Add -Wtsan.

gcc/ChangeLog:

	PR sanitizer/97868
	* common.opt: Add new warning -Wtsan.
	* doc/invoke.texi: Likewise.
	* tsan.c (instrument_builtin_call): Warn users about unsupported
	std::atomic_thread_fence.

gcc/testsuite/ChangeLog:

	PR sanitizer/97868
	* gcc.dg/tsan/atomic-fence.c: New test.

gcc/common.opt

@@ -842,6 +842,10 @@ Wvector-operation-performance
 Common Var(warn_vector_operation_performance) Warning
 Warn when a vector operation is compiled outside the SIMD.
 
+Wtsan
+Common Var(warn_tsan) Init(1) Warning
+Warn about unsupported features in ThreadSanitizer.
+
 Xassembler
 Driver Separate
 

gcc/doc/invoke.texi

@@ -387,7 +387,7 @@ Objective-C and Objective-C++ Dialects}.
 -Wswitch  -Wno-switch-bool  -Wswitch-default  -Wswitch-enum @gol
 -Wno-switch-outside-range  -Wno-switch-unreachable  -Wsync-nand @gol
 -Wsystem-headers  -Wtautological-compare  -Wtrampolines  -Wtrigraphs @gol
--Wtype-limits  -Wundef @gol
+-Wtsan -Wtype-limits  -Wundef @gol
 -Wuninitialized  -Wunknown-pragmas @gol
 -Wunsuffixed-float-constants  -Wunused @gol
 -Wunused-but-set-parameter  -Wunused-but-set-variable @gol
@@ -8023,6 +8023,16 @@ Note that the code above is invalid in C++11.
 
 This warning is enabled by default.
 
+@item -Wtsan
+@opindex Wtsan
+@opindex Wno-tsan
+Warn about unsupported features in ThreadSanitizer.
+
+ThreadSanitizer does not support @code{std::atomic_thread_fence} and
+can report false positives.
+
+This warning is enabled by default.
+
 @item -Wtype-limits
 @opindex Wtype-limits
 @opindex Wno-type-limits

gcc/testsuite/gcc.dg/tsan/atomic-fence.c

@@ -0,0 +1,11 @@
+/* PR sanitizer/97868 */
+/* { dg-do compile } */
+/* { dg-options "-fsanitize=thread" } */
+
+int
+main ()
+{
+  __atomic_thread_fence (__ATOMIC_RELAXED); /* { dg-warning ".atomic_thread_fence. is not supported with .-fsanitize=thread." } */
+  return 0;
+}
+

gcc/tsan.c

@@ -45,6 +45,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "asan.h"
 #include "builtins.h"
 #include "target.h"
+#include "diagnostic-core.h"
 
 /* Number of instrumented memory accesses in the current function.  */
 
@@ -500,6 +501,11 @@ instrument_builtin_call (gimple_stmt_iterator *gsi)
       continue;
     else
       {
+	if (fcode == BUILT_IN_ATOMIC_THREAD_FENCE)
+	  warning_at (gimple_location (stmt), OPT_Wtsan,
+		      "%qs is not supported with %qs", "atomic_thread_fence",
+		      "-fsanitize=thread");
+
 	tree decl = builtin_decl_implicit (tsan_atomic_table[i].tsan_fcode);
 	if (decl == NULL_TREE)
 	  return;