c++: Implement -Wself-move warning [PR81159]
Commit Message
About 5 years ago we got a request to implement -Wself-move, which
warns about useless moves like this:
int x;
x = std::move (x);
This patch implements that warning.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
PR c++/81159
gcc/c-family/ChangeLog:
* c.opt (Wself-move): New option.
gcc/cp/ChangeLog:
* typeck.cc (maybe_warn_self_move): New.
(cp_build_modify_expr): Call maybe_warn_self_move.
gcc/ChangeLog:
* doc/invoke.texi: Document -Wself-move.
gcc/testsuite/ChangeLog:
* g++.dg/warn/Wself-move1.C: New test.
---
gcc/c-family/c.opt | 4 ++
gcc/cp/typeck.cc | 48 +++++++++++++-
gcc/doc/invoke.texi | 23 ++++++-
gcc/testsuite/g++.dg/warn/Wself-move1.C | 87 +++++++++++++++++++++++++
4 files changed, 160 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/warn/Wself-move1.C
base-commit: 9385cd9c74cf6662f43038aafe4d2467899f322e
Comments
On 8/9/22 09:37, Marek Polacek wrote:
> About 5 years ago we got a request to implement -Wself-move, which
> warns about useless moves like this:
>
> int x;
> x = std::move (x);
>
> This patch implements that warning.
>
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> PR c++/81159
>
> gcc/c-family/ChangeLog:
>
> * c.opt (Wself-move): New option.
>
> gcc/cp/ChangeLog:
>
> * typeck.cc (maybe_warn_self_move): New.
> (cp_build_modify_expr): Call maybe_warn_self_move.
>
> gcc/ChangeLog:
>
> * doc/invoke.texi: Document -Wself-move.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/warn/Wself-move1.C: New test.
> ---
> gcc/c-family/c.opt | 4 ++
> gcc/cp/typeck.cc | 48 +++++++++++++-
> gcc/doc/invoke.texi | 23 ++++++-
> gcc/testsuite/g++.dg/warn/Wself-move1.C | 87 +++++++++++++++++++++++++
> 4 files changed, 160 insertions(+), 2 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/warn/Wself-move1.C
>
> diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
> index 44e1a60ce24..a098ae1830d 100644
> --- a/gcc/c-family/c.opt
> +++ b/gcc/c-family/c.opt
> @@ -1229,6 +1229,10 @@ Wselector
> ObjC ObjC++ Var(warn_selector) Warning
> Warn if a selector has multiple methods.
>
> +Wself-move
> +C++ ObjC++ Var(warn_self_move) Warning LangEnabledBy(C++ ObjC++, Wall)
> +Warn when a value is moved to itself with std::move.
> +
> Wsequence-point
> C ObjC C++ ObjC++ Var(warn_sequence_point) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
> Warn about possible violations of sequence point rules.
> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
> index 6e4f23af982..f05913c0fac 100644
> --- a/gcc/cp/typeck.cc
> +++ b/gcc/cp/typeck.cc
> @@ -8897,7 +8897,51 @@ cp_build_c_cast (location_t loc, tree type, tree expr,
>
> return error_mark_node;
> }
> -
> +
> +/* Warn when a value is moved to itself with std::move. LHS is the target,
> + RHS may be the std::move call, and LOC is the location of the whole
> + assignment. */
> +
> +static void
> +maybe_warn_self_move (location_t loc, tree lhs, tree rhs)
> +{
> + if (!warn_self_move)
> + return;
> +
> + /* C++98 doesn't know move. */
> + if (cxx_dialect < cxx11)
> + return;
> +
> + if (processing_template_decl)
> + return;
> +
> + /* We're looking for *std::move<T&> ((T &) &arg), or
> + *std::move<T&> ((T &) (T *) r) if the argument it a reference. */
> + if (!REFERENCE_REF_P (rhs)
> + || TREE_CODE (TREE_OPERAND (rhs, 0)) != CALL_EXPR)
> + return;
> + tree fn = TREE_OPERAND (rhs, 0);
> + if (!is_std_move_p (fn))
> + return;
> + tree arg = CALL_EXPR_ARG (fn, 0);
> + if (TREE_CODE (arg) != NOP_EXPR)
> + return;
> + /* Strip the (T &). */
> + arg = TREE_OPERAND (arg, 0);
> + /* Strip the (T *) or &. */
> + arg = TREE_OPERAND (arg, 0);
Are you sure these are the only two expressions that can make it here?
