Laurent Pinchart af7d6a4c2d meson: Ignore -Wredundant-move with gcc-13 and newer ...

Starting from 13.1, gcc 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. This is the case when a function returns an object whose type
is a class derived from the class type the function returns:

struct U { };
struct T : U { };

U f()
{
	T t;
	return t;
}

Up to C++20, the automatic move from local variables selects the move
constructor of class U, which is not the move constructor of the
expression. Overload resolution is then performed a second time, with t
considered as an lvalue. An explicit `std::move(t)` is needed in the
return statement to select the U move constructor.

Starting from C++23, `t` is treated as an xvalue, and the U move
constructor is selected without the need for an explicit `std::move(t)`.
An explicit `std:move()` then generates a redundant-move warning, as in
the valueOrTuple() function in src/py/libcamera/py_helpers.cpp.

Omitting the `std::move()` silences the warning, but selects the copy
constructor of U with older gcc versions and with clang, which
negatively impacts performance.

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.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

2023-05-01 11:17:36 +01:00

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