Re: [PATCH] c++: Extend -Wpessimizing-move to other contexts

[Marek Polacek <polacek@redhat.com>]

On Mon, Aug 22, 2022 at 01:48:34PM +0200, Stephan Bergmann wrote:
> On 16/08/2022 14:27, Marek Polacek via Gcc-patches wrote:
> > Ping.  (The other std::move patches depend on this one.)
> 
> <https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=8d22c7cb8b1a6f9b67c54a798dd5504244614e51>
> "c++: Extend -Wpessimizing-move to other contexts" started to cause false
> positive

Thanks for reporting the problem.  I'm testing a simple patch (appended
below) and will post it once testing finishes.
 
> > $ cat test.cc
> > #include <utility>
> > struct S1 {
> >     S1();
> >     S1(S1 const &) = delete;
> >     S1(S1 &&);
> >     S1 operator =(S1 const &) = delete;
> >     S1 operator =(S1 &&);
> > };
> > struct S2 { S2(S1); };
> > S2 f() {
> >     S1 s;
> >     return { std::move(s) };
> > }
> > 
> > $ g++ -fsyntax-only -Wredundant-move test.cc
> > test.cc: In function ‘S2 f()’:
> > test.cc:12:27: warning: redundant move in return statement [-Wredundant-move]
> >    12 |     return { std::move(s) };
> >       |                           ^
> > test.cc:12:27: note: remove ‘std::move’ call

--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -10447,7 +10447,7 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
     return;

   /* A a = std::move (A());  */
-  if (TREE_CODE (expr) == TREE_LIST)
+  if (TREE_CODE (expr) == TREE_LIST && !return_p)
     {
       if (list_length (expr) == 1)
    expr = TREE_VALUE (expr);
@@ -10456,7 +10456,7 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
     }
   /* A a = {std::move (A())};
      A a{std::move (A())};  */
-  else if (TREE_CODE (expr) == CONSTRUCTOR)
+  else if (TREE_CODE (expr) == CONSTRUCTOR && !return_p)
     {
       if (CONSTRUCTOR_NELTS (expr) == 1)
    expr = CONSTRUCTOR_ELT (expr, 0)->value;

Marek