What if the argument to move is *Tptr?
> + arg = convert_from_reference (arg);
> + /* So that we catch (i) = std::move (i);. */
> + lhs = maybe_undo_parenthesized_ref (lhs);
> + STRIP_ANY_LOCATION_WRAPPER (lhs);
> + if (cp_tree_equal (lhs, arg))
> + {
> + auto_diagnostic_group d;
> + if (warning_at (loc, OPT_Wself_move, "moving a variable to itself"))
> + inform (loc, "remove %<std::move%> call");
> + }
> +}
> +
> /* For use from the C common bits. */
> tree
> build_modify_expr (location_t location,
> @@ -9101,6 +9145,8 @@ cp_build_modify_expr (location_t loc, tree lhs, enum tree_code modifycode,
>
> if (modifycode == NOP_EXPR)
> {
> + maybe_warn_self_move (loc, lhs, rhs);
> +
> if (c_dialect_objc ())
> {
> result = objc_maybe_build_modify_expr (lhs, rhs);
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index f3e9429b2ca..28cf36b94c6 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -264,7 +264,7 @@ in the following sections.
> -Wreorder -Wregister @gol
> -Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates @gol
> -Wno-non-template-friend -Wold-style-cast @gol
> --Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo @gol
> +-Woverloaded-virtual -Wno-pmf-conversions -Wself-move -Wsign-promo @gol
> -Wsized-deallocation -Wsuggest-final-methods @gol
> -Wsuggest-final-types -Wsuggest-override @gol
> -Wno-terminate -Wuseless-cast -Wno-vexing-parse @gol
> @@ -5841,6 +5841,7 @@ Options} and @ref{Objective-C and Objective-C++ Dialect Options}.
> -Wreorder @gol
> -Wrestrict @gol
> -Wreturn-type @gol
> +-Wself-move @r{(only for C++)} @gol
> -Wsequence-point @gol
> -Wsign-compare @r{(only in C++)} @gol
> -Wsizeof-array-div @gol
> @@ -6826,6 +6827,26 @@ of a declaration:
>
> This warning is enabled by @option{-Wall}.
>
> +@item -Wno-self-move @r{(C++ and Objective-C++ only)}
> +@opindex Wself-move
> +@opindex Wno-self-move
> +This warning warns when a value is moved to itself with @code{std::move}.
> +Such a @code{std::move} has no effect.
...unless it naively breaks the object, like
T(T&& ot): data(ot.data) { ot.data = nullptr; } // oops
> +@smallexample
> +struct T @{
> +@dots{}
> +@};
> +void fn()
> +@{
> + T t;
> + @dots{}
> + t = std::move (t);
> +@}
> +@end smallexample
> +
> +This warning is enabled by @option{-Wall}.
