[pushed] c++: NRV and goto [PR92407]

[pushed] c++: NRV and goto [PR92407]

[Jason Merrill]

Tested x86_64-pc-linux-gnu, applying to trunk.

-- 8< --

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.
---
 gcc/cp/typeck.cc                 |  3 +++
 gcc/testsuite/g++.dg/opt/nrv22.C | 30 ++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/opt/nrv22.C

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 ();
+}

base-commit: 29c82c6ca929e0f5eccfe038dea71177d814c6b7
prerequisite-patch-id: aed53cdac161144c31cb1433282e1ad1d49d3770
prerequisite-patch-id: 1098cb4457a5eff90fa8176f9b0d8d2e9477596e
prerequisite-patch-id: 823f2ce422455c6c7ccbaa9938b670a600b376df
-- 
2.31.1