[PATCH] libatomic: Provide gthr.h default implementation

[Sebastian Huber <sebastian.huber@embedded-brains.de>]

Build libatomic for all targets.  Use gthr.h to provide a default
implementation.  If the thread model is "single", then this implementation will
not work if for example atomic operations are used for thread/interrupt
synchronization.

libatomic/ChangeLog:

	* Makefile.am (BUILT_SOURCES): New.
	(gthr.h): Link from libgcc source file if USE_CONFIG_GTHR is enabled.
	(gthr-default.h): Likewise.
	* Makefile.in: Regenerate.
	* configure: Likewise.
	* configure.ac: Map thread model to thread header.
	(thread_header): New substitution.
	(USE_CONFIG_GTHR): New automake conditional.
	* configure.tgt (*-*-elf*): Delete.
	(UNSUPPORTED): Likewise.
	(USE_CONFIG_GTHR): Define as default.
	* testsuite/Makefile.in: Regenerate.
	* config/gthr/host-config.h: New file.
	* config/gthr/lock.c: Likewise.
---
 libatomic/Makefile.am               |  12 +++
 libatomic/Makefile.in               |  40 +++++---
 libatomic/config/gthr/host-config.h |  55 +++++++++++
 libatomic/config/gthr/lock.c        | 136 ++++++++++++++++++++++++++++
 libatomic/configure                 |  40 +++++++-
 libatomic/configure.ac              |  10 +-
 libatomic/configure.tgt             |  16 +---
 libatomic/testsuite/Makefile.in     |   4 +-
 8 files changed, 281 insertions(+), 32 deletions(-)
 create mode 100644 libatomic/config/gthr/host-config.h
 create mode 100644 libatomic/config/gthr/lock.c

diff --git a/libatomic/Makefile.am b/libatomic/Makefile.am
index 41e5da28512..d56d553c9d5 100644
--- a/libatomic/Makefile.am
+++ b/libatomic/Makefile.am
@@ -34,6 +34,18 @@ search_path = $(addprefix $(top_srcdir)/config/, $(config_path)) \
 
 vpath % $(strip $(search_path))
 
+BUILT_SOURCES =
+
+if USE_CONFIG_GTHR
+gthr.h: $(top_srcdir)/../libgcc/gthr.h
+	-$(LN_S) $< $@
+
+gthr-default.h: $(top_srcdir)/../libgcc/$(thread_header)
+	-$(LN_S) $< $@
+
+BUILT_SOURCES += gthr.h gthr-default.h
+endif
+
 DEFAULT_INCLUDES = $(addprefix -I, $(search_path))
 AM_CFLAGS = $(XCFLAGS)
 AM_CCASFLAGS = $(XCFLAGS)
diff --git a/libatomic/Makefile.in b/libatomic/Makefile.in
index a0fa3dfc8cc..2e3ddd014f8 100644
--- a/libatomic/Makefile.in
+++ b/libatomic/Makefile.in
@@ -89,15 +89,16 @@ POST_UNINSTALL = :
 build_triplet = @build@
 host_triplet = @host@
 target_triplet = @target@
-@ARCH_AARCH64_LINUX_TRUE@@HAVE_IFUNC_TRUE@am__append_1 = $(foreach s,$(SIZES),$(addsuffix _$(s)_1_.lo,$(SIZEOBJS)))
-@ARCH_AARCH64_LINUX_TRUE@@HAVE_IFUNC_TRUE@am__append_2 = atomic_16.S
-@ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@am__append_3 = $(foreach \
+@USE_CONFIG_GTHR_TRUE@am__append_1 = gthr.h gthr-default.h
+@ARCH_AARCH64_LINUX_TRUE@@HAVE_IFUNC_TRUE@am__append_2 = $(foreach s,$(SIZES),$(addsuffix _$(s)_1_.lo,$(SIZEOBJS)))
+@ARCH_AARCH64_LINUX_TRUE@@HAVE_IFUNC_TRUE@am__append_3 = atomic_16.S
+@ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@am__append_4 = $(foreach \
 @ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@	s,$(SIZES),$(addsuffix \
 @ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@	_$(s)_1_.lo,$(SIZEOBJS))) \
 @ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@	$(addsuffix \
 @ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@	_8_2_.lo,$(SIZEOBJS))
-@ARCH_I386_TRUE@@HAVE_IFUNC_TRUE@am__append_4 = $(addsuffix _8_1_.lo,$(SIZEOBJS))
-@ARCH_X86_64_TRUE@@HAVE_IFUNC_TRUE@am__append_5 = $(addsuffix _16_1_.lo,$(SIZEOBJS)) \
+@ARCH_I386_TRUE@@HAVE_IFUNC_TRUE@am__append_5 = $(addsuffix _8_1_.lo,$(SIZEOBJS))
+@ARCH_X86_64_TRUE@@HAVE_IFUNC_TRUE@am__append_6 = $(addsuffix _16_1_.lo,$(SIZEOBJS)) \
 @ARCH_X86_64_TRUE@@HAVE_IFUNC_TRUE@		       $(addsuffix _16_2_.lo,$(SIZEOBJS))
 
