[gcc r14-1592] c++: NRV and goto [PR92407]

[gcc r14-1592] c++: NRV and goto [PR92407]

[Jason Merrill]

https://gcc.gnu.org/g:b192e2007e1c98b548f4aa878523b485968d24a4

commit r14-1592-gb192e2007e1c98b548f4aa878523b485968d24a4
Author: Jason Merrill <jason@redhat.com>
Date:   Sun Jun 4 12:00:55 2023 -0400

    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:
---
 gcc/cp/typeck.cc                 |  3 +++
 gcc/testsuite/g++.dg/opt/nrv22.C | 30 ++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 11fcc7fcd3b..6618c6a2021 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 00000000000..eb889fa615b
--- /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 ();
+}