From: Jason Merrill <jason@redhat.com>
Date: Sun, 4 Jun 2023 16:00:55 +0000 (-0400)
Subject: c++: NRV and goto [PR92407]
X-Git-Tag: basepoints/gcc-15~8547
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=b192e2007e1c98b548f4aa878523b485968d24a4;p=thirdparty%2Fgcc.git

c++: NRV and goto [PR92407]

Here our named return value optimization was breaking the required
destructor when the goto takes 'a' out of scope.  The simplest fix is to
disable the optimization in the presence of user labels.

We could do better by disabling the optimization only if there is a backward
goto across the variable declaration, but we don't currently track that.

	PR c++/92407

gcc/cp/ChangeLog:

	* typeck.cc (check_return_expr): Prevent NRV in the presence of
	named labels.

gcc/testsuite/ChangeLog:

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

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 11fcc7fcd3b3..6618c6a20218 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -11155,6 +11155,9 @@ 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->x_named_labels
           && (current_function_return_value == NULL_TREE
 	      || current_function_return_value == bare_retval))
 	current_function_return_value = bare_retval;
diff --git a/gcc/testsuite/g++.dg/opt/nrv22.C b/gcc/testsuite/g++.dg/opt/nrv22.C
new file mode 100644
index 000000000000..eb889fa615b1
--- /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 ();
+}