From af7d6a4c2df6c836ecc704ea44b39ea07418d832 Mon Sep 17 00:00:00 2001 From: Laurent Pinchart Date: Mon, 1 May 2023 05:57:11 +0300 Subject: 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 Reviewed-by: Kieran Bingham Signed-off-by: Kieran Bingham --- meson.build | 15 +++++++++++++++ 1 file changed, 15 insertions(+) diff --git a/meson.build b/meson.build index 8628e6ac..6e363a90 100644 --- a/meson.build +++ b/meson.build @@ -136,6 +136,21 @@ if cc.get_id() == 'gcc' ] endif + # 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. + if cc.version().version_compare('>=13') + cpp_arguments += [ + '-Wno-redundant-move', + ] + endif + # gcc 7.1 introduced processor-specific ABI breakages related to parameter # passing on ARM platforms. This generates a large number of messages # during compilation. Silence them. -- cgit v1.2.1