From: Jason Merrill <jason@redhat.com>
To: Marek Polacek <polacek@redhat.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH v2] c++: Extend -Wredundant-move for const-qual objects [PR90428]
Date: Mon, 15 Aug 2022 15:43:38 -0400 [thread overview]
Message-ID: <219680be-9ebc-a2aa-c6c6-4f1d2d8f1e8f@redhat.com> (raw)
In-Reply-To: <YvFxnuEhiqRkdMeu@redhat.com>
On 8/8/22 13:27, Marek Polacek wrote:
> On Sat, Aug 06, 2022 at 03:58:13PM -0800, Jason Merrill wrote:
>> On 8/6/22 11:13, Marek Polacek wrote:
>>> In this PR, Jon suggested extending the -Wredundant-move warning
>>> to warn when the user is moving a const object as in:
>>>
>>> struct T { };
>>>
>>> T f(const T& t)
>>> {
>>> return std::move(t);
>>> }
>>>
>>> where the std::move is redundant, because T does not have
>>> a T(const T&&) constructor (which is very unlikely). Even with
>>> the std::move, T(T&&) would not be used because it would mean
>>> losing the const. Instead, T(const T&) will be called.
>>>
>>> I had to restructure the function a bit, but it's better now. This patch
>>> depends on my other recent patches to maybe_warn_pessimizing_move.
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> PR c++/90428
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> * typeck.cc (maybe_warn_pessimizing_move): Extend the
>>> -Wredundant-move warning to warn about std::move on a
>>> const-qualified object.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> * g++.dg/cpp0x/Wredundant-move1.C: Adjust dg-warning.
>>> * g++.dg/cpp0x/Wredundant-move9.C: Likewise.
>>> * g++.dg/cpp0x/Wredundant-move10.C: New test.
>>> ---
>>> gcc/cp/typeck.cc | 157 +++++++++++-------
>>> gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C | 3 +-
>>> .../g++.dg/cpp0x/Wredundant-move10.C | 61 +++++++
>>> gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C | 3 +-
>>> 4 files changed, 162 insertions(+), 62 deletions(-)
>>> create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C
>>>
>>> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
>>> index 70a5efc45de..802bc9c43fb 100644
>>> --- a/gcc/cp/typeck.cc
>>> +++ b/gcc/cp/typeck.cc
>>> @@ -10411,72 +10411,109 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
>>> return;
>>> }
>>> - /* We're looking for *std::move<T&> ((T &) &arg). */
>>> - if (REFERENCE_REF_P (expr)
>>> - && TREE_CODE (TREE_OPERAND (expr, 0)) == CALL_EXPR)
>>> - {
>>> - tree fn = TREE_OPERAND (expr, 0);
>>> - if (is_std_move_p (fn))
>>> - {
>>> - tree arg = CALL_EXPR_ARG (fn, 0);
>>> - tree moved;
>>> - if (TREE_CODE (arg) != NOP_EXPR)
>>> - return;
>>> - arg = TREE_OPERAND (arg, 0);
>>> - if (TREE_CODE (arg) != ADDR_EXPR)
>>> - return;
>>> - arg = TREE_OPERAND (arg, 0);
>>> - arg = convert_from_reference (arg);
>>> - if (can_do_rvo_p (arg, type))
>>> - {
>>> - auto_diagnostic_group d;
>>> - if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
>>> - && warning_at (loc, OPT_Wpessimizing_move,
>>> - "moving a temporary object prevents copy "
>>> - "elision"))
>>> - inform (loc, "remove %<std::move%> call");
>>> - }
>>> - /* 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))
>>> + /* First, check if this is a call to std::move. */
>>> + if (!REFERENCE_REF_P (expr)
>>> + || TREE_CODE (TREE_OPERAND (expr, 0)) != CALL_EXPR)
>>> + return;
>>> + tree fn = TREE_OPERAND (expr, 0);
>>> + if (!is_std_move_p (fn))
>>> + return;
>>> + tree arg = CALL_EXPR_ARG (fn, 0);
>>> + if (TREE_CODE (arg) != NOP_EXPR)
>>> + return;
>>> + /* If we're looking at *std::move<T&> ((T &) &arg), do the pessimizing N/RVO
>>> + and implicitly-movable warnings. */
>>> + if (TREE_CODE (TREE_OPERAND (arg, 0)) == ADDR_EXPR)
>>> + {
>>> + arg = TREE_OPERAND (arg, 0);
>>> + arg = TREE_OPERAND (arg, 0);
>>> + arg = convert_from_reference (arg);
>>> + if (can_do_rvo_p (arg, type))
>>
>> Incidentally, this function should probably have a different name if we're
>> checking it in non-return situations.
