Re: [PATCH] c++: REF_PARENTHESIZED_P wrapper inhibiting NRVO [PR67302]

[Jason Merrill via Gcc-patches]

On 6/19/21 3:45 PM, Patrick Palka wrote:

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.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

OK.

        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.
---
  gcc/cp/typeck.c                  |  9 ++++-----
  gcc/testsuite/g++.dg/opt/nrv21.C | 14 ++++++++++++++
  2 files changed, 18 insertions(+), 5 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/opt/nrv21.C

diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c
index dbb2370510c..aa014c3812a 100644
--- a/gcc/cp/typeck.c
+++ b/gcc/cp/typeck.c
@@ -10306,7 +10306,10 @@ 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)
@@ -10340,10 +10343,6 @@ 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..31bff79afc1
--- /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);
+}