On Mon, Aug 15, 2022 at 03:54:05PM -0400, Jason Merrill wrote:
> On 8/9/22 09:37, Marek Polacek wrote:
> > + /* 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?
Not 100% sure but I couldn't find any other form. For *Tptr we get
*std::move<int*&> ((int * &) &Tptr)
so it works as expected as well. I've extended the existing test to test this
too.
> > @@ -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
"If you try to move me I'll disappear!"
I've added the weasel word: "typically has no effect." Or do we want to say
more?
-- >8 --
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.
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 | 105 ++++++++++++++++++++++++
4 files changed, 178 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 dfdebd596ef..f776efd39d8 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 992ebfd99fb..cbc32a7c8ca 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);
+ 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 f65d351a5fc..5dea3fee124 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
@@ -5843,6 +5843,7 @@ 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
@@ -6828,6 +6829,26 @@ 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} typically 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
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..b12f0d43dad
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wself-move1.C
@@ -0,0 +1,105 @@
+// 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" }
+}
+
+// Test various other arguments to std::move.
+template<typename T>
+void
+testargs (T *Tptr, T& Tref, T&& Trref, const T *Tcptr)
+{
+ Tptr = std::move (Tptr); // { dg-warning "moving a variable to itself" }
+ Tref = std::move (Tref); // { dg-warning "moving a variable to itself" }
+ Trref = std::move (Trref); // { dg-warning "moving a variable to itself" }
+ Tcptr = std::move (Tcptr); // { dg-warning "moving a variable to itself" }
+}
+
+void
+call_testargs ()
+{
+ int i = 42;
+ testargs<int>(&i, i, 42, &i);
+}
base-commit: ca170ed9f8a086ca7e1eec841882b6bed9ec1a3a
--
2.37.2