>
> I've renamed it to can_elide_copy_prvalue_p in this patch.
>
>>> + {
>>> + auto_diagnostic_group d;
>>> + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
>>> + && warning_at (loc, OPT_Wpessimizing_move,
>>> + "moving a temporary object prevents copy elision"))
>>> + inform (loc, "remove %<std::move%> call");
>>> + }
>>> + /* The rest of the warnings is only relevant for when we are returning
>>> + from a function. */
>>> + if (!return_p)
>>> + return;
>>> +
>>> + tree moved;
>>> + /* Warn if we could do copy elision were it not for the move. */
>>> + if (can_do_nrvo_p (arg, type))
>>> + {
>>> + auto_diagnostic_group d;
>>> + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
>>> + && warning_at (loc, OPT_Wpessimizing_move,
>>> + "moving a local object in a return statement "
>>> + "prevents copy elision"))
>>> + inform (loc, "remove %<std::move%> call");
>>> + }
>>> + /* 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
>>> + && !warning_suppressed_p (expr, OPT_Wredundant_move)
>>> + && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true)))
>>> + {
>>> + /* Make sure that overload resolution would actually succeed
>>> + if we removed the std::move call. */
>>> + tree t = convert_for_initialization (NULL_TREE, type,
>>> + moved,
>>> + (LOOKUP_NORMAL
>>> + | LOOKUP_ONLYCONVERTING
>>> + | LOOKUP_PREFER_RVALUE),
>>> + ICR_RETURN, NULL_TREE, 0,
>>> + tf_none);
>>> + /* If this worked, implicit rvalue would work, so the call to
>>> + std::move is redundant. */
>>> + if (t != error_mark_node
>>> + /* Trying to move something const will never succeed unless
>>> + there's T(const T&&), which it almost never is, and if
>>> + so, T wouldn't be error_mark_node now. So do warn. */
>>> + || (CP_TYPE_CONST_P (TREE_TYPE (arg))
>>> + && same_type_ignoring_top_level_qualifiers_p
>>> + (TREE_TYPE (arg), type)))
>>
>> I'm confused by the || case here. Overload resolution fails if we drop the
>> move call, but we're still going to warn?
>
> This is to warn about this:
>
> T f5(const T t)
> {
> return std::move(t); // { dg-warning "redundant move" }
> }
>
> where OR fails because there's no T(const T&&) (or it's deleted in which case
> convert_for_initialization also returns error_mark_node). This OR is going to
> fail with std::move but also without std::move when we're trying to treat an lvalue
> as an rvalue. So the std::move has no effect, because T(const T&) will be
> called in either case.
>
> Now, if there was a T(const T&&), we'd *still* warn, because the std::move would
> still be redundant. Does that make sense?
Ah, so this is because LOOKUP_PREFER_RVALUE returns an error if a const
T& overload is selected. The comment should mention that. OK with that
change.
Since you've been thinking about this area, I wonder if you want to look
at implementing P2266?
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> -- >8 --
> In this PR, Jon suggested extending the -Wredundant-move warning
> to warn when the user is moving a const object as in:
>
> struct T { };
>
> T f(const T& t)
> {
> return std::move(t);
> }
>
> where the std::move is redundant, because T does not have
> a T(const T&&) constructor (which is very unlikely). Even with
> the std::move, T(T&&) would not be used because it would mean
> losing the const. Instead, T(const T&) will be called.
>
> I had to restructure the function a bit, but it's better now. This patch
> depends on my other recent patches to maybe_warn_pessimizing_move.
>
> PR c++/90428
>
> gcc/cp/ChangeLog:
>
> * typeck.cc (can_do_rvo_p): Rename to ...
> (can_elide_copy_prvalue_p): ... this.
> (maybe_warn_pessimizing_move): Extend the
> -Wredundant-move warning to warn about std::move on a
> const-qualified object.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/Wredundant-move1.C: Adjust dg-warning.
> * g++.dg/cpp0x/Wredundant-move9.C: Likewise.
> * g++.dg/cpp0x/Wredundant-move10.C: New test.