 subdir = .
@@ -115,7 +116,8 @@ am__aclocal_m4_deps = $(top_srcdir)/../config/acx.m4 \
 	$(top_srcdir)/../ltversion.m4 $(top_srcdir)/../lt~obsolete.m4 \
 	$(top_srcdir)/acinclude.m4 $(top_srcdir)/../libtool.m4 \
 	$(top_srcdir)/../config/enable.m4 \
-	$(top_srcdir)/../config/cet.m4 $(top_srcdir)/configure.ac
+	$(top_srcdir)/../config/cet.m4 $(top_srcdir)/../config/gthr.m4 \
+	$(top_srcdir)/configure.ac
 am__configure_deps = $(am__aclocal_m4_deps) $(CONFIGURE_DEPENDENCIES) \
 	$(ACLOCAL_M4)
 DIST_COMMON = $(srcdir)/Makefile.am $(top_srcdir)/configure \
@@ -392,6 +394,7 @@ target_alias = @target_alias@
 target_cpu = @target_cpu@
 target_os = @target_os@
 target_vendor = @target_vendor@
+thread_header = @thread_header@
 tmake_file = @tmake_file@
 toolexecdir = @toolexecdir@
 toolexeclibdir = @toolexeclibdir@
@@ -404,6 +407,7 @@ gcc_version := $(shell @get_gcc_base_ver@ $(top_srcdir)/../gcc/BASE-VER)
 search_path = $(addprefix $(top_srcdir)/config/, $(config_path)) \
 	$(top_srcdir) $(top_builddir)
 
+BUILT_SOURCES = $(am__append_1)
 DEFAULT_INCLUDES = $(addprefix -I, $(search_path))
 AM_CFLAGS = $(XCFLAGS)
 AM_CCASFLAGS = $(XCFLAGS)
@@ -419,7 +423,7 @@ noinst_LTLIBRARIES = libatomic_convenience.la
 libatomic_version_info = -version-info $(libtool_VERSION)
 libatomic_la_LDFLAGS = $(libatomic_version_info) $(libatomic_version_script) $(lt_host_flags)
 libatomic_la_SOURCES = gload.c gstore.c gcas.c gexch.c glfree.c lock.c \
-	init.c fenv.c fence.c flag.c $(am__append_2)
+	init.c fenv.c fence.c flag.c $(am__append_3)
 SIZEOBJS = load store cas exch fadd fsub fand fior fxor fnand tas
 EXTRA_libatomic_la_SOURCES = $(addsuffix _n.c,$(SIZEOBJS))
 libatomic_la_DEPENDENCIES = $(libatomic_la_LIBADD) $(libatomic_version_dep)
@@ -445,8 +449,8 @@ all_c_files := $(foreach dir,$(search_path),$(wildcard $(dir)/*.c))
 # Then sort through them to find the one we want, and select the first.
 M_SRC = $(firstword $(filter %/$(M_FILE), $(all_c_files)))
 libatomic_la_LIBADD = $(foreach s,$(SIZES),$(addsuffix \
-	_$(s)_.lo,$(SIZEOBJS))) $(am__append_1) $(am__append_3) \
-	$(am__append_4) $(am__append_5)
+	_$(s)_.lo,$(SIZEOBJS))) $(am__append_2) $(am__append_4) \
+	$(am__append_5) $(am__append_6)
 @ARCH_AARCH64_LINUX_TRUE@@HAVE_IFUNC_TRUE@IFUNC_OPTIONS = -march=armv8-a+lse
 @ARCH_ARM_LINUX_TRUE@@HAVE_IFUNC_TRUE@IFUNC_OPTIONS = -march=armv7-a+fp -DHAVE_KERNEL64
 @ARCH_I386_TRUE@@HAVE_IFUNC_TRUE@IFUNC_OPTIONS = -march=i586
@@ -459,7 +463,7 @@ MULTIDIRS =
 MULTISUBDIR = 
 MULTIDO = true
 MULTICLEAN = true
-all: auto-config.h
+all: $(BUILT_SOURCES) auto-config.h
 	$(MAKE) $(AM_MAKEFLAGS) all-recursive
 
