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++: Tweak for -Wpessimizing-move in templates [PR89780]
Date: Thu, 11 Aug 2022 14:24:26 -0800 [thread overview]
Message-ID: <f7409f56-3c22-4761-22f1-0e3f58af1d01@redhat.com> (raw)
In-Reply-To: <YvFpVOf/UFUqbaM1@redhat.com>
On 8/8/22 12:51, Marek Polacek wrote:
> On Sat, Aug 06, 2022 at 04:02:13PM -0700, Jason Merrill wrote:
>> On 8/4/22 11:46, Marek Polacek wrote:
>>> In my previous patches I've been extending our std::move warnings,
>>> but this tweak actually dials it down a little bit. As reported in
>>> bug 89780, it's questionable to warn about expressions in templates
>>> that were type-dependent, but aren't anymore because we're instantiating
>>> the template. As in,
>>>
>>> template <typename T>
>>> Dest withMove() {
>>> T x;
>>> return std::move(x);
>>> }
>>>
>>> template Dest withMove<Dest>(); // #1
>>> template Dest withMove<Source>(); // #2
>>>
>>> Saying that the std::move is pessimizing for #1 is not incorrect, but
>>> it's not useful, because removing the std::move would then pessimize #2.
>>> So the user can't really win. At the same time, disabling the warning
>>> just because we're in a template would be going too far, I still want to
>>> warn for
>>>
>>> template <typename>
>>> Dest withMove() {
>>> Dest x;
>>> return std::move(x);
>>> }
>>>
>>> because the std::move therein will be pessimizing for any instantiation.
>>>
>>> So I'm using the suppress_warning machinery to that effect.
>>> Problem: I had to add a new group to nowarn_spec_t, otherwise
>>> suppressing the -Wpessimizing-move warning would disable a whole bunch
>>> of other warnings, which we really don't want.
>>>
>>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>>
>>> PR c++/89780
>>>
>>> gcc/cp/ChangeLog:
>>>
>>> * pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Maybe suppress
>>> -Wpessimizing-move.
>>> * typeck.cc (maybe_warn_pessimizing_move): Don't issue warnings
>>> if they are suppressed.
>>> (check_return_expr): Disable -Wpessimizing-move when returning
>>> a dependent expression.
>>>
>>> gcc/ChangeLog:
>>>
>>> * diagnostic-spec.cc (nowarn_spec_t::nowarn_spec_t): Handle
>>> OPT_Wpessimizing_move and OPT_Wredundant_move.
>>> * diagnostic-spec.h (nowarn_spec_t): Add NW_REDUNDANT enumerator.
>>>
>>> gcc/testsuite/ChangeLog:
>>>
>>> * g++.dg/cpp0x/Wpessimizing-move3.C: Remove dg-warning.
>>> * g++.dg/cpp0x/Wpessimizing-move7.C: Likewise.
>>> * g++.dg/cpp0x/Wredundant-move2.C: Likewise.
>>> * g++.dg/cpp0x/Wpessimizing-move9.C: New test.
