Ping. (The other std::move patches depend on this one.)
(can_do_rvo_p is renamed to can_elide_copy_prvalue_p in the PR90428 patch.)
On Tue, Aug 02, 2022 at 07:04:47PM -0400, Marek Polacek via Gcc-patches wrote:
> In my recent patch which enhanced -Wpessimizing-move so that it warns
> about class prvalues too I said that I'd like to extend it so that it
> warns in more contexts where a std::move can prevent copy elision, such
> as:
>
> T t = std::move(T());
> T t(std::move(T()));
> T t{std::move(T())};
> T t = {std::move(T())};
> void foo (T);
> foo (std::move(T()));
>
> This patch does that by adding two maybe_warn_pessimizing_move calls.
> These must happen before we've converted the initializers otherwise the
> std::move will be buried in a TARGET_EXPR.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> PR c++/106276
>
> gcc/cp/ChangeLog:
>
> * call.cc (build_over_call): Call maybe_warn_pessimizing_move.
> * cp-tree.h (maybe_warn_pessimizing_move): Declare.
> * decl.cc (build_aggr_init_full_exprs): Call
> maybe_warn_pessimizing_move.
> * typeck.cc (maybe_warn_pessimizing_move): Handle TREE_LIST and
> CONSTRUCTOR. Add a bool parameter and use it. Adjust a diagnostic
> message.
> (check_return_expr): Adjust the call to maybe_warn_pessimizing_move.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/Wpessimizing-move7.C: Add dg-warning.
> * g++.dg/cpp0x/Wpessimizing-move8.C: New test.
> ---
> gcc/cp/call.cc | 5 +-
> gcc/cp/cp-tree.h | 1 +
> gcc/cp/decl.cc | 3 +-
> gcc/cp/typeck.cc | 58 ++++++++++++-----
> .../g++.dg/cpp0x/Wpessimizing-move7.C | 16 ++---
> .../g++.dg/cpp0x/Wpessimizing-move8.C | 65 +++++++++++++++++++
> 6 files changed, 120 insertions(+), 28 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move8.C
>
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index 01a7be10077..370137ebd6d 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -9627,10 +9627,13 @@ build_over_call (struct z_candidate *cand, int flags,
> tsubst_flags_t complain)
> if (!conversion_warning)
> arg_complain &= ~tf_warning;
>
> + if (arg_complain & tf_warning)
> + maybe_warn_pessimizing_move (arg, type, /*return_p*/false);
> +
> val = convert_like_with_context (conv, arg, fn, i - is_method,
> arg_complain);
> val = convert_for_arg_passing (type, val, arg_complain);
> -
> +
> if (val == error_mark_node)
> return error_mark_node;
> else
> diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
> index 3278b4114bd..5a8af22b509 100644
> --- a/gcc/cp/cp-tree.h
> +++ b/gcc/cp/cp-tree.h
> @@ -8101,6 +8101,7 @@ extern tree finish_right_unary_fold_expr (tree,
> int);
> extern tree finish_binary_fold_expr (tree, tree, int);
> extern tree treat_lvalue_as_rvalue_p (tree, bool);
> extern bool decl_in_std_namespace_p (tree);
> +extern void maybe_warn_pessimizing_move (tree, tree, bool);
>
> /* in typeck2.cc */
> extern void require_complete_eh_spec_types (tree, tree);
> diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> index 70ad681467e..dc6853a7de1 100644
> --- a/gcc/cp/decl.cc
> +++ b/gcc/cp/decl.cc
> @@ -7220,9 +7220,10 @@ check_array_initializer (tree decl, tree type, tree
> init)
>
> static tree
> build_aggr_init_full_exprs (tree decl, tree init, int flags)
> -
> {
> gcc_assert (stmts_are_full_exprs_p ());
> + if (init)
> + maybe_warn_pessimizing_move (init, TREE_TYPE (decl), /*return_p*/false);
> return build_aggr_init (decl, init, flags, tf_warning_or_error);
> }
>
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 9500c4e2fe8..2650beb780e 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -10368,17 +10368,17 @@ treat_lvalue_as_rvalue_p (tree expr, bool return_p)
> }
> }
>
> -/* Warn about wrong usage of std::move in a return statement. RETVAL
> - is the expression we are returning; FUNCTYPE is the type the function
> - is declared to return. */
> +/* Warn about dubious usage of std::move (in a return statement, if RETURN_P
> + is true). EXPR is the std::move expression; TYPE is the type of the
> object
> + being initialized. */
>
> -static void
> -maybe_warn_pessimizing_move (tree retval, tree functype)
