Re: [PATCH] c++: Quash bogus -Wredundant-move warning

[Jason Merrill via Gcc-patches]

On 8/22/22 17:10, Marek Polacek wrote:

This patch fixes a pretty stoopid thinko.  When I added code to warn
about pessimizing std::move in initializations like

   T t{std::move(T())};

I also added code to unwrap the expression from { }.  But when we have

   return {std::move(t)};

we cannot warn about a redundant std::move because the implicit move
wouldn't happen for "return {t};" because the expression isn't just
a name.  However, we still want to warn about

   return {std::move(T())};

so let's not disable the -Wpessimizing-move warning.  Tests added for
both cases.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

gcc/cp/ChangeLog:

        * typeck.cc (maybe_warn_pessimizing_move): Don't warn about
        redundant std::move when the expression was wrapped in { }.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/Wpessimizing-move10.C: New test.
        * g++.dg/cpp0x/Wredundant-move12.C: New test.
---
  gcc/cp/typeck.cc                              | 13 +++++--
  .../g++.dg/cpp0x/Wpessimizing-move10.C        | 30 ++++++++++++++++
  .../g++.dg/cpp0x/Wredundant-move12.C          | 36 +++++++++++++++++++
  3 files changed, 77 insertions(+), 2 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move10.C
  create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wredundant-move12.C

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 992ebfd99fb..7fde65adaa4 100644
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -10397,11 +10397,15 @@ maybe_warn_pessimizing_move (tree expr, tree type, 
bool return_p)
    if (!CLASS_TYPE_P (type))
      return;

+ bool wrapped_p = false;

    /* A a = std::move (A());  */
    if (TREE_CODE (expr) == TREE_LIST)
      {
        if (list_length (expr) == 1)
-       expr = TREE_VALUE (expr);
+       {
+         expr = TREE_VALUE (expr);
+         wrapped_p = true;
+       }
        else
        return;
      }
@@ -10410,7 +10414,10 @@ maybe_warn_pessimizing_move (tree expr, tree type, 
bool return_p)
    else if (TREE_CODE (expr) == CONSTRUCTOR)
      {
        if (CONSTRUCTOR_NELTS (expr) == 1)
-       expr = CONSTRUCTOR_ELT (expr, 0)->value;
+       {
+         expr = CONSTRUCTOR_ELT (expr, 0)->value;
+         wrapped_p = true;
+       }
        else
        return;
      }
@@ -10458,6 +10465,8 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool 
return_p)
        /* Warn if the move is redundant.  It is redundant when we would
         do maybe-rvalue overload resolution even without std::move.  */
        else if (warn_redundant_move
+              /* This doesn't apply for return {std::move (t)};.  */
+              && !wrapped_p
               && !warning_suppressed_p (expr, OPT_Wredundant_move)
               && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true)))
        {
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move10.C 
b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move10.C
new file mode 100644
index 00000000000..77314141da3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move10.C
@@ -0,0 +1,30 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wpessimizing-move" }
+
+// Define std::move.
+namespace std {
+  template<typename _Tp>
+    struct remove_reference
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    constexpr typename std::remove_reference<_Tp>::type&&
+    move(_Tp&& __t) noexcept
+    { return static_cast<typename std::remove_reference<_Tp>::type&&>(__t); }
+}
+
+struct S { };
+
+S
+f ()
+{
+   return {std::move(S())}; // { dg-warning "moving a temporary object prevents 
copy elision" }
+}
diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move12.C 
b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move12.C
new file mode 100644
index 00000000000..192d981ba5e
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move12.C
@@ -0,0 +1,36 @@
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wredundant-move" }
+
+// Define std::move.
+namespace std {
+  template<typename _Tp>
+    struct remove_reference
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    struct remove_reference<_Tp&&>
+    { typedef _Tp   type; };
+
+  template<typename _Tp>
+    constexpr typename std::remove_reference<_Tp>::type&&
+    move(_Tp&& __t) noexcept
+    { return static_cast<typename std::remove_reference<_Tp>::type&&>(__t); }
+}
+
+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) }; // { dg-bogus "redundant move" }
+}

base-commit: cc4fa7a210b638d6a46f14dab17f2361389d18e1