错过优化：std::vector<t> ::pop_back() 不是限定析构函数调用？ </t>

Question

. Answers to this question are eligible for a +200 reputation bounty. John Zwinck is looking for an :

An acceptable answer to claim the bounty might point out some existing compiler which does this optimization, or cite some part of the C++ standard which makes this optimization impossible, or cite parts of the standard which indicate this optimization is possible even though it's not implemented yet.

In an std::vector<T> the vector owns the allocated storage and it constructs T s and destructs T s. Regardless of T 's class hierarchy, std::vector<T> knows that it has only created a T and thus when .pop_back() is called it only has to destroy a T (not some derived class of T ). Take the following code:

#include <vector>

struct Bar {
    virtual ~Bar() noexcept = default;
};

struct FooOpen : Bar {
    int a;
};

struct FooFinal final : Bar {
    int a;
};

void popEm(std::vector<FooOpen>& v) {
    v.pop_back();
}

void popEm(std::vector<FooFinal>& v) {
    v.pop_back();
}

https://godbolt.org/z/G5ceGe6rq

The PopEm for FooFinal simply just reduces the vector's size by 1 (element). This makes sense. But PopEm for FooOpen calls the virtual destructor that the class got by extending Bar . Given that FooOpen is not final, if a normal delete fooOpen was called on a FooOpen* pointer, it would need to do the virtual destructor, but in the case of std::vector it knows that it only made a FooOpen and no derived class of it was constructed. Therefore, couldn't std::vector<FooOpen> treat the class as final and omit the call to the virtual destructor on the pop_back() ?

Answer 1

Long story short - compiler doesn't have enough context information to deduce it https://godbolt.org/z/roq7sYdvT

Boring part:

The results are similar for all 3: msvc, clang, and gcc, so I guess the problem is general. I analysed the libstdc++ code just to find pop_back() runs like this:

void pop_back() // a bit more convoluted but boils-down to this
{
    --back;
    back->~T();
}

Not any surprise. It's like in C++ textbooks. But it shows the problem - virtual call to a destructor from a pointer. What we're looking for is the 'devirtualisation' technique described here: Is final used for optimisation in C++ - it states devirtualisation is 'as-if' behaviour, so it looks like it is open for optimisation if the compiler has enough information to do it.

My opinion:

I meddled with the code a little and i think optimisation doesn't happen because the compiler cannot deduce the only objects pointed by "back" are FooOpen