> +void
> +maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
> {
> if (!(warn_pessimizing_move || warn_redundant_move))
> return;
>
> - location_t loc = cp_expr_loc_or_input_loc (retval);
> + const location_t loc = cp_expr_loc_or_input_loc (expr);
>
> /* C++98 doesn't know move. */
> if (cxx_dialect < cxx11)
> @@ -10390,14 +10390,32 @@ maybe_warn_pessimizing_move (tree retval, tree
> functype)
> return;
>
> /* This is only interesting for class types. */
> - if (!CLASS_TYPE_P (functype))
> + if (!CLASS_TYPE_P (type))
> return;
>
> + /* A a = std::move (A()); */
> + if (TREE_CODE (expr) == TREE_LIST)
> + {
> + if (list_length (expr) == 1)
> + expr = TREE_VALUE (expr);
> + else
> + return;
> + }
> + /* A a = {std::move (A())};
> + A a{std::move (A())}; */
> + else if (TREE_CODE (expr) == CONSTRUCTOR)
> + {
> + if (CONSTRUCTOR_NELTS (expr) == 1)
> + expr = CONSTRUCTOR_ELT (expr, 0)->value;
> + else
> + return;
> + }
> +
> /* We're looking for *std::move<T&> ((T &) &arg). */
> - if (REFERENCE_REF_P (retval)
> - && TREE_CODE (TREE_OPERAND (retval, 0)) == CALL_EXPR)
> + if (REFERENCE_REF_P (expr)
> + && TREE_CODE (TREE_OPERAND (expr, 0)) == CALL_EXPR)
> {
> - tree fn = TREE_OPERAND (retval, 0);
> + tree fn = TREE_OPERAND (expr, 0);
> if (is_std_move_p (fn))
> {
> tree arg = CALL_EXPR_ARG (fn, 0);
> @@ -10409,20 +10427,24 @@ maybe_warn_pessimizing_move (tree retval, tree
> functype)
> return;
> arg = TREE_OPERAND (arg, 0);
> arg = convert_from_reference (arg);
> - /* Warn if we could do copy elision were it not for the move. */
> - if (can_do_nrvo_p (arg, functype))
> + if (can_do_rvo_p (arg, type))
> {
> auto_diagnostic_group d;
> if (warning_at (loc, OPT_Wpessimizing_move,
> - "moving a local object in a return statement "
> - "prevents copy elision"))
> + "moving a temporary object prevents copy "
> + "elision"))
> inform (loc, "remove %<std::move%> call");
> }
> - else if (can_do_rvo_p (arg, functype))
> + /* The rest of the warnings is only relevant for when we are
> + returning from a function. */
> + else if (!return_p)
> + return;
> + /* Warn if we could do copy elision were it not for the move. */
> + else if (can_do_nrvo_p (arg, type))
> {
> auto_diagnostic_group d;
> if (warning_at (loc, OPT_Wpessimizing_move,
> - "moving a temporary object in a return statement "
> + "moving a local object in a return statement "
> "prevents copy elision"))
> inform (loc, "remove %<std::move%> call");
> }
> @@ -10433,7 +10455,7 @@ maybe_warn_pessimizing_move (tree retval, tree
> functype)
> {
> /* Make sure that overload resolution would actually succeed
> if we removed the std::move call. */
> - tree t = convert_for_initialization (NULL_TREE, functype,
> + tree t = convert_for_initialization (NULL_TREE, type,
> moved,
> (LOOKUP_NORMAL
> | LOOKUP_ONLYCONVERTING
> @@ -10718,7 +10740,7 @@ check_return_expr (tree retval, bool *no_warning)
> return NULL_TREE;
>
> if (!named_return_value_okay_p)
> - maybe_warn_pessimizing_move (retval, functype);
> + maybe_warn_pessimizing_move (retval, functype, /*return_p*/true);
>
> /* Do any required conversions. */
> if (bare_retval == result || DECL_CONSTRUCTOR_P (current_function_decl))
> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
> b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
> index cd4eaa09aae..a17c7a87b7d 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
> @@ -30,23 +30,23 @@ static A foo ();
> A
> fn1 ()
> {
> - return std::move (A{}); // { dg-warning "moving a temporary object in a
> return statement prevents copy elision" }
> - return std::move (A()); // { dg-warning "moving a temporary object in a
> return statement prevents copy elision" }
> - return std::move (foo ()); // { dg-warning "moving a temporary object in a
> return statement prevents copy elision" }
> + return std::move (A{}); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + return std::move (A()); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + return std::move (foo ()); // { dg-warning "moving a temporary object
