Why can't the C++ compiler elide the move when moving a POD into an optional with RVO?

Question

Consider the following code ( godbolt ):

#include <optional>
#include <array>

struct LargeType {
    std::array<int, 256> largeContents;
};

LargeType doSomething();

std::optional<LargeType> wrapIntoOptional(){
    return std::optional<LargeType> {doSomething()};
}

As you see, there is a function returning a large POD and then a function wrapping it into a std::optional . As visible in godbolt, the compiler creates a memcpy here, so it cannot fully elide moving the object. Why is this?

If I understand it correctly, the C++ language would allow eliding the move due to the as-if rule, as there are no visible side effects to it. So it seems that the compiler really cannot avoid it. But why?

My (probably incorrect) understanding how the compiler could optimize the memcpy out is to hand a reference to the storage inside the optional to doSomething() (as I guess such large objects get passed by hidden reference anyway). The optional itself would already lie on the stack of the caller of wrapIntoOptional due to RVO. As the definition of the constructor of std::optional is in the header, it is available to the compiler, so it should be able to inline it, so it can hand that storage location to doSomething in the first place. So what's wrong about my intuition here?

To clarify: I don't argue that the C++ language requires the compiler to inline this. I just thought it would be a reasonable optimization and given that wrapping things into optionals is a common operation, it would be an optimization that is implemented in modern compilers.

Answer 1

It is impossible to elide any copy/move through a constructor call into storage managed by the constructed object.

The constructor takes the object as a reference. In order to bind the reference to something there must be an object, so the prvalue from doSomething() must be materialized into a temporary object to bind to the reference and the constructor then must copy/move from that temporary into its own storage.

It is impossible to elide through function parameters. That would require knowing the implementation of the function and the way C++ is specified it is possible to compile each function only knowing the declarations of other functions (aside from constant expression evaluation). This would break that or require a new type of annotation in the declaration.

None of this prevents the compiler from optimizing in a way that doesn't affect the observable behavior though. If your compiler is not figuring out that the extra copy can be avoided and has no observable side effects when seeing all relevant function/constructor definitions, then that's something you could complain to your compiler vendor about. The concept of copy elision is about allowing the compiler to optimize away a copy/move even though it would have had observable side effects .

Answer 2

You can add noexcept to elide copy:

https://godbolt.org/z/rrGEfrdzc