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C++ PATCH for c++/86981, implement -Wpessimizing-move


This patch implements -Wpessimizing-move, a C++-specific warning that warns
when using std::move in a return statement precludes the NRVO.  Consider:

struct T { };

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

where we could elide the copy were it not for the move call; the standard
requires that the expression be the name of a non-volatile automatic object, so
no function call would work there.
Had 't' been a parameter, the move would have been merely redundant, but that's
for another warning, -Wredundant-move, which should be a fairly easy extension
of this one.

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

2018-08-20  Marek Polacek  <polacek@redhat.com>

	PR c++/86981, Implement -Wpessimizing-move.
	* c.opt (Wpessimizing-move): New option.

	* typeck.c (decl_in_std_namespace_p): New.
	(is_std_move_p): New.
	(can_do_nrvo_p): New, factored out of ...
	(check_return_expr): ... here.  Warn about potentially harmful
	std::move in a return statement.

	* doc/invoke.texi: Document -Wpessimizing-move.

	* g++.dg/cpp0x/Wpessimizing-move1.C: New test.
	* g++.dg/cpp0x/Wpessimizing-move2.C: New test.
	* g++.dg/cpp0x/Wpessimizing-move3.C: New test.
	* g++.dg/cpp0x/Wpessimizing-move4.C: New test.
	* g++.dg/cpp1z/Wpessimizing-move1.C: New test.

diff --git gcc/c-family/c.opt gcc/c-family/c.opt
index 9980bfac11c..76840dd77ad 100644
--- gcc/c-family/c.opt
+++ gcc/c-family/c.opt
@@ -937,6 +937,10 @@ Wpedantic
 C ObjC C++ ObjC++ CPP(cpp_pedantic) CppReason(CPP_W_PEDANTIC) Warning
 ; Documented in common.opt
 
+Wpessimizing-move
+C++ ObjC++ Var(warn_pessimizing_move) Warning LangEnabledBy(C++ ObjC++, Wall)
+Warn about calling std::move on a local object in a return statement preventing copy elision.
+
 Wpmf-conversions
 C++ ObjC++ Var(warn_pmf2ptr) Init(1) Warning
 Warn when converting the type of pointers to member functions.
diff --git gcc/cp/typeck.c gcc/cp/typeck.c
index 8c13ae9b19b..e7504d5a246 100644
--- gcc/cp/typeck.c
+++ gcc/cp/typeck.c
@@ -9113,6 +9113,58 @@ maybe_warn_about_returning_address_of_local (tree retval)
   return false;
 }
 
+/* Returns true if DECL is in the std namespace.  */
+
+static bool
+decl_in_std_namespace_p (tree decl)
+{
+  return (decl != NULL_TREE
+	  && DECL_NAMESPACE_STD_P (decl_namespace_context (decl)));
+}
+
+/* Returns true if FN, a CALL_EXPR, is a call to std::move.  */
+
+static bool
+is_std_move_p (tree fn)
+{
+  /* std::move only takes one argument.  */
+  if (call_expr_nargs (fn) != 1)
+    return false;
+
+  tree fndecl = cp_get_callee_fndecl_nofold (fn);
+  if (!decl_in_std_namespace_p (fndecl))
+    return false;
+
+  tree name = DECL_NAME (fndecl);
+  return name && id_equal (name, "move");
+}
+
+/* Returns true if RETVAL is a good candidate for the NRVO as per
+   [class.copy.elision].  FUNCTYPE is the type the function is declared
+   to return.  */
+
+static bool
+can_do_nrvo_p (tree retval, tree functype)
+{
+  tree result = DECL_RESULT (current_function_decl);
+  return (retval != NULL_TREE
+	  && !processing_template_decl
+	  /* Must be a local, automatic variable.  */
+	  && VAR_P (retval)
+	  && DECL_CONTEXT (retval) == current_function_decl
+	  && !TREE_STATIC (retval)
+	  /* And not a lambda or anonymous union proxy.  */
+	  && !DECL_HAS_VALUE_EXPR_P (retval)
+	  && (DECL_ALIGN (retval) <= DECL_ALIGN (result))
+	  /* The cv-unqualified type of the returned value must be the
+	     same as the cv-unqualified return type of the
+	     function.  */
+	  && same_type_p ((TYPE_MAIN_VARIANT (TREE_TYPE (retval))),
+			  (TYPE_MAIN_VARIANT (functype)))
+	  /* And the returned value must be non-volatile.  */
+	  && !TYPE_VOLATILE (TREE_TYPE (retval)));
+}
+
 /* Check that returning RETVAL from the current function is valid.
    Return an expression explicitly showing all conversions required to
    change RETVAL into the function return type, and to assign it to
@@ -9130,7 +9182,6 @@ check_return_expr (tree retval, bool *no_warning)
      the declared type is incomplete.  */
   tree functype;
   int fn_returns_value_p;
-  bool named_return_value_okay_p;
 