> prevents copy elision" }
> }
>
> B fn2 ()
> {
> - return std::move (A());
> - return std::move (A{});
> - return std::move (foo ());
> + return std::move (A()); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + return std::move (A{}); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + return std::move (foo ()); // { dg-warning "moving a temporary object
> prevents copy elision" }
> }
>
> template <typename T1, typename T2>
> T1
> fn3 ()
> {
> - return std::move (T2{}); // { dg-warning "moving a temporary object in a
> return statement prevents copy elision" }
> + return std::move (T2{}); // { dg-warning "moving a temporary object
> prevents copy elision" }
> }
>
> void
> @@ -58,6 +58,6 @@ do_fn3 ()
>
> char take_buffer;
> struct label_text {
> - label_text take() { return std::move(label_text(&take_buffer)); } // {
> dg-warning "moving a temporary object in a return statement prevents copy
> elision" }
> + label_text take() { return std::move(label_text(&take_buffer)); } // {
> dg-warning "moving a temporary object prevents copy elision" }
> label_text(char *);
> };
> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move8.C
> b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move8.C
> new file mode 100644
> index 00000000000..51406c8f97f
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move8.C
> @@ -0,0 +1,65 @@
> +// PR c++/106276
> +// { 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 A { A(); A(const A&) = delete; A(A&&); };
> +struct B { B(A); };
> +struct X { };
> +
> +void foo (A);
> +void bar (X);
> +
> +void
> +fn1 ()
> +{
> + A a1 = std::move (A()); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + A a2 = std::move (A{}); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + A a3(std::move (A())); // { dg-warning "moving a temporary object prevents
> copy elision" }
> + A a4(std::move (A{})); // { dg-warning "moving a temporary object prevents
> copy elision" }
> + A a5{std::move (A())}; // { dg-warning "moving a temporary object prevents
> copy elision" }
> + A a6{std::move (A{})}; // { dg-warning "moving a temporary object prevents
> copy elision" }
> + A a7 = {std::move (A())}; // { dg-warning "moving a temporary object
> prevents copy elision" }
> + A a8 = {std::move (A{})}; // { dg-warning "moving a temporary object
> prevents copy elision" }
> +
> + B b1 = std::move (A()); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + B b2(std::move (A())); // { dg-warning "moving a temporary object prevents
> copy elision" }
> + B b3{std::move (A())}; // { dg-warning "moving a temporary object prevents
> copy elision" }
> + B b4 = {std::move (A())}; // { dg-warning "moving a temporary object
> prevents copy elision" }
> +
> + X x1 = std::move (X()); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + X x2 = std::move (X{}); // { dg-warning "moving a temporary object
> prevents copy elision" }
> + X x3(std::move (X())); // { dg-warning "moving a temporary object prevents
> copy elision" }
> + X x4(std::move (X{})); // { dg-warning "moving a temporary object prevents
> copy elision" }
> + X x5{std::move (X())}; // { dg-warning "moving a temporary object prevents
> copy elision" }
> + X x6{std::move (X{})}; // { dg-warning "moving a temporary object prevents
> copy elision" }
> + X x7 = {std::move (X())}; // { dg-warning "moving a temporary object
> prevents copy elision" }
> + X x8 = {std::move (X{})}; // { dg-warning "moving a temporary object
> prevents copy elision" }
> +
> + foo (std::move (A())); // { dg-warning "moving a temporary object prevents
> copy elision" }
> + foo (std::move (A{})); // { dg-warning "moving a temporary object prevents
> copy elision" }
> + bar (std::move (X())); // { dg-warning "moving a temporary object prevents
> copy elision" }
> + bar (std::move (X{})); // { dg-warning "moving a temporary object prevents
> copy elision" }
> +
> + foo (std::move (a1));
> + bar (std::move (x1));
> +}
>
> base-commit: 502605a277d36cee1b0570982a16d97a43eace67
> prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5
> --
> 2.37.1
>
Marek
