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?
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
--
2.37.1
