Re: [PATCH] c++: Extend -Wpessimizing-move for class prvalues [PR106276]

[Jason Merrill <jason@redhat.com>]

On 8/6/22 15:49, Jason Merrill wrote:
> On 7/27/22 17:14, Marek Polacek wrote:
>> We already have a warning that warns about pessimizing std::move
>> in a return statement, when it prevents the NRVO:
>>
>>    T fn()
>>    {
>>      T t;
>>      return std::move (t); // warning \o/
>>    }
>>
>> However, the warning doesn't warn when what we are returning is a class
>> prvalue, that is, when std::move prevents the RVO:
>>
>>    T fn()
>>    {
>>      T t;
>>      return std::move (T{}); // no warning :-(
>>    }
>>
>> This came up recently in GCC:
>> <https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598177.html>.
>>
>> This patch fixes that.  I would like to extend the warning further, so
>> that it warns in more contexts, e.g.:
>>
>>    T t = std::move(T());
>>
>> or
>>
>>    void foo (T);
>>    foo (std::move(T()));
>>
>> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>>
>>     PR c++/106276
>>
>> gcc/cp/ChangeLog:
>>
>>     * typeck.cc (can_do_rvo_p): New.
>>     (maybe_warn_pessimizing_move): Warn when moving a temporary object
>>     in a return statement prevents copy elision.
>>
>> gcc/testsuite/ChangeLog:
>>
>>     * g++.dg/cpp0x/Wpessimizing-move7.C: New test.
>> ---
>>   gcc/cp/typeck.cc                              | 31 ++++++++-
>>   .../g++.dg/cpp0x/Wpessimizing-move7.C         | 63 +++++++++++++++++++
>>   2 files changed, 91 insertions(+), 3 deletions(-)
>>   create mode 100644 gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
>>
>> diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
>> index 6e4f23af982..9500c4e2fe8 100644
>> --- a/gcc/cp/typeck.cc
>> +++ b/gcc/cp/typeck.cc
>> @@ -10287,12 +10287,29 @@ can_do_nrvo_p (tree retval, tree functype)
>>         /* The cv-unqualified type of the returned value must be the
>>            same as the cv-unqualified return type of the
>>            function.  */
>> -      && same_type_p ((TYPE_MAIN_VARIANT (TREE_TYPE (retval))),
>> -              (TYPE_MAIN_VARIANT (functype)))
>> +      && same_type_p (TYPE_MAIN_VARIANT (TREE_TYPE (retval)),
>> +              TYPE_MAIN_VARIANT (functype))
>>         /* And the returned value must be non-volatile.  */
>>         && !TYPE_VOLATILE (TREE_TYPE (retval)));
>>   }
>> +/* Like can_do_nrvo_p, but we check if we're trying to move a class
>> +   prvalue.  */
>> +
>> +static bool
>> +can_do_rvo_p (tree retval, tree functype)
>> +{
>> +  if (functype == error_mark_node)
>> +    return false;
>> +  if (retval)
>> +    STRIP_ANY_LOCATION_WRAPPER (retval);
>> +  return (retval != NULL_TREE
>> +      && TREE_CODE (retval) == TARGET_EXPR
> 
> Maybe use !glvalue_p instead of specifically checking for TARGET_EXPR? I 
> don't feel strongly about this.
> 
>> +      && same_type_p (TYPE_MAIN_VARIANT (TREE_TYPE (retval)),
>> +              TYPE_MAIN_VARIANT (functype))
>> +      && !TYPE_VOLATILE (TREE_TYPE (retval)));
>> +}
>> +
>>   /* If we should treat RETVAL, an expression being returned, as if it 
>> were
>>      designated by an rvalue, returns it adjusted accordingly; 
>> otherwise, returns
>>      NULL_TREE.  See [class.copy.elision].  RETURN_P is true if this 
>> is a return
>> @@ -10401,12 +10418,20 @@ maybe_warn_pessimizing_move (tree retval, 
>> tree functype)
>>                     "prevents copy elision"))
>>           inform (loc, "remove %<std::move%> call");
>>           }
>> +      else if (can_do_rvo_p (arg, functype))
>> +        {
>> +          auto_diagnostic_group d;
>> +          if (warning_at (loc, OPT_Wpessimizing_move,
>> +                  "moving a temporary object in a return statement "
>> +                  "prevents copy elision"))
> 
> It doesn't just prevent copy elision, it produces a dangling reference. 
>   This is a special case of the warning we talked about passing a 
> temporary to a function that returns a reference argument unchanged, and 
> should probably use a different warning flag.

Wait, no, I'm confused, the temporary does live long enough to get copied.

I still don't think we want to suppress this warning in the other patch.

>> +        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
>>              && (moved = treat_lvalue_as_rvalue_p (arg, /*return*/true)))
>>           {
>> -          /* Make sure that the overload resolution would actually 
>> succeed
>> +          /* Make sure that overload resolution would actually succeed
>>            if we removed the std::move call.  */
>>             tree t = convert_for_initialization (NULL_TREE, functype,
>>                              moved,
>> diff --git a/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C 
>> b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
>> new file mode 100644
>> index 00000000000..cd4eaa09aae
>> --- /dev/null
>> +++ b/gcc/testsuite/g++.dg/cpp0x/Wpessimizing-move7.C
>> @@ -0,0 +1,63 @@
>> +// PR c++/106276
>> +// { dg-do compile { target c++11 } }
>> +// { dg-options "-Wpessimizing-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 A { A(); A(const A&) = delete; A(A&&); };
>> +struct B { B(A); };
>> +
>> +static A foo ();
>> +
>> +A
>> +fn1 ()
>> +{
>> +  return std::move (A{}); // { dg-warning "moving a temporary object 
>> in a return statement prevents copy elision" }
>> +  return std::move (A()); // { dg-warning "moving a temporary object 
>> in a return statement prevents copy elision" }
>> +  return std::move (foo ()); // { dg-warning "moving a temporary 
>> object in a return statement prevents copy elision" }
>> +}
>> +
>> +B fn2 ()
>> +{
>> +  return std::move (A());
>> +  return std::move (A{});
>> +  return std::move (foo ());
>> +}
>> +
>> +template <typename T1, typename T2>
>> +T1
>> +fn3 ()
>> +{
>> +  return std::move (T2{}); // { dg-warning "moving a temporary object 
>> in a return statement prevents copy elision" }
>> +}
>> +
>> +void
>> +do_fn3 ()
>> +{
>> +  fn3<A, A>();
>> +  fn3<B, A>();
>> +}
>> +
>> +char take_buffer;
>> +struct label_text {
>> +  label_text take() { return std::move(label_text(&take_buffer)); } 
>> // { dg-warning "moving a temporary object in a return statement 
>> prevents copy elision" }
>> +  label_text(char *);
>> +};
>>
>> base-commit: 219f86495791fd27b012678f13e10cada211daab
>