[RFC,v1,2/5] meson: Ignore `Wredundant-move` with GCC 11 and above

25684 diff mbox series

In C++20 mode, object slicing in a return statement triggers automatic move
since GCC 11, and using `std::move()` emits `-Wredundant-move` in those
same GCC versions. So disable the warning in those as well.

This is relevant for `valueOrTuple()` in `py_helpers.cpp`, which
returns a `py::tuple` as `py::object`.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
---
 meson.build | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Hi Barnabás,

Thank you for the patch.

On Wed, Jan 07, 2026 at 08:36:04PM +0100, Barnabás Pőcze wrote:
> In C++20 mode, object slicing in a return statement triggers automatic move
> since GCC 11, and using `std::move()` emits `-Wredundant-move` in those
> same GCC versions. So disable the warning in those as well.

Reading https://en.cppreference.com/w/cpp/language/return.html, do I
understand correctly that in C++20 the following clause stops applying,
and that's what causes the difference in behaviour due to the "formally"
part ?

  or it succeeded, but did not select the move constructor (formally,
  the first parameter of the selected constructor was not an rvalue
  reference to the (possibly cv-qualified) type of expression)

> This is relevant for `valueOrTuple()` in `py_helpers.cpp`, which
> returns a `py::tuple` as `py::object`.
> 
> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
> ---
>  meson.build | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/meson.build b/meson.build
> index 4aa7ecc96..1d8e1d994 100644
> --- a/meson.build
> +++ b/meson.build
> @@ -168,7 +168,7 @@ if cc.get_id() == 'gcc'
>      # disable the warning. With -Wpessimizing-move enabled, the compiler will
>      # still warn of pessimizing moves, only the redundant but not pessimizing
>      # moves will be ignored.

Expanding the context:

    # gcc 13 implements the C++23 version of automatic move from local
    # variables in return statements (see
    # https://en.cppreference.com/w/cpp/language/return). As a result, some
    # previously required explicit std::move() in return statements generate
    # warnings. Those moves can't be removed as older compiler versions could
    # use copy constructors instead of move constructors. The easiest fix is to
    # disable the warning. With -Wpessimizing-move enabled, the compiler will
    # still warn of pessimizing moves, only the redundant but not pessimizing
    # moves will be ignored.

This would look odd compared to the version check below. We can't just
s/13/11/ as that wouldn't be correct, so we probably need to list both
versions with their corresponding behaviour.

> -    if cc.version().version_compare('>=13')
> +    if cc.version().version_compare('>=11')
>          cpp_arguments += [
>              '-Wno-redundant-move',
>          ]

2026. 01. 07. 21:25 keltezéssel, Laurent Pinchart írta:
> Hi Barnabás,
> 
> Thank you for the patch.
> 
> On Wed, Jan 07, 2026 at 08:36:04PM +0100, Barnabás Pőcze wrote:
>> In C++20 mode, object slicing in a return statement triggers automatic move
>> since GCC 11, and using `std::move()` emits `-Wredundant-move` in those
>> same GCC versions. So disable the warning in those as well.
> 
> Reading https://en.cppreference.com/w/cpp/language/return.html, do I
> understand correctly that in C++20 the following clause stops applying,
> and that's what causes the difference in behaviour due to the "formally"
> part ?
> 
>    or it succeeded, but did not select the move constructor (formally,
>    the first parameter of the selected constructor was not an rvalue
>    reference to the (possibly cv-qualified) type of expression)

I believe the relevant document is https://wg21.link/p1155r3#slicing


> 
>> This is relevant for `valueOrTuple()` in `py_helpers.cpp`, which
>> returns a `py::tuple` as `py::object`.
>>
>> Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
>> ---
>>   meson.build | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/meson.build b/meson.build
>> index 4aa7ecc96..1d8e1d994 100644
>> --- a/meson.build
>> +++ b/meson.build
>> @@ -168,7 +168,7 @@ if cc.get_id() == 'gcc'
>>       # disable the warning. With -Wpessimizing-move enabled, the compiler will
>>       # still warn of pessimizing moves, only the redundant but not pessimizing
>>       # moves will be ignored.
> 
> Expanding the context:
> 
>      # gcc 13 implements the C++23 version of automatic move from local
>      # variables in return statements (see
>      # https://en.cppreference.com/w/cpp/language/return). As a result, some
>      # previously required explicit std::move() in return statements generate
>      # warnings. Those moves can't be removed as older compiler versions could
>      # use copy constructors instead of move constructors. The easiest fix is to
>      # disable the warning. With -Wpessimizing-move enabled, the compiler will
>      # still warn of pessimizing moves, only the redundant but not pessimizing
>      # moves will be ignored.
> 
> This would look odd compared to the version check below. We can't just
> s/13/11/ as that wouldn't be correct, so we probably need to list both
> versions with their corresponding behaviour.

This is the behaviour of when object slicing in the return statement:

      -std=c++17       -std=c++20
   8:   move             move
   9:   copy             copy
  10:   copy             copy
  11:   copy             move
  12:   copy             move
  13:   move             move
  14:   move             move
  15:   move             move

Since GCC 8 is not supported. I think it is enough to just state that
since GCC 11 automatic move happens when the object is sliced, which
is I think the problematic case for libcamera.


> 
>> -    if cc.version().version_compare('>=13')
>> +    if cc.version().version_compare('>=11')
>>           cpp_arguments += [
>>               '-Wno-redundant-move',
>>           ]
>