 .SUFFIXES:
@@ -741,14 +745,16 @@ distclean-tags:
 	-rm -f TAGS ID GTAGS GRTAGS GSYMS GPATH tags
 	-rm -f cscope.out cscope.in.out cscope.po.out cscope.files
 check-am: all-am
-check: check-recursive
+check: $(BUILT_SOURCES)
+	$(MAKE) $(AM_MAKEFLAGS) check-recursive
 all-am: Makefile $(LTLIBRARIES) auto-config.h all-local
 installdirs: installdirs-recursive
 installdirs-am:
 	for dir in "$(DESTDIR)$(toolexeclibdir)"; do \
 	  test -z "$$dir" || $(MKDIR_P) "$$dir"; \
 	done
-install: install-recursive
+install: $(BUILT_SOURCES)
+	$(MAKE) $(AM_MAKEFLAGS) install-recursive
 install-exec: install-exec-recursive
 install-data: install-data-recursive
 uninstall: uninstall-recursive
@@ -778,6 +784,7 @@ distclean-generic:
 maintainer-clean-generic:
 	@echo "This command is intended for maintainers to use"
 	@echo "it deletes files that may require special tools to rebuild."
+	-test -z "$(BUILT_SOURCES)" || rm -f $(BUILT_SOURCES)
 clean: clean-recursive
 
 clean-am: clean-generic clean-libtool clean-local \
@@ -854,7 +861,8 @@ ps-am:
 
 uninstall-am: uninstall-toolexeclibLTLIBRARIES
 
-.MAKE: $(am__recursive_targets) all install-am install-strip
+.MAKE: $(am__recursive_targets) all check install install-am \
+	install-strip
 
 .PHONY: $(am__recursive_targets) CTAGS GTAGS TAGS all all-am all-local \
 	am--refresh check check-am clean clean-cscope clean-generic \
@@ -878,6 +886,12 @@ uninstall-am: uninstall-toolexeclibLTLIBRARIES
 
