testsuite: Add and use thread_fence effective-target

Some targets like arm-eabi with newlib and default settings rely on
__sync_synchronize() to ensure synchronization.  Newlib does not
implement it by default, to make users aware they have to take special
care.

This makes a few tests fail to link.

This patch adds a new thread_fence effective target (similar to the
corresponding one in libstdc++ testsuite), and uses it in the tests
that need it, making them UNSUPPORTED instead of FAIL and UNRESOLVED.

2023-09-10  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* doc/sourcebuild.texi (Other attributes): Document thread_fence
	effective-target.

	gcc/testsuite/
	* g++.dg/init/array54.C: Require thread_fence.
	* gcc.dg/c2x-nullptr-1.c: Likewise.
	* gcc.dg/pr103721-2.c: Likewise.
	* lib/target-supports.exp (check_effective_target_thread_fence):
	New.

5 files changed, 19 insertions, 0 deletions

diff --git a/gcc/doc/sourcebuild.texi b/gcc/doc/sourcebuild.texi
index c20af31..c8611a0 100644
--- a/gcc/doc/sourcebuild.texi
+++ b/gcc/doc/sourcebuild.texi
@@ -2875,6 +2875,10 @@ Compiler has been configured to support link-time optimization (LTO).
 Compiler and linker support link-time optimization relocatable linking
 with @option{-r} and @option{-flto} options.
 
+@item thread_fence
+Target implements @code{__atomic_thread_fence} without relying on
+non-implemented @code{__sync_synchronize()}.
+
 @item naked_functions
 Target supports the @code{naked} function attribute.
 
diff --git a/gcc/testsuite/g++.dg/init/array54.C b/gcc/testsuite/g++.dg/init/array54.C
index f6be350..5241e45 100644
--- a/gcc/testsuite/g++.dg/init/array54.C
+++ b/gcc/testsuite/g++.dg/init/array54.C
@@ -1,5 +1,6 @@
 // PR c++/90947
 // { dg-do run { target c++11 } }
+// { dg-require-effective-target thread_fence }
 
 #include <atomic>
 
diff --git a/gcc/testsuite/gcc.dg/c2x-nullptr-1.c b/gcc/testsuite/gcc.dg/c2x-nullptr-1.c
index 4e44023..97a31c2 100644
--- a/gcc/testsuite/gcc.dg/c2x-nullptr-1.c
+++ b/gcc/testsuite/gcc.dg/c2x-nullptr-1.c
@@ -1,5 +1,6 @@
 /* Test valid usage of C23 nullptr.  */
 /* { dg-do run } */
+// { dg-require-effective-target thread_fence }
 /* { dg-options "-std=c2x -pedantic-errors -Wall -Wextra -Wno-unused-variable" } */
 
 #include <stdarg.h>
diff --git a/gcc/testsuite/gcc.dg/pr103721-2.c b/gcc/testsuite/gcc.dg/pr103721-2.c
index aefa1f0..e059b1c 100644
--- a/gcc/testsuite/gcc.dg/pr103721-2.c
+++ b/gcc/testsuite/gcc.dg/pr103721-2.c
@@ -1,4 +1,5 @@
 // { dg-do run }
+// { dg-require-effective-target thread_fence }
 // { dg-options "-O2" }
 
 extern void abort ();
diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index 024939e..17a87db 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -9514,6 +9514,18 @@ proc check_effective_target_sync_char_short { } {
 	     || [check_effective_target_mips_llsc] }}]
 }
 
+# Return 1 if thread_fence does not rely on __sync_synchronize
+# library function
+
+proc check_effective_target_thread_fence {} {
+    return [check_no_compiler_messages thread_fence executable {
+	int main () {
+	    __atomic_thread_fence (__ATOMIC_SEQ_CST);
+	    return 0;
+	}
+    } ""]
+}
+
 # Return 1 if the target uses a ColdFire FPU.
 
 proc check_effective_target_coldfire_fpu { } {