[gcc(refs/users/marxin/heads/PR97868-tsan-fences)] Add -Wtsan.

[Martin Liska <marxin@gcc.gnu.org>]

https://gcc.gnu.org/g:a644a9bb8ddb9803c9aa033dc86bb03ceac9e895

commit a644a9bb8ddb9803c9aa033dc86bb03ceac9e895
Author: Martin Liska <mliska@suse.cz>
Date:   Mon Dec 7 15:55:59 2020 +0100

    Add -Wtsan.

Diff:
---
 gcc/common.opt | 4 ++++
 gcc/tsan.c     | 6 ++++++
 2 files changed, 10 insertions(+)

diff --git a/gcc/common.opt b/gcc/common.opt
index 6645539f5e5..6c24c7bbffb 100644
--- a/gcc/common.opt
+++ b/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 the ThreadSanitizer.
+
 Xassembler
 Driver Separate
 
diff --git a/gcc/tsan.c b/gcc/tsan.c
index 4d6223454b5..be9fabea62a 100644
--- a/gcc/tsan.c
+++ b/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 by ThreadSanitizer and may "
+		      "lead to false positives", "atomic_thread_fence");
+
 	tree decl = builtin_decl_implicit (tsan_atomic_table[i].tsan_fcode);
 	if (decl == NULL_TREE)
 	  return;