 
 vpath % $(strip $(search_path))
+
+@USE_CONFIG_GTHR_TRUE@gthr.h: $(top_srcdir)/../libgcc/gthr.h
+@USE_CONFIG_GTHR_TRUE@	-$(LN_S) $< $@
+
+@USE_CONFIG_GTHR_TRUE@gthr-default.h: $(top_srcdir)/../libgcc/$(thread_header)
+@USE_CONFIG_GTHR_TRUE@	-$(LN_S) $< $@
 @LIBAT_BUILD_VERSIONED_SHLIB_SUN_TRUE@@LIBAT_BUILD_VERSIONED_SHLIB_TRUE@libatomic.map-sun : $(top_srcdir)/libatomic.map \
 @LIBAT_BUILD_VERSIONED_SHLIB_SUN_TRUE@@LIBAT_BUILD_VERSIONED_SHLIB_TRUE@		$(top_srcdir)/../contrib/make_sunver.pl \
 @LIBAT_BUILD_VERSIONED_SHLIB_SUN_TRUE@@LIBAT_BUILD_VERSIONED_SHLIB_TRUE@		$(libatomic_la_OBJECTS) $(libatomic_la_LIBADD)
diff --git a/libatomic/config/gthr/host-config.h b/libatomic/config/gthr/host-config.h
new file mode 100644
index 00000000000..5e529945eee
--- /dev/null
+++ b/libatomic/config/gthr/host-config.h
@@ -0,0 +1,55 @@
+/* Copyright (C) 2012-2022 Free Software Foundation, Inc.
+   Contributed by Richard Henderson <rth@redhat.com>.
+
+   This file is part of the GNU Atomic Library (libatomic).
+
+   Libatomic 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 of the License, or
+   (at your option) any later version.
+
+   Libatomic 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/>.  */
+
+/* Included after all more target-specific host-config.h.  */
+
+
+#ifndef protect_start_end
+# ifdef HAVE_ATTRIBUTE_VISIBILITY
+#  pragma GCC visibility push(hidden)
+# endif
+
+void libat_lock_1 (void *ptr);
+void libat_unlock_1 (void *ptr);
+
+static inline UWORD
+protect_start (void *ptr)
+{
+  libat_lock_1 (ptr);
+  return 0;
+}
+
+static inline void
+protect_end (void *ptr, UWORD dummy UNUSED)
+{
+  libat_unlock_1 (ptr);
+}
+
+# define protect_start_end 1
+# ifdef HAVE_ATTRIBUTE_VISIBILITY
+#  pragma GCC visibility pop
+# endif
+#endif /* protect_start_end */
+
+#include_next <host-config.h>
diff --git a/libatomic/config/gthr/lock.c b/libatomic/config/gthr/lock.c
new file mode 100644
index 00000000000..2d495ed3b56
--- /dev/null
+++ b/libatomic/config/gthr/lock.c
@@ -0,0 +1,136 @@
+/* Copyright (C) 2012-2022 Free Software Foundation, Inc.
+   Contributed by Richard Henderson <rth@redhat.com>.
+
+   This file is part of the GNU Atomic Library (libatomic).
+
+   Libatomic 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 of the License, or
+   (at your option) any later version.
+
+   Libatomic 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 "libatomic_i.h"
+#include "gthr.h"
+
+
+/* The target page size.  Must be no larger than the runtime page size,
+   lest locking fail with virtual address aliasing (i.e. a page mmaped
+   at two locations).  */
+#ifndef PAGE_SIZE
+#define PAGE_SIZE	4096
+#endif
+
+/* The target cacheline size.  This is an optimization; the padding that
+   should be applied to the locks to keep them from interfering.  */
+#ifndef CACHLINE_SIZE
+#define CACHLINE_SIZE	64
+#endif
+
+/* The granularity at which locks are applied.  Almost certainly the
+   cachline size is the right thing to use here.  */
+#ifndef WATCH_SIZE
+#define WATCH_SIZE	CACHLINE_SIZE
+#endif
+
+struct lock
+{
+  __gthread_mutex_t mutex;
+  char pad[sizeof(__gthread_mutex_t) < CACHLINE_SIZE
+	   ? CACHLINE_SIZE - sizeof(__gthread_mutex_t)
+	   : 0];
+};
+
+#define NLOCKS		(PAGE_SIZE / WATCH_SIZE)
+#ifdef __GTHREAD_MUTEX_INIT
+static struct lock locks[NLOCKS] = {
+  [0 ... NLOCKS-1].mutex = __GTHREAD_MUTEX_INIT
+};
+#else
+static __gthread_once_t locks_once = __GTHREAD_ONCE_INIT;
+
+static struct lock locks[NLOCKS];
+
+static void
+locks_init (void)
+{
+  size_t i;
+  for (i = 0; i < NLOCKS; ++i)
+    __GTHREAD_MUTEX_INIT_FUNCTION (&locks[i].mutex);
+}
+#endif
+
+static inline uintptr_t
+addr_hash (void *ptr)
+{
+  return ((uintptr_t)ptr / WATCH_SIZE) % NLOCKS;
+}
+
+void
+libat_lock_1 (void *ptr)
+{
+#ifndef __GTHREAD_MUTEX_INIT
+  __gthread_once (&locks_once, locks_init);
+#endif
+  __gthread_mutex_lock (&locks[addr_hash (ptr)].mutex);
+}
+
+void
+libat_unlock_1 (void *ptr)
+{
+  __gthread_mutex_unlock (&locks[addr_hash (ptr)].mutex);
+}
+
+void
+libat_lock_n (void *ptr, size_t n)
+{
+  uintptr_t h = addr_hash (ptr);
+  size_t i = 0;
+
+  /* Don't lock more than all the locks we have.  */
+  if (n > PAGE_SIZE)
+    n = PAGE_SIZE;
+
+  do
+    {
+#ifndef __GTHREAD_MUTEX_INIT
+      __gthread_once (&locks_once, locks_init);
+#endif
+      __gthread_mutex_lock (&locks[h].mutex);
+      if (++h == NLOCKS)
+	h = 0;
+      i += WATCH_SIZE;
+    }
+  while (i < n);
+}
+
+void
+libat_unlock_n (void *ptr, size_t n)
+{
+  uintptr_t h = addr_hash (ptr);
+  size_t i = 0;
+
+  if (n > PAGE_SIZE)
+    n = PAGE_SIZE;
+
+  do
+    {
+      __gthread_mutex_unlock (&locks[h].mutex);
+      if (++h == NLOCKS)
+	h = 0;
+      i += WATCH_SIZE;
+    }
+  while (i < n);
+}
diff --git a/libatomic/configure b/libatomic/configure
index e47d2d7fb35..cec874ebabe 100755
--- a/libatomic/configure
+++ b/libatomic/configure
@@ -656,7 +656,10 @@ LIBAT_BUILD_VERSIONED_SHLIB_FALSE
 LIBAT_BUILD_VERSIONED_SHLIB_TRUE
 OPT_LDFLAGS
 SECTION_LDFLAGS
+USE_CONFIG_GTHR_FALSE
+USE_CONFIG_GTHR_TRUE
 enable_aarch64_lse
+thread_header
 libtool_VERSION
 MAINT
 MAINTAINER_MODE_FALSE
@@ -11406,7 +11409,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 11409 "configure"
+#line 11412 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -11512,7 +11515,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 11515 "configure"
+#line 11518 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -11808,6 +11811,26 @@ target_thread_file=`$CC -v 2>&1 | sed -n 's/^Thread model: //p'`
 { $as_echo "$as_me:${as_lineno-$LINENO}: result: $target_thread_file" >&5
 $as_echo "$target_thread_file" >&6; }
 