   *no_warning = false;
 
@@ -9342,24 +9393,7 @@ check_return_expr (tree retval, bool *no_warning)
 
      See finish_function and finalize_nrv for the rest of this optimization.  */
 
-  named_return_value_okay_p = 
-    (retval != NULL_TREE
-     && !processing_template_decl
-     /* Must be a local, automatic variable.  */
-     && VAR_P (retval)
-     && DECL_CONTEXT (retval) == current_function_decl
-     && ! TREE_STATIC (retval)
-     /* And not a lambda or anonymous union proxy.  */
-     && !DECL_HAS_VALUE_EXPR_P (retval)
-     && (DECL_ALIGN (retval) <= DECL_ALIGN (result))
-     /* The cv-unqualified type of the returned value must be the
-        same as the cv-unqualified return type of the
-        function.  */
-     && same_type_p ((TYPE_MAIN_VARIANT (TREE_TYPE (retval))),
-                     (TYPE_MAIN_VARIANT (functype)))
-     /* And the returned value must be non-volatile.  */
-     && ! TYPE_VOLATILE (TREE_TYPE (retval)));
-     
+  bool named_return_value_okay_p = can_do_nrvo_p (retval, functype);
   if (fn_returns_value_p && flag_elide_constructors)
     {
       if (named_return_value_okay_p
@@ -9375,6 +9409,32 @@ check_return_expr (tree retval, bool *no_warning)
   if (!retval)
     return NULL_TREE;
 
+  /* Warn about wrong usage of std::move in a return statement.  */
+  if (!named_return_value_okay_p
+      && warn_pessimizing_move
+      /* C++98 doesn't know move.  */
+      && (cxx_dialect >= cxx11)
+      /* This is only interesting for class type.  */
+      && CLASS_TYPE_P (functype)
+      /* We're looking for *std::move<T&> (&arg).  */
+      && REFERENCE_REF_P (retval)
+      && TREE_CODE (TREE_OPERAND (retval, 0)) == CALL_EXPR)
+    {
+      tree fn = TREE_OPERAND (retval, 0);
+      if (is_std_move_p (fn))
+	{
+	  tree arg = CALL_EXPR_ARG (fn, 0);
+	  STRIP_NOPS (arg);
+	  if (TREE_CODE (arg) == ADDR_EXPR)
+	    arg = TREE_OPERAND (arg, 0);
+	  /* Warn if we could do copy elision were it not for the move.  */
+	  if (can_do_nrvo_p (arg, functype)
+	      && warning (OPT_Wpessimizing_move, "moving a local object "
+			  "in a return statement prevents copy elision"))
+	    inform (input_location, "remove %<std::move%> call");
+	}
+    }
+
   /* Do any required conversions.  */
   if (retval == result || DECL_CONSTRUCTOR_P (current_function_decl))
     /* No conversions are required.  */
diff --git gcc/doc/invoke.texi gcc/doc/invoke.texi
index f8287153be1..7a79ba32fc7 100644
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -231,6 +231,7 @@ in the following sections.
 -Wdelete-non-virtual-dtor -Wdeprecated-copy  -Wliteral-suffix @gol
 -Wmultiple-inheritance @gol
 -Wnamespaces  -Wnarrowing @gol
+-Wpessimizing-move @gol
 -Wnoexcept  -Wnoexcept-type  -Wclass-memaccess @gol
 -Wnon-virtual-dtor  -Wreorder  -Wregister @gol
 -Weffc++  -Wstrict-null-sentinel  -Wtemplates @gol
@@ -3131,6 +3132,31 @@ The compiler rearranges the member initializers for @code{i}
 and @code{j} to match the declaration order of the members, emitting
 a warning to that effect.  This warning is enabled by @option{-Wall}.
 