>>> ---
>>> gcc/cp/pt.cc | 3 +
>>> gcc/cp/typeck.cc | 20 +++--
>>> gcc/diagnostic-spec.cc | 7 +-
>>> gcc/diagnostic-spec.h | 4 +-
>>> .../g++.dg/cpp0x/Wpessimizing-move3.C | 2 +-
>>> .../g++.dg/cpp0x/Wpessimizing-move7.C | 2 +-
>>> .../g++.dg/cpp0x/Wpessimizing-move9.C | 89 +++++++++++++++++++
>>> gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C | 4 +-
>>> 8 files changed, 119 insertions(+), 12 deletions(-)
>>> create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C
>>>
>>> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
>>> index 6c581fe0fb7..fe7e809fc2d 100644
>>> --- a/gcc/cp/pt.cc
>>> +++ b/gcc/cp/pt.cc
>>> @@ -21215,6 +21215,9 @@ tsubst_copy_and_build (tree t,
>>> CALL_EXPR_ORDERED_ARGS (call) = ord;
>>> CALL_EXPR_REVERSE_ARGS (call) = rev;
>>> }
>>> + if (warning_suppressed_p (t, OPT_Wpessimizing_move))
>>> + /* This also suppresses -Wredundant-move. */
>>> + suppress_warning (ret, OPT_Wpessimizing_move);
>>> }
>>> RETURN (ret);
>>> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
>>> index 2650beb780e..70a5efc45de 100644
>>> --- a/gcc/cp/typeck.cc
>>> +++ b/gcc/cp/typeck.cc
>>> @@ -10430,9 +10430,10 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
>>> if (can_do_rvo_p (arg, type))
>>> {
>>> auto_diagnostic_group d;
>>> - if (warning_at (loc, OPT_Wpessimizing_move,
>>> - "moving a temporary object prevents copy "
>>> - "elision"))
>>> + if (!warning_suppressed_p (expr, OPT_Wpessimizing_move)
>>
>> I don't think we ever want to suppress this warning; moving it to a
>> different warning flag (as I suggested on the other patch) would accomplish
>> that.
>
> Agreed. I just removed the warning_suppressed_p check though, a new flag would
> need another NW_ group etc.
>
>>> + && 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
>>> @@ -10443,14 +10444,16 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
>>> else if (can_do_nrvo_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"))
>>> + 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
>>> @@ -10695,6 +10698,11 @@ check_return_expr (tree retval, bool *no_warning)
>>> /* We don't know if this is an lvalue or rvalue use, but
>>> either way we can mark it as read. */
>>> mark_exp_read (retval);
>>> + /* Disable our std::move warnings when we're returning
>>> + a dependent expression (c++/89780). */
>>> + if (retval && TREE_CODE (retval) == CALL_EXPR)
>>
>> Should this check type_dependent_expression_p?
>
> We already check that, this block is guarded by:
>
> if (dependent_type_p (functype)
> || type_dependent_expression_p (retval))
> {
> dependent:
>
>>> --- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
>>> +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
>>> @@ -46,7 +46,7 @@ template <typename T1, typename T2>
>>> T1
>>> fn3 ()
>>> {
>>> - return std::move (T2{}); // { dg-warning "moving a temporary object prevents copy elision" }
>>> + return std::move (T2{});
>>
>> As mentioned above, we want to keep this warning; the dangling reference
>> problem is not type-dependent.
>
> Fixed.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
OK.
> -- >8 --
> In my previous patches I've been extending our std::move warnings,
> but this tweak actually dials it down a little bit. As reported in
> bug 89780, it's questionable to warn about expressions in templates
> that were type-dependent, but aren't anymore because we're instantiating
> the template. As in,
>
> template <typename T>
> Dest withMove() {
> T x;
> return std::move(x);
> }
>
> template Dest withMove<Dest>(); // #1
> template Dest withMove<Source>(); // #2
>
> Saying that the std::move is pessimizing for #1 is not incorrect, but
> it's not useful, because removing the std::move would then pessimize #2.
> So the user can't really win. At the same time, disabling the warning
> just because we're in a template would be going too far, I still want to
> warn for
>
> template <typename>
> Dest withMove() {
> Dest x;
> return std::move(x);
> }
>
> because the std::move therein will be pessimizing for any instantiation.
>
> So I'm using the suppress_warning machinery to that effect.
> Problem: I had to add a new group to nowarn_spec_t, otherwise
> suppressing the -Wpessimizing-move warning would disable a whole bunch
> of other warnings, which we really don't want.
>
> PR c++/89780
>
> gcc/cp/ChangeLog:
>
> * pt.cc (tsubst_copy_and_build) <case CALL_EXPR>: Maybe suppress
> -Wpessimizing-move.
> * typeck.cc (maybe_warn_pessimizing_move): Don't issue warnings
> if they are suppressed.