+# Map from thread model to thread header.
+
+case $target_thread_file in
+    aix)	thread_header=config/rs6000/gthr-aix.h ;;
+    dce)	thread_header=config/pa/gthr-dce.h ;;
+    gcn)	thread_header=config/gcn/gthr-gcn.h ;;
+    lynx)	thread_header=config/gthr-lynx.h ;;
+    mipssde)	thread_header=config/mips/gthr-mipssde.h ;;
+    posix)	thread_header=gthr-posix.h ;;
+    rtems)	thread_header=config/gthr-rtems.h ;;
+    single)	thread_header=gthr-single.h ;;
+    tpf)	thread_header=config/s390/gthr-tpf.h ;;
+    vxworks)	thread_header=config/gthr-vxworks.h ;;
+    win32)	thread_header=config/i386/gthr-win32.h ;;
+    mcf)	thread_header=config/i386/gthr-mcf.h ;;
+esac
+
+
+
+
 case "$target" in
  *aarch64*)
     ac_ext=c
@@ -11872,10 +11895,15 @@ esac
 
 # Get target configury.
 . ${srcdir}/configure.tgt
-if test -n "$UNSUPPORTED"; then
-  as_fn_error $? "Configuration ${target} is unsupported." "$LINENO" 5
+ if test -n "$USE_CONFIG_GTHR"; then
+  USE_CONFIG_GTHR_TRUE=
+  USE_CONFIG_GTHR_FALSE='#'
+else
+  USE_CONFIG_GTHR_TRUE='#'
+  USE_CONFIG_GTHR_FALSE=
 fi
 
+
 # Write out the ifunc resolver arg type.
 
 cat >>confdefs.h <<_ACEOF
@@ -15924,6 +15952,10 @@ if test -z "${MAINTAINER_MODE_TRUE}" && test -z "${MAINTAINER_MODE_FALSE}"; then
   as_fn_error $? "conditional \"MAINTAINER_MODE\" was never defined.
 Usually this means the macro was only invoked conditionally." "$LINENO" 5
 fi
+if test -z "${USE_CONFIG_GTHR_TRUE}" && test -z "${USE_CONFIG_GTHR_FALSE}"; then
+  as_fn_error $? "conditional \"USE_CONFIG_GTHR\" was never defined.
+Usually this means the macro was only invoked conditionally." "$LINENO" 5
+fi
 
 if test -z "${LIBAT_BUILD_VERSIONED_SHLIB_TRUE}" && test -z "${LIBAT_BUILD_VERSIONED_SHLIB_FALSE}"; then
   as_fn_error $? "conditional \"LIBAT_BUILD_VERSIONED_SHLIB\" was never defined.
diff --git a/libatomic/configure.ac b/libatomic/configure.ac
index 5563551aaae..78a0b533c79 100644
--- a/libatomic/configure.ac
+++ b/libatomic/configure.ac
@@ -22,6 +22,8 @@
 #  see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
 #  <http://www.gnu.org/licenses/>.
 