+@item -Wno-pessimizing-move @r{(C++ and Objective-C++ only)}
+@opindex Wpessimizing-move
+@opindex Wno-pessimizing-move
+This warning warns when a call to @code{std::move} prevents copy
+elision.  A typical scenario when copy elision can occur is when returning in
+a function with a class return type, when the expression being returned is the
+name of a non-volatile automatic object, and is not a function parameter, and
+has the same type as the function return type.
+
+@smallexample
+struct T @{
+@dots{}
+@};
+T fn()
+@{
+  T t;
+  @dots{}
+  return std::move (t);
+@}
+@end smallexample
+
+But in this example, the @code{std::move} call prevents copy elision.
+
+This warning is enabled by @option{-Wall}.
+
 @item -fext-numeric-literals @r{(C++ and Objective-C++ only)}
 @opindex fext-numeric-literals
 @opindex fno-ext-numeric-literals
@@ -4036,6 +4062,7 @@ Options} and @ref{Objective-C and Objective-C++ Dialect Options}.
 -Wnonnull-compare  @gol
 -Wopenmp-simd @gol
 -Wparentheses  @gol
+-Wpessimizing-move @r{(only for C++)}  @gol
 -Wpointer-sign  @gol
 -Wreorder   @gol
 -Wrestrict   @gol
diff --git gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move1.C gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move1.C
index e69de29bb2d..858bed6065e 100644
--- gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move1.C
+++ gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move1.C
@@ -0,0 +1,132 @@
+// PR c++/86981
+// { 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 T {
+  T() { }
+  T(const T&) { }
+  T(T&&) { }
+};
+struct U {
+  U() { }
+  U(const U&) { }
+  U(U&&) { }
+  U(T) { }
+};
+
+T g;
+
+T
+fn1 ()
+{
+  T t;
+  return std::move (t); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+}
+
+T
+fn2 ()
+{
+  // Not a local variable.
+  return std::move (g);
+}
+
+int
+fn3 ()
+{
+  int i = 42;
+  // Not a class type.
+  return std::move (i);
+}
+
+T
+fn4 (bool b)
+{
+  T t;
+  if (b)
+    throw std::move (t);
+  return std::move (t); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+}
+
+T
+fn5 (T t)
+{
+  // Function parameter; std::move is redundant but not pessimizing.
+  return std::move (t);
+}
+
+U
+fn6 (T t, U u, bool b)
+{
+  if (b)
+    return std::move (t);
+  else
+    // Function parameter; std::move is redundant but not pessimizing.
+    return std::move (u);
+}
+
+U
+fn6 (bool b)
+{
+  T t;
+  U u;
+  if (b)
+    return std::move (t);
+  else
+    return std::move (u); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+}
+
+T
+fn7 ()
+{
+  static T t;
+  // Non-local; don't warn.
+  return std::move (t);
+}
+
+T
+fn8 ()
+{
+  return T();
+}
+
+T
+fn9 (int i)
+{
+  T t;
+
+  switch (i)
+    {
+    case 1:
+      return std::move ((t)); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+    case 2:
+      return (std::move (t)); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+    default:
+      return (std::move ((t))); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+    }
+}
+
+int
+fn10 ()
+{
+  return std::move (42);
+}
diff --git gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move2.C gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move2.C
index e69de29bb2d..0ee6e0535dc 100644
--- gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move2.C
+++ gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move2.C
@@ -0,0 +1,14 @@
+// PR c++/86981
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wpessimizing-move" }
+
+#include <string>
+#include <tuple>
+#include <utility>
+
+std::tuple<std::string, std::string>
+foo ()
+{
+  std::pair<std::string, std::string> p;
+  return std::move (p);
+}
diff --git gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
index e69de29bb2d..a1af1230b68 100644
--- gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
+++ gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
@@ -0,0 +1,59 @@
+// PR c++/86981
+// { 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 T { };
+struct U { U(T); };
+
+template<typename Tp>
+T
+fn1 ()
+{
+  T t;
+  // Non-dependent type.
+  return std::move (t); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+}
+
+template<typename Tp1, typename Tp2>
+Tp1
+fn2 ()
+{
+  Tp2 t;
+  return std::move (t); // { dg-warning "moving a local object in a return statement prevents copy elision" }
+}
+
+template<typename Tp1, typename Tp2>
+Tp1
+fn3 ()
+{
+  Tp2 t;
+  return std::move (t);
+}
+
+int
+main ()
+{
+  fn1<T>();
+  fn2<T, T>();
+  fn3<U, T>();
+}
diff --git gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move4.C gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move4.C
index e69de29bb2d..59e148e9f91 100644
--- gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move4.C
+++ gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move4.C
@@ -0,0 +1,46 @@
+// PR c++/86981
+// { 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 T { };
+
+T
+fn1 ()
+{
+  T t;
+  return (1, std::move (t));
+}
+
+T
+fn2 ()
+{
+  T t;
+  return [&](){ return std::move (t); }();
+}
+
+T
+fn3 ()
+{
+  T t;
+  return [=](){ return std::move (t); }();
+}
diff --git gcc/testsuite/g++.dg/cpp1z/Wpessimizing-move1.C gcc/testsuite/g++.dg/cpp1z/Wpessimizing-move1.C
index e69de29bb2d..59741889707 100644
--- gcc/testsuite/g++.dg/cpp1z/Wpessimizing-move1.C
+++ gcc/testsuite/g++.dg/cpp1z/Wpessimizing-move1.C
@@ -0,0 +1,18 @@
+// PR c++/86981
+// { dg-options "-Wpessimizing-move -std=c++17" }
+
+#include <utility>
+#include <optional>
+
+struct T {
+  T() { }
+  T(const T&) { }
+  T(T&&) { }
+};
+
+std::optional<T>
+fn ()
+{
+  T t;
+  return std::move (t);
+}


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