> ---
> gcc/cp/typeck.cc | 157 +++++++++++-------
> gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C | 3 +-
> .../g++.dg/cpp0x/Wredundant-move10.C | 61 +++++++
> gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C | 3 +-
> 4 files changed, 162 insertions(+), 62 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C
>
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 70a5efc45de..5be79b83871 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -10297,7 +10297,7 @@ can_do_nrvo_p (tree retval, tree functype)
> prvalue. */
>
> static bool
> -can_do_rvo_p (tree retval, tree functype)
> +can_elide_copy_prvalue_p (tree retval, tree functype)
> {
> if (functype == error_mark_node)
> return false;
> @@ -10411,72 +10411,109 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
> return;
> }
>
> - /* We're looking for *std::move<T&> ((T &) &arg). */
> - if (REFERENCE_REF_P (expr)
> - && TREE_CODE (TREE_OPERAND (expr, 0)) == CALL_EXPR)
> + /* First, check if this is a call to std::move. */
> + if (!REFERENCE_REF_P (expr)
> + || TREE_CODE (TREE_OPERAND (expr, 0)) != CALL_EXPR)
> + return;
> + tree fn = TREE_OPERAND (expr, 0);
> + if (!is_std_move_p (fn))
> + return;
> + tree arg = CALL_EXPR_ARG (fn, 0);
> + if (TREE_CODE (arg) != NOP_EXPR)
> + return;
> + /* If we're looking at *std::move<T&> ((T &) &arg), do the pessimizing N/RVO
> + and implicitly-movable warnings. */
> + if (TREE_CODE (TREE_OPERAND (arg, 0)) == ADDR_EXPR)
> {
> - tree fn = TREE_OPERAND (expr, 0);
> - if (is_std_move_p (fn))
> - {
> - tree arg = CALL_EXPR_ARG (fn, 0);
> - tree moved;
> - if (TREE_CODE (arg) != NOP_EXPR)
> - return;
> - arg = TREE_OPERAND (arg, 0);
> - if (TREE_CODE (arg) != ADDR_EXPR)
> - return;
> - arg = TREE_OPERAND (arg, 0);
> - arg = convert_from_reference (arg);
> - if (can_do_rvo_p (arg, type))
> - {
> - auto_diagnostic_group d;
> - if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
> - && warning_at (loc, OPT_Wpessimizing_move,
> - "moving a temporary object prevents copy "
> - "elision"))
> - inform (loc, "remove %<std::move%> call");
> - }
> - /* 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))
> + arg = TREE_OPERAND (arg, 0);
> + arg = TREE_OPERAND (arg, 0);
> + arg = convert_from_reference (arg);
> + if (can_elide_copy_prvalue_p (arg, type))
> + {
> + auto_diagnostic_group d;
> + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
> + && warning_at (loc, OPT_Wpessimizing_move,
> + "moving a temporary object prevents copy elision"))
> + inform (loc, "remove %<std::move%> call");
> + }
> + /* The rest of the warnings is only relevant for when we are returning
> + from a function. */
> + if (!return_p)
> + return;
> +
> + tree moved;
> + /* Warn if we could do copy elision were it not for the move. */
> + if (can_do_nrvo_p (arg, type))
> + {
> + auto_diagnostic_group d;
> + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
> + && warning_at (loc, OPT_Wpessimizing_move,
> + "moving a local object in a return statement "
> + "prevents copy elision"))
> + inform (loc, "remove %<std::move%> call");
> + }
> + /* 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
> + && !warning_suppressed_p (expr, OPT_Wredundant_move)
> + && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true)))
> + {
> + /* Make sure that overload resolution would actually succeed
> + if we removed the std::move call. */
> + tree t = convert_for_initialization (NULL_TREE, type,
> + moved,
> + (LOOKUP_NORMAL
> + | LOOKUP_ONLYCONVERTING
> + | LOOKUP_PREFER_RVALUE),
> + ICR_RETURN, NULL_TREE, 0,
> + tf_none);
> + /* If this worked, implicit rvalue would work, so the call to
> + std::move is redundant. */
> + if (t != error_mark_node
> + /* Trying to move something const will never succeed unless
> + there's T(const T&&), which it almost never is, and if
> + so, T wouldn't be error_mark_node now. So do warn. */
> + || (CP_TYPE_CONST_P (TREE_TYPE (arg))
> + && same_type_ignoring_top_level_qualifiers_p
> + (TREE_TYPE (arg), type)))
> {
> auto_diagnostic_group d;
> - if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
> - && warning_at (loc, OPT_Wpessimizing_move,
> - "moving a local object in a return statement "
> - "prevents copy elision"))
> + if (warning_at (loc, OPT_Wredundant_move,
> + "redundant move in return statement"))
> inform (loc, "remove %<std::move%> call");
> }
> - /* 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
> - && !warning_suppressed_p (expr, OPT_Wredundant_move)
> - && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true)))
> - {
