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?
-- >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