> (check_return_expr): Disable -Wpessimizing-move when returning
> a dependent expression.
>
> gcc/ChangeLog:
>
> * diagnostic-spec.cc (nowarn_spec_t::nowarn_spec_t): Handle
> OPT_Wpessimizing_move and OPT_Wredundant_move.
> * diagnostic-spec.h (nowarn_spec_t): Add NW_REDUNDANT enumerator.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/Wpessimizing-move3.C: Remove dg-warning.
> * g++.dg/cpp0x/Wredundant-move2.C: Likewise.
> ---
> gcc/cp/pt.cc | 3 +
> gcc/cp/typeck.cc | 13 ++-
> gcc/diagnostic-spec.cc | 7 +-
> gcc/diagnostic-spec.h | 4 +-
> .../g++.dg/cpp0x/Wpessimizing-move3.C | 2 +-
> .../g++.dg/cpp0x/Wpessimizing-move9.C | 89 +++++++++++++++++++
> gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C | 4 +-
> 7 files changed, 114 insertions(+), 8 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C
>
> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> index 6c581fe0fb7..fe7e809fc2d 100644
> --- a/gcc/cp/pt.cc
> +++ b/gcc/cp/pt.cc
> @@ -21215,6 +21215,9 @@ tsubst_copy_and_build (tree t,
> CALL_EXPR_ORDERED_ARGS (call) = ord;
> CALL_EXPR_REVERSE_ARGS (call) = rev;
> }
> + if (warning_suppressed_p (t, OPT_Wpessimizing_move))
> + /* This also suppresses -Wredundant-move. */
> + suppress_warning (ret, OPT_Wpessimizing_move);
> }
>
> RETURN (ret);
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 2650beb780e..f4104c75fbe 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -10443,14 +10443,16 @@ maybe_warn_pessimizing_move (tree expr, tree type, bool return_p)
> else if (can_do_nrvo_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"))
> + 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
> @@ -10695,6 +10697,11 @@ check_return_expr (tree retval, bool *no_warning)
> /* We don't know if this is an lvalue or rvalue use, but
> either way we can mark it as read. */
> mark_exp_read (retval);
> + /* Disable our std::move warnings when we're returning
> + a dependent expression (c++/89780). */
> + if (retval && TREE_CODE (retval) == CALL_EXPR)
> + /* This also suppresses -Wredundant-move. */
> + suppress_warning (retval, OPT_Wpessimizing_move);
> return retval;
> }
>
> diff --git a/gcc/diagnostic-spec.cc b/gcc/diagnostic-spec.cc
> index 4341ccfaae9..aece89619e7 100644
> --- a/gcc/diagnostic-spec.cc
> +++ b/gcc/diagnostic-spec.cc
> @@ -96,7 +96,7 @@ nowarn_spec_t::nowarn_spec_t (opt_code opt)
> case OPT_Winit_self:
> case OPT_Wuninitialized:
> case OPT_Wmaybe_uninitialized:
> - m_bits = NW_UNINIT;
> + m_bits = NW_UNINIT;
> break;
>
> case OPT_Wdangling_pointer_:
> @@ -105,6 +105,11 @@ nowarn_spec_t::nowarn_spec_t (opt_code opt)
> m_bits = NW_DANGLING;
> break;
>
> + case OPT_Wpessimizing_move:
> + case OPT_Wredundant_move:
> + m_bits = NW_REDUNDANT;
> + break;
> +
> default:
> /* A catchall group for everything else. */
> m_bits = NW_OTHER;
> diff --git a/gcc/diagnostic-spec.h b/gcc/diagnostic-spec.h
> index 28e5e5ccc75..e5f1c127d4f 100644
> --- a/gcc/diagnostic-spec.h
> +++ b/gcc/diagnostic-spec.h
> @@ -45,9 +45,11 @@ public:
> NW_DANGLING = 1 << 5,
> /* All other unclassified warnings. */
> NW_OTHER = 1 << 6,
> + /* Warnings about redundant calls. */
> + NW_REDUNDANT = 1 << 7,
> /* All groups of warnings. */
> NW_ALL = (NW_ACCESS | NW_LEXICAL | NW_NONNULL
> - | NW_UNINIT | NW_VFLOW | NW_DANGLING | NW_OTHER)
> + | NW_UNINIT | NW_VFLOW | NW_DANGLING | NW_REDUNDANT | NW_OTHER)
> };
>
> nowarn_spec_t (): m_bits () { }
> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
> index a1af1230b68..c81f29a4ba6 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move3.C
> @@ -39,7 +39,7 @@ Tp1
> fn2 ()
> {
> Tp2 t;
> - return std::move (t); // { dg-warning "moving a local object in a return statement prevents copy elision" }
> + return std::move (t);
> }
>
> template<typename Tp1, typename Tp2>
> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C
> new file mode 100644
> index 00000000000..898040e6bfc
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move9.C
> @@ -0,0 +1,89 @@
> +// PR c++/89780
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wpessimizing-move -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 Dest {
> + Dest() = default;
> + Dest(Dest&&);
> + Dest(const Dest&);
> +};
> +struct Source : Dest {};
> +
> +template <typename T>
> +Dest withMove() {
> + T x;
> + return std::move(x);
> +}
> +
> +template Dest withMove<Dest>();
> +template Dest withMove<Source>();
> +
> +template<typename T>
> +Dest bar () {
> + return std::move(T()); // { dg-warning "moving a temporary object prevents copy elision" }
> +}
> +
> +template Dest bar<Dest>();
> +template Dest bar<Source>();
> +
> +template<typename T>
> +Dest baz (T x) {
> + return std::move(x);
> +}
> +
> +void
> +call_baz ()
> +{
> + Dest d;
> + Source s;
> + baz (d);
> + baz (s);
> +}
> +
> +template<typename>
> +Dest foo ()
> +{
> + Dest d;
> + return std::move(d); // { dg-warning "moving a local object in a return statement prevents copy elision" }
> +}
> +
> +template Dest foo<int>();
> +
> +template<typename>
> +Dest qux () {
> + return std::move(Dest()); // { dg-warning "moving a temporary object prevents copy elision" }
> +}
> +
> +template Dest qux<int>();
> +
> +template<typename>
> +Dest qui (Dest x) {
> + return std::move(x); // { dg-warning "redundant move in return statement" }
> +}
> +
> +void
> +call_qui ()
> +{
> + Dest d;
> + qui<int> (d);
> +}
> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C
> index f181afeeb84..6e0aa4b0277 100644
> --- a/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C
> +++ b/gcc/testsuite/g++.dg/cpp0x/Wredundant-move2.C
> @@ -37,14 +37,14 @@ template<typename Tp1, typename Tp2>
> Tp1
> fn2 (Tp2 t)
> {
> - return std::move (t); // { dg-warning "redundant move in return statement" }
> + return std::move (t);
> }
>
> template<typename Tp1, typename Tp2>
> Tp1
> fn3 (Tp2 t)
> {
> - return std::move (t); // { dg-warning "redundant move in return statement" }
> + return std::move (t);
> }
>
> int
>
> base-commit: 4b0253b019943abf2cc5f4db0b7ed67caedffe4a
> prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5
> prerequisite-patch-id: ce490c3c0b991fa7f27315387949c145c66223a4
prev parent reply other threads:[~2022-08-11 22:25 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-08-04 18:46 [PATCH] " Marek Polacek
2022-08-06 23:02 ` Jason Merrill
2022-08-08 19:51 ` [PATCH v2] " Marek Polacek
2022-08-11 22:24 ` Jason Merrill [this message]
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=f7409f56-3c22-4761-22f1-0e3f58af1d01@redhat.com \
--to=jason@redhat.com \
--cc=gcc-patches@gcc.gnu.org \
--cc=polacek@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).