c++: NRV and goto [PR92407]

Here our named return value optimization was breaking the required
destructor when the goto takes 'a' out of scope.  A simple fix for the
release branches is to disable the optimization in the presence of backward
goto.

We could do better by disabling the optimization only if there is a backward
goto across the variable declaration, but we don't track that, and in GCC 14
we instead make the goto work with NRV.

	PR c++/92407

gcc/cp/ChangeLog:

	* cp-tree.h (struct language_function): Add backward_goto.
	* decl.c (check_goto): Set it.
	* typeck.c (check_return_expr): Prevent NRV if set.

gcc/testsuite/ChangeLog:

	* g++.dg/opt/nrv22.C: New test.

(cherry picked from commit a645347c19b07cc7abd7bf276c6769fc41afc932)

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 8887e946d5379a844b890bbc1d1a8f7360087bb3..7e04a080f3a162d8809663846dedb0d07182eca1 100644 (file)
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -2024,6 +2024,7 @@ struct GTY(()) language_function {
 
   BOOL_BITFIELD invalid_constexpr : 1;
   BOOL_BITFIELD throwing_cleanup : 1;
+  BOOL_BITFIELD backward_goto : 1;
 
   hash_table<named_label_hash> *x_named_labels;
 

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 712e875edd04101a87ef9c3301ed89091ae5f63d..52088a480fe5ab6e01776ebc0837858c059d8768 100644 (file)
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -3563,6 +3563,8 @@ check_goto (tree decl)
       return;
     }
 
+  cp_function_chain->backward_goto = true;
+
   bool saw_catch = false, complained = false;
   int identified = 0;
   tree bad;

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 6e5a35202c71a2e104cabc12250faf593c3e9f44..61e3222c6c620985069aa48e5c0053a6633b30d8 100644 (file)
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -10307,6 +10307,9 @@ check_return_expr (tree retval, bool *no_warning)
   if (fn_returns_value_p && flag_elide_constructors)
     {
       if (named_return_value_okay_p
+         /* The current NRV implementation breaks if a backward goto needs to
+            destroy the object (PR92407).  */
+         && !cp_function_chain->backward_goto
           && (current_function_return_value == NULL_TREE
               || current_function_return_value == retval))
        current_function_return_value = retval;

diff --git a/gcc/testsuite/g++.dg/opt/nrv22.C b/gcc/testsuite/g++.dg/opt/nrv22.C
new file mode 100644 (file)
index 0000000..eb889fa
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/nrv22.C
@@ -0,0 +1,30 @@
+// PR c++/92407
+// { dg-do run }
+
+struct A
+{
+  A () { a++; }
+  A (const A &) { a++; }
+  ~A () { a--; }
+  static int a;
+};
+int A::a = 0;
+
+A
+foo ()
+{
+  int cnt = 10;
+lab:
+  A a;
+  if (cnt--)
+    goto lab;
+  return a;
+}
+
+int
+main ()
+{
+  foo ();
+  if (A::a)
+    __builtin_abort ();
+}