c++: Drop TREE_READONLY on vars (possibly) initialized by tls wrapper [PR109164]

The following two testcases are miscompiled, because we keep TREE_READONLY
on the vars even when they are (possibly) dynamically initialized by a TLS
wrapper function.  Normally cp_finish_decl drops TREE_READONLY from vars
which need dynamic initialization, but for TLS we do this kind of
initialization upon every access to those variables.  Keeping them
TREE_READONLY means e.g. PRE can hoist loads from those before loops
which contain the TLS wrapper calls, so we can access the TLS variables
before they are initialized.

2023-03-20  Jakub Jelinek  <jakub@redhat.com>

	PR c++/109164
	* cp-tree.h (var_needs_tls_wrapper): Declare.
	* decl2.cc (var_needs_tls_wrapper): No longer static.
	* decl.cc (cp_finish_decl): Clear TREE_READONLY on TLS variables
	for which a TLS wrapper will be needed.

	* g++.dg/tls/thread_local13.C: New test.
	* g++.dg/tls/thread_local13-aux.cc: New file.
	* g++.dg/tls/thread_local14.C: New test.
	* g++.dg/tls/thread_local14-aux.cc: New file.

7 files changed, 115 insertions, 1 deletions

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b74c18b..0e37d40 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6989,6 +6989,7 @@ extern void copy_linkage			(tree, tree);
 extern tree get_guard				(tree);
 extern tree get_guard_cond			(tree, bool);
 extern tree set_guard				(tree);
+extern bool var_needs_tls_wrapper		(tree);
 extern tree maybe_get_tls_wrapper_call		(tree);
 extern void mark_needed				(tree);
 extern bool decl_needed_p			(tree);
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index f578533..20b980f 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -8706,6 +8706,18 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	  if (!decl_maybe_constant_destruction (decl, type))
 	    TREE_READONLY (decl) = 0;
 	}
+      else if (VAR_P (decl)
+	       && CP_DECL_THREAD_LOCAL_P (decl)
+	       && (!DECL_EXTERNAL (decl) || flag_extern_tls_init)
+	       && (was_readonly || TREE_READONLY (decl))
+	       && var_needs_tls_wrapper (decl))
+	{
+	  /* TLS variables need dynamic initialization by the TLS wrapper
+	     function, we don't want to hoist accesses to it before the
+	     wrapper.  */
+	  was_readonly = 0;
+	  TREE_READONLY (decl) = 0;
+	}
 
       make_rtl_for_nonlocal_decl (decl, init, asmspec);
 
diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 387e245..0002d21 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -3623,7 +3623,7 @@ var_defined_without_dynamic_init (tree var)
 /* Returns true iff VAR is a variable that needs uses to be
    wrapped for possible dynamic initialization.  */
 
-static bool
+bool
 var_needs_tls_wrapper (tree var)
 {
   return (!error_operand_p (var)
diff --git a/gcc/testsuite/g++.dg/tls/thread_local13-aux.cc b/gcc/testsuite/g++.dg/tls/thread_local13-aux.cc
new file mode 100644
index 0000000..691f308
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tls/thread_local13-aux.cc
@@ -0,0 +1,35 @@
+// PR c++/109164
+
+struct S { virtual void foo (); int s; };
+extern bool baz ();
+
+void
+S::foo ()
+{
+  if (s != 42)
+    __builtin_abort ();
+}
+
+S s;
+
+S &
+qux ()
+{
+  s.s = 42;
+  return s;
+}
+
+thread_local S &t = qux ();
+
+bool
+bar ()
+{
+  return false;
+}
+
+int
+main ()
+{
+  if (baz ())
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/tls/thread_local13.C b/gcc/testsuite/g++.dg/tls/thread_local13.C
new file mode 100644
index 0000000..b424b9f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tls/thread_local13.C
@@ -0,0 +1,21 @@
+// PR c++/109164
+// { dg-do run { target c++11 } }
+// { dg-options "-O2" }
+// { dg-add-options tls }
+// { dg-require-effective-target tls_runtime }
+// { dg-additional-sources "thread_local13-aux.cc" }
+
+struct S { virtual void foo (); int s; };
+extern thread_local S &t;
+bool bar ();
+
+bool
+baz ()
+{
+  while (1)
+    {
+      t.foo ();
+      if (!bar ())
+        return false;
+    }
+}
diff --git a/gcc/testsuite/g++.dg/tls/thread_local14-aux.cc b/gcc/testsuite/g++.dg/tls/thread_local14-aux.cc
new file mode 100644
index 0000000..e8f2a24
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tls/thread_local14-aux.cc
@@ -0,0 +1,26 @@
+// PR c++/109164
+
+extern bool baz ();
+
+int
+qux ()
+{
+  return 42;
+}
+
+extern thread_local const int t = qux ();
+
+bool
+bar (int x)
+{
+  if (x != 42)
+    __builtin_abort ();
+  return false;
+}
+
+int
+main ()
+{
+  if (baz ())
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/tls/thread_local14.C b/gcc/testsuite/g++.dg/tls/thread_local14.C
new file mode 100644
index 0000000..a7402d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tls/thread_local14.C
@@ -0,0 +1,19 @@
+// PR c++/109164
+// { dg-do run { target c++11 } }
+// { dg-options "-O2" }
+// { dg-add-options tls }
+// { dg-require-effective-target tls_runtime }
+// { dg-additional-sources "thread_local14-aux.cc" }
+
+extern thread_local const int t;
+bool bar (int);
+
+bool
+baz ()
+{
+  while (1)
+    {
+      if (!bar (t))
+        return false;
+    }
+}