+sinclude(../config/gthr.m4)
+
 AC_INIT([GNU Atomic Library], 1.0,,[libatomic])
 AC_CONFIG_HEADER(auto-config.h)
 
@@ -165,6 +167,10 @@ AC_MSG_CHECKING([for thread model used by GCC])
 target_thread_file=`$CC -v 2>&1 | sed -n 's/^Thread model: //p'`
 AC_MSG_RESULT([$target_thread_file])
 
+# Map from thread model to thread header.
+GCC_AC_THREAD_HEADER([$target_thread_file])
+AC_SUBST(thread_header)
+
 case "$target" in
  *aarch64*)
     ACX_PROG_CC_WARNING_OPTS([-march=armv8-a+lse],[enable_aarch64_lse])
@@ -173,9 +179,7 @@ esac
 
 # Get target configury.
 . ${srcdir}/configure.tgt
-if test -n "$UNSUPPORTED"; then
-  AC_MSG_ERROR([Configuration ${target} is unsupported.])
-fi
+AM_CONDITIONAL(USE_CONFIG_GTHR, test -n "$USE_CONFIG_GTHR")
 
 # Write out the ifunc resolver arg type.
 AC_DEFINE_UNQUOTED(IFUNC_RESOLVER_ARGS, $IFUNC_RESOLVER_ARGS,
diff --git a/libatomic/configure.tgt b/libatomic/configure.tgt
index 57f093603bb..a4463c5a7cb 100644
--- a/libatomic/configure.tgt
+++ b/libatomic/configure.tgt
@@ -174,21 +174,15 @@ case "${target}" in
 	config_path="rtems"
 	;;
 
-  *-*-elf*)
-	# ??? No target OS.  We could be targeting bare-metal kernel-mode,
-	# or user-mode for some custom OS.  If the target supports TAS,
-	# we can build our own spinlocks, given there are no signals.
-	# If the target supports disabling interrupts, we can work in
-	# kernel-mode, given the system is not multi-processor.
-	UNSUPPORTED=1
-	;;
-
   nvptx*-*-*)
 	;;
 
   *)
-	# Who are you?
-	UNSUPPORTED=1
+	# Use gthr.h by default.  If the thread model is "single", then this
+	# implementation will not work if for example atomic operations are
+	# used for thread/interrupt synchronization.
+	config_path="${config_path} gthr"
+	USE_CONFIG_GTHR=1
 	;;
 esac
 
diff --git a/libatomic/testsuite/Makefile.in b/libatomic/testsuite/Makefile.in
index 8bc70562e5b..80652d1ffb1 100644
--- a/libatomic/testsuite/Makefile.in
+++ b/libatomic/testsuite/Makefile.in
@@ -103,7 +103,8 @@ am__aclocal_m4_deps = $(top_srcdir)/../config/acx.m4 \
 	$(top_srcdir)/../ltversion.m4 $(top_srcdir)/../lt~obsolete.m4 \
 	$(top_srcdir)/acinclude.m4 $(top_srcdir)/../libtool.m4 \
 	$(top_srcdir)/../config/enable.m4 \
-	$(top_srcdir)/../config/cet.m4 $(top_srcdir)/configure.ac
+	$(top_srcdir)/../config/cet.m4 $(top_srcdir)/../config/gthr.m4 \
+	$(top_srcdir)/configure.ac
 am__configure_deps = $(am__aclocal_m4_deps) $(CONFIGURE_DEPENDENCIES) \
 	$(ACLOCAL_M4)
 DIST_COMMON = $(srcdir)/Makefile.am
@@ -262,6 +263,7 @@ target_alias = @target_alias@
 target_cpu = @target_cpu@
 target_os = @target_os@
 target_vendor = @target_vendor@
+thread_header = @thread_header@
 tmake_file = @tmake_file@
 toolexecdir = @toolexecdir@
 toolexeclibdir = @toolexeclibdir@
-- 
2.35.3