> - /* Make sure that overload resolution would actually succeed
> - if we removed the std::move call. */
> - tree t = convert_for_initialization (NULL_TREE, type,
> - moved,
> - (LOOKUP_NORMAL
> - | LOOKUP_ONLYCONVERTING
> - | LOOKUP_PREFER_RVALUE),
> - ICR_RETURN, NULL_TREE, 0,
> - tf_none);
> - /* If this worked, implicit rvalue would work, so the call to
> - std::move is redundant. */
> - if (t != error_mark_node)
> - {
> - auto_diagnostic_group d;
> - if (warning_at (loc, OPT_Wredundant_move,
> - "redundant move in return statement"))
> - inform (loc, "remove %<std::move%> call");
> - }
> - }
> }
> }
> + /* Also try to warn about redundant std::move in code such as
> + T f (const T& t)
> + {
> + return std::move(t);
> + }
> + for which EXPR will be something like
> + *std::move<const T&> ((const struct T &) (const struct T *) t)
> + and where the std::move does nothing if T does not have a T(const T&&)
> + constructor, because the argument is const. It will not use T(T&&)
> + because that would mean losing the const. */
> + else if (TREE_CODE (TREE_TYPE (arg)) == REFERENCE_TYPE
> + && CP_TYPE_CONST_P (TREE_TYPE (TREE_TYPE (arg))))
> + {
> + tree rtype = TREE_TYPE (TREE_TYPE (arg));
> + if (!same_type_ignoring_top_level_qualifiers_p (rtype, type))
> + return;
> + /* Check for the unlikely case there's T(const T&&) (we don't care if
> + it's deleted). */
> + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (rtype)))
> + if (move_fn_p (fn))
> + {
> + tree t = TREE_VALUE (FUNCTION_FIRST_USER_PARMTYPE (fn));
> + if (UNLIKELY (CP_TYPE_CONST_P (TREE_TYPE (t))))
> + return;
> + }
> + auto_diagnostic_group d;
> + if (warning_at (loc, OPT_Wredundant_move,
> + "redundant move in return statement"))
> + inform (loc, "remove %<std::move%> call");
> + }
> }
>
> /* Check that returning RETVAL from the current function is valid.
> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C
> index ce4087b476f..c227019cce1 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move1.C
> @@ -60,7 +60,8 @@ fn4 (const T t)
> {
> // t is const: will decay into copy despite std::move, so it's redundant.
> // We used to warn about this, but no longer since c++/87378.
> - return std::move (t); // { dg-warning "redundant move" "" { target c++20 } }
> + // Now we warn again since c++/90428.
> + return std::move (t); // { dg-warning "redundant move" }
> }
>
> int
> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C
> new file mode 100644
> index 00000000000..a215a4774d6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move10.C
> @@ -0,0 +1,61 @@
> +// PR c++/90428
> +// { 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 T { T(); T(const T&); T(T&&) = delete; };
> +struct S : T { };
> +struct W { W(const W&); W(W&&) = delete; W(const W&&); };
> +
> +T f1(T t)
> +{
> + const T& rt = t;
> + return std::move(rt); // { dg-warning "redundant move" }
> +}
> +
> +T f2(const T& t)
> +{
> + return std::move(t); // { dg-warning "redundant move" }
> +}
> +
> +W f3(const W& w)
> +{
> + return std::move(w);
> +}
> +
> +T f4(const S& s)
> +{
> + return std::move(s);
> +}
> +
> +T f5(const T t)
> +{
> + return std::move(t); // { dg-warning "redundant move" }
> +}
> +
> +struct S1 { S1(S1 &&) = delete; S1(const S1&); };
> +struct S2: S1 {};
> +
> +S1 f3(const S2 s)
> +{
> + return std::move(s); // { dg-warning "redundant move" "" { target c++20 } }
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C
> index ca1e23b7a4b..489ecd2b9c9 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move9.C
> @@ -61,7 +61,8 @@ fn4 (const T<int> t)
> {
> // t is const: will decay into copy despite std::move, so it's redundant.
> // We used to warn about this, but no longer since c++/87378.
> - return std::move (t); // { dg-warning "redundant move" "" { target c++20 } }
> + // Now we warn again since c++/90428.
> + return std::move (t); // { dg-warning "redundant move" }
> }
>
> int
>
> base-commit: 053876cdbe8057210e6f4da4eec2df58f92ccd4c
> prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5
> prerequisite-patch-id: ce490c3c0b991fa7f27315387949c145c66223a4
> prerequisite-patch-id: bf551f0fe75fa8f6775fe53469b61ae94d8cab81
next prev parent reply other threads:[~2022-08-15 19:43 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-06 18:13 [PATCH] " Marek Polacek
2022-08-06 23:58 ` Jason Merrill
2022-08-08 20:27 ` [PATCH v2] " Marek Polacek
2022-08-09 15:08 ` Marek Polacek
2022-08-15 19:43 ` Jason Merrill [this message]
2022-08-17 19:09 ` Marek Polacek
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