| Message ID | YvFxnuEhiqRkdMeu@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | [v2] c++: Extend -Wredundant-move for const-qual objects [PR90428] | expand |
On Mon, Aug 08, 2022 at 04:27:10PM -0400, Marek Polacek wrote: > + /* 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 This is TYPE_REF_P so I'll fix that up. > + && CP_TYPE_CONST_P (TREE_TYPE (TREE_TYPE (arg)))) Marek
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
On Mon, Aug 15, 2022 at 03:43:38PM -0400, Jason Merrill wrote: > On 8/8/22 13:27, Marek Polacek wrote: > > 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. Thanks, pushed. > Since you've been thinking about this area, I wonder if you want to look at > implementing P2266? Yup, I can take a look. It's also come up recently in a discussion with Davis in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89780#c3. Since P2266 was approved, I've updated our C++23 status table as well. Marek
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