> +
> @item -Wsequence-point
> @opindex Wsequence-point
> @opindex Wno-sequence-point
> diff --git a/gcc/testsuite/g++.dg/warn/Wself-move1.C b/gcc/testsuite/g++.dg/warn/Wself-move1.C
> new file mode 100644
> index 00000000000..4a6dbad4533
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wself-move1.C
> @@ -0,0 +1,87 @@
> +// PR c++/81159
> +// { dg-do compile { target c++11 } }
> +// { dg-options "-Wself-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); }
> +}
> +
> +int g;
> +
> +struct S {
> + int x;
> + S(S&& o) {
> + x = std::move (x); // { dg-warning "moving a variable to itself" }
> + x = std::move (o.x);
> + o.x = std::move (x);
> + o.x = std::move (o.x); // { dg-warning "moving a variable to itself" }
> + }
> + void foo (int x) {
> + x = std::move (x); // { dg-warning "moving a variable to itself" }
> + }
> +};
> +
> +struct X {
> + int x;
> + X(int x) : x(std::move (x)) { }
> +};
> +
> +struct A {};
> +struct B { A a; };
> +struct C { C(); ~C(); };
> +struct D { D(); D(const D&); D(D&&); D& operator=(const D&); };
> +
> +void
> +test ()
> +{
> + int i = 42;
> + i = std::move (i); // { dg-warning "moving a variable to itself" }
> + (i) = std::move (i); // { dg-warning "moving a variable to itself" }
> +
> + g = std::move (g); // { dg-warning "moving a variable to itself" }
> + (g) = std::move (g); // { dg-warning "moving a variable to itself" }
> +
> + A a;
> + a = std::move (a); // { dg-warning "moving a variable to itself" }
> +
> + B b;
> + b = std::move (b); // { dg-warning "moving a variable to itself" }
> + b.a = std::move (b.a); // { dg-warning "moving a variable to itself" }
> +
> + C c;
> + c = std::move (c); // { dg-warning "moving a variable to itself" }
> + D d;
> + d = std::move (d); // { dg-warning "moving a variable to itself" }
> +}
> +
> +template<typename T>
> +void ttest ()
> +{
> + T t;
> + t = std::move (t); // { dg-warning "moving a variable to itself" }
> +}
> +
> +template void ttest<A>();
> +
> +void
> +testref (int &r, int &&rr)
> +{
> + r = std::move (r); // { dg-warning "moving a variable to itself" }
> + rr = std::move (rr); // { dg-warning "moving a variable to itself" }
> +}
>
> base-commit: 9385cd9c74cf6662f43038aafe4d2467899f322e
@@ -1229,6 +1229,10 @@ Wselector
ObjC ObjC++ Var(warn_selector) Warning
Warn if a selector has multiple methods.
+Wself-move
+C++ ObjC++ Var(warn_self_move) Warning LangEnabledBy(C++ ObjC++, Wall)
+Warn when a value is moved to itself with std::move.
+
Wsequence-point
C ObjC C++ ObjC++ Var(warn_sequence_point) Warning LangEnabledBy(C ObjC C++ ObjC++,Wall)
Warn about possible violations of sequence point rules.
@@ -8897,7 +8897,51 @@ cp_build_c_cast (location_t loc, tree type, tree expr,
return error_mark_node;
}
-
+
+/* Warn when a value is moved to itself with std::move. LHS is the target,
+ RHS may be the std::move call, and LOC is the location of the whole
+ assignment. */
+
+static void
+maybe_warn_self_move (location_t loc, tree lhs, tree rhs)
+{
+ if (!warn_self_move)
+ return;
+
+ /* C++98 doesn't know move. */
+ if (cxx_dialect < cxx11)
+ return;
+
+ if (processing_template_decl)
+ return;
+
+ /* We're looking for *std::move<T&> ((T &) &arg), or
+ *std::move<T&> ((T &) (T *) r) if the argument it a reference. */
+ if (!REFERENCE_REF_P (rhs)
+ || TREE_CODE (TREE_OPERAND (rhs, 0)) != CALL_EXPR)
+ return;
+ tree fn = TREE_OPERAND (rhs, 0);
+ if (!is_std_move_p (fn))
+ return;
+ tree arg = CALL_EXPR_ARG (fn, 0);
+ if (TREE_CODE (arg) != NOP_EXPR)
+ return;
+ /* Strip the (T &). */
+ arg = TREE_OPERAND (arg, 0);
+ /* Strip the (T *) or &. */
+ arg = TREE_OPERAND (arg, 0);
+ arg = convert_from_reference (arg);
+ /* So that we catch (i) = std::move (i);. */
+ lhs = maybe_undo_parenthesized_ref (lhs);
+ STRIP_ANY_LOCATION_WRAPPER (lhs);
+ if (cp_tree_equal (lhs, arg))
+ {
+ auto_diagnostic_group d;
+ if (warning_at (loc, OPT_Wself_move, "moving a variable to itself"))
+ inform (loc, "remove %<std::move%> call");
+ }
+}
+
/* For use from the C common bits. */
tree
build_modify_expr (location_t location,
@@ -9101,6 +9145,8 @@ cp_build_modify_expr (location_t loc, tree lhs, enum tree_code modifycode,
if (modifycode == NOP_EXPR)
{
+ maybe_warn_self_move (loc, lhs, rhs);
+
if (c_dialect_objc ())
{
result = objc_maybe_build_modify_expr (lhs, rhs);
@@ -264,7 +264,7 @@ in the following sections.
-Wreorder -Wregister @gol
-Wstrict-null-sentinel -Wno-subobject-linkage -Wtemplates @gol
-Wno-non-template-friend -Wold-style-cast @gol
--Woverloaded-virtual -Wno-pmf-conversions -Wsign-promo @gol
+-Woverloaded-virtual -Wno-pmf-conversions -Wself-move -Wsign-promo @gol
-Wsized-deallocation -Wsuggest-final-methods @gol
-Wsuggest-final-types -Wsuggest-override @gol
-Wno-terminate -Wuseless-cast -Wno-vexing-parse @gol
@@ -5841,6 +5841,7 @@ Options} and @ref{Objective-C and Objective-C++ Dialect Options}.
-Wreorder @gol
-Wrestrict @gol
-Wreturn-type @gol
+-Wself-move @r{(only for C++)} @gol
-Wsequence-point @gol
-Wsign-compare @r{(only in C++)} @gol
-Wsizeof-array-div @gol
@@ -6826,6 +6827,26 @@ of a declaration:
This warning is enabled by @option{-Wall}.
+@item -Wno-self-move @r{(C++ and Objective-C++ only)}
+@opindex Wself-move
+@opindex Wno-self-move
+This warning warns when a value is moved to itself with @code{std::move}.
+Such a @code{std::move} has no effect.
+
+@smallexample
+struct T @{
+@dots{}
+@};
+void fn()
+@{
+ T t;
+ @dots{}
+ t = std::move (t);
+@}
+@end smallexample
+
+This warning is enabled by @option{-Wall}.
+
@item -Wsequence-point
@opindex Wsequence-point
@opindex Wno-sequence-point
new file mode 100644
@@ -0,0 +1,87 @@
+// PR c++/81159
+// { dg-do compile { target c++11 } }
+// { dg-options "-Wself-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); }
+}
+
+int g;
+
+struct S {
+ int x;
+ S(S&& o) {
+ x = std::move (x); // { dg-warning "moving a variable to itself" }
+ x = std::move (o.x);
+ o.x = std::move (x);
+ o.x = std::move (o.x); // { dg-warning "moving a variable to itself" }
+ }
+ void foo (int x) {
+ x = std::move (x); // { dg-warning "moving a variable to itself" }
+ }
+};
+
+struct X {
+ int x;
+ X(int x) : x(std::move (x)) { }
+};
+
+struct A {};
+struct B { A a; };
+struct C { C(); ~C(); };
+struct D { D(); D(const D&); D(D&&); D& operator=(const D&); };
+
+void
+test ()
+{
+ int i = 42;
+ i = std::move (i); // { dg-warning "moving a variable to itself" }
+ (i) = std::move (i); // { dg-warning "moving a variable to itself" }
+
+ g = std::move (g); // { dg-warning "moving a variable to itself" }
+ (g) = std::move (g); // { dg-warning "moving a variable to itself" }
+
+ A a;
+ a = std::move (a); // { dg-warning "moving a variable to itself" }
+
+ B b;
+ b = std::move (b); // { dg-warning "moving a variable to itself" }
+ b.a = std::move (b.a); // { dg-warning "moving a variable to itself" }
+
+ C c;
+ c = std::move (c); // { dg-warning "moving a variable to itself" }
+ D d;
+ d = std::move (d); // { dg-warning "moving a variable to itself" }
+}
+
+template<typename T>
+void ttest ()
+{
+ T t;
+ t = std::move (t); // { dg-warning "moving a variable to itself" }
+}
+
+template void ttest<A>();
+
+void
+testref (int &r, int &&rr)
+{
+ r = std::move (r); // { dg-warning "moving a variable to itself" }
+ rr = std::move (rr); // { dg-warning "moving a variable to itself" }
+}