Re: [PATCH v2] c++: Implement -Wself-move warning [PR81159]

[Jason Merrill <jason@redhat.com>]

On 8/18/22 13:19, Marek Polacek wrote:
> 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)

That likes like what you'd get when the argument is Tptr, not when it's 
*Tptr.  And indeed that's what I see in the testcase:

> +  Tptr = std::move (Tptr); // { dg-warning "moving a variable to itself" }

is missing the *

>>> @@ -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;
>   }
> -\f
> +
> +/* 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