool> );
+
+int main()
+{
+  VERIFY( ! std::is_debugger_present() );
+}