c++: Tweak for -Wpessimizing-move in templates [PR89780]
Commit Message
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
base-commit: be58bf98e98bb431ed26ca8be84586075fe8be82
prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5
prerequisite-patch-id: ce490c3c0b991fa7f27315387949c145c66223a4
Comments
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.
> + && 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?
> + /* 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-move7.C b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
> index a17c7a87b7d..ebbcb2122a6 100644
> --- 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.
> }
>
> void
> 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..9519695eda7
> --- /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());
> +}
> +
> +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: be58bf98e98bb431ed26ca8be84586075fe8be82
> prerequisite-patch-id: f4862bc588ce5fed1d21016fecc4b61feb71eba5
> prerequisite-patch-id: ce490c3c0b991fa7f27315387949c145c66223a4
@@ -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);
@@ -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)
+ && 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)
+ /* This also suppresses -Wredundant-move. */
+ suppress_warning (retval, OPT_Wpessimizing_move);
return retval;
}
@@ -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;
@@ -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 () { }
@@ -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>
@@ -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{});
}
void
new file mode 100644
@@ -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());
+}
+
+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);
+}
@@ -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