[libcamera-devel] [PATCH] py: Drop redundant std::move()

[Laurent Pinchart]

On Fri, Apr 28, 2023 at 05:51:07PM +0300, Laurent Pinchart via libcamera-devel wrote:
> Hi Tomi,
> 
> On Wed, Apr 05, 2023 at 09:49:45AM +0300, Tomi Valkeinen wrote:
> > On 25/01/2023 11:42, Laurent Pinchart via libcamera-devel wrote:
> > > On Wed, Jan 25, 2023 at 12:09:40AM +0000, Kieran Bingham wrote:
> > >> Quoting Laurent Pinchart via libcamera-devel (2023-01-24 23:36:24)
> > >>> gcc-13 warns that the valueOrTuple() function has a redundant
> > >>> std::move() in a return statement:
> > >>>
> > >>> ../../src/py/libcamera/py_helpers.cpp: In instantiation of ‘pybind11::object valueOrTuple(const libcamera::ControlValue&) [with T = bool]’:
> > >>> ../../src/py/libcamera/py_helpers.cpp:38:28:   required from here
> > >>> ../../src/py/libcamera/py_helpers.cpp:28:35: error: redundant move in return statement [-Werror=redundant-move]
> > >>>     28 |                 return std::move(t);
> > >>
> > >> ohhh - this may be just too pedantic for me. Explicitly stating
> > >> std::move(t) when the compiler knows it is a move may be redundant to
> > >> the compiler, but it's not redundant to the reader?!
> > >>
> > >> Doesn't this help make it clear that the t is being moved... in which
> > >> case it's helpful self documenting code?
> > >>
> > >> I'm normally all for warnings, but this one is annoying.
> > >>
> > >> https://developers.redhat.com/blog/2019/04/12/understanding-when-not-to-stdmove-in-c
> > >> states that this isn't a 'pessimizing' operation, it's just redundant,
> > >> but it does make it clearer that a move is expected to occur?
> > > 
> > > Adding more context, the function is implemented as
> > > 
> > > 	if (cv.isArray()) {
> > > 		const T *v = reinterpret_cast<const T *>(cv.data().data());
> > > 		auto t = py::tuple(cv.numElements());
> > > 
> > > 		for (size_t i = 0; i < cv.numElements(); ++i)
> > > 			t[i] = v[i];
> > > 
> > > 		return std::move(t);
> > > 	}
> > > 
> > > 	return py::cast(cv.get<T>());
> > > 
> > > The type of 't' is py::tuple (replacing 'auto' with 'py::tuple' still
> > > produces the same warning), which inherits from py::object. We thus seem
> > > to be in the last case described by the above link:
> > > 
> > >      There are situations where returning std::move (expr) makes sense,
> > >      however. The rules for the implicit move require that the selected
> > >      constructor take an rvalue reference to the returned object's type.
> > >      Sometimes that isn't the case. For example, when a function returns
> > >      an object whose type is a class derived from the class type the
> > >      function returns. In that case, overload resolution is performed a
> > >      second time, this time treating the object as an lvalue:
> > > 
> > >      struct U { };
> > >      struct T : U { };
> > > 
> > >      U f()
> > >      {
> > >              T t;
> > >              return std::move (t);
> > >      }
> > > 
> > > g++-13 produces a warning when compiling that code:
> > > 
> > > move.cpp: In function ‘U f()’:
> > > move.cpp:9:26: warning: redundant move in return statement [-Wredundant-move]
> > >      9 |         return std::move (t);
> > >        |                ~~~~~~~~~~^~~
> > > move.cpp:9:26: note: remove ‘std::move’ call
> > > 
> > > This may also be a false positive of gcc ?
> > 
> > I don't have gcc 13, nor does godbolt.org, but other gcc nor clang 
> > versions don't complain.
> > 
> > With some testing on godbolt, with and without std::move the end result 
> > is the same (with -O2) on the compilers I tested.
> > 
> > So... I don't know. The text you pasted seems to suggest that 
> > std::move() would be needed there, but I don't see a diff (then again, 
> > my test code is just test code, not the actual py code we have). I'm 
> > fine either way, but if gcc 13 is not much used yet, maybe we should 
> > wait a bit?
> > 
> > Also, a bit beside the point, I'm actually a bit surprised that
> > 
> > U f()
> > {
> > 	return T();
> > }
> > 
> > works without warnings (even if I add fields to U and T). It's silently 
> > throwing away the T specific parts, only keeping the U parts.
> 
> I tried this test code:
> 
> --------
> #include <functional>
> #include <iostream>
> 
> struct Base {
> 	Base()
> 	{
> 		std::cout << "Base::Base()" << std::endl;
> 	}
> 
> 	Base(const Base &)
> 	{
> 		std::cout << "Base::Base(const Base &)" << std::endl;
> 	}
> 
> 	Base(Base &&)
> 	{
> 		std::cout << "Base::Base(Base &&)" << std::endl;
> 	}
> };
> 
> struct Derived : Base {
> };
> 
> Base f()
> {
> 	Derived t;
> 	return std::move(t);
> }
> 
> Base g()
> {
> 	Derived t;
> 	return t;
> }
> 
> int main()
> {
> 	f();
> 	g();
> 
> 	return 0;
> }
> --------
> 
> $ g++-12 -W -Wall -o move2 move2.cpp && ./move2 
> Base::Base()
> Base::Base(Base &&)
> Base::Base()
> Base::Base(const Base &)
> 
> $ g++-13 -W -Wall -o move2 move2.cpp && ./move2 
> move2.cpp: In function ‘Base f()’:
> move2.cpp:27:25: warning: redundant move in return statement [-Wredundant-move]
>    27 |         return std::move(t);
>       |                ~~~~~~~~~^~~
> move2.cpp:27:25: note: remove ‘std::move’ call
> Base::Base()
> Base::Base(Base &&)
> Base::Base()
> Base::Base(Base &&)
> 
> This is annoying. The move seems to be redundant indeed with g++-13, but
> dropping it results in the copy constructor being used with g++-12 and
> earlier.

Worse, I tried

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wredundant-move"
		/*
		 * gcc-13 calls the py::object move constructor even without a
		 * std::move(), making it redundant, but earlier gcc versions
		 * call the copy constructor. Keep the move and silence the
		 * warning.
		 */
		return std::move(t);
#pragma GCC diagnostic pop

which gave me

../../src/py/libcamera/py_helpers.cpp: In function ‘pybind11::object valueOrTuple(const libcamera::ControlValue&)’:
../../src/py/libcamera/py_helpers.cpp:29:32: error: unknown option after ‘#pragma GCC diagnostic’ kind [-Werror=pragmas]
 #pragma GCC diagnostic ignored "-Wredundant-move"
                                ^~~~~~~~~~~~~~~~~~

with gcc-8.5.0.

-- 
Regards,

Laurent Pinchart