[gcc r12-1698] c++: REF_PARENTHESIZED_P wrapper inhibiting NRVO [PR67302]

[gcc r12-1698] c++: REF_PARENTHESIZED_P wrapper inhibiting NRVO [PR67302]

[Patrick Palka]

https://gcc.gnu.org/g:21761d2b2b01f6cef4287c646845f6b3006546aa

commit r12-1698-g21761d2b2b01f6cef4287c646845f6b3006546aa
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Jun 21 07:54:29 2021 -0400

    c++: REF_PARENTHESIZED_P wrapper inhibiting NRVO [PR67302]
    
    Here, in C++14 or later, we remember the parentheses around 'a' in the
    return statement by using a REF_PARENTHESIZED_P wrapper, which ends up
    inhibiting NRVO because we don't look through this wrapper before
    checking the conditions for NRVO.  This patch fixes this by calling
    maybe_undo_parenthesized_ref sooner in check_return_expr.
    
            PR c++/67302
    
    gcc/cp/ChangeLog:
    
            * typeck.c (check_return_expr): Call maybe_undo_parenthesized_ref
            sooner, before the NRVO handling.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/opt/nrv21.C: New test.

Diff:
---
 gcc/cp/typeck.c                  |  9 ++++-----
 gcc/testsuite/g++.dg/opt/nrv21.C | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index 5a9331b36e6..937581ae83c 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -10311,7 +10311,10 @@ check_return_expr (tree retval, bool *no_warning)
 
      See finish_function and finalize_nrv for the rest of this optimization.  */
   if (retval)
-    STRIP_ANY_LOCATION_WRAPPER (retval);
+    {
+      retval = maybe_undo_parenthesized_ref (retval);
+      STRIP_ANY_LOCATION_WRAPPER (retval);
+    }
 
   bool named_return_value_okay_p = can_do_nrvo_p (retval, functype);
   if (fn_returns_value_p && flag_elide_constructors)
@@ -10345,10 +10348,6 @@ check_return_expr (tree retval, bool *no_warning)
       if (VOID_TYPE_P (functype))
 	return error_mark_node;
 
-      /* If we had an id-expression obfuscated by force_paren_expr, we need
-	 to undo it so we can try to treat it as an rvalue below.  */
-      retval = maybe_undo_parenthesized_ref (retval);
-
       if (processing_template_decl)
 	retval = build_non_dependent_expr (retval);
 
diff --git a/gcc/testsuite/g++.dg/opt/nrv21.C b/gcc/testsuite/g++.dg/opt/nrv21.C
new file mode 100644
index 00000000000..ff338527456
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/nrv21.C
@@ -0,0 +1,14 @@
+// PR c++/67302
+// { dg-additional-options -fdump-tree-gimple }
+// { dg-final { scan-tree-dump-not "<retval> = a" "gimple" } }
+
+struct A
+{
+  int ar[42];
+  A();
+};
+
+A f() {
+  A a;
+  return (a); // The parens should not inhibit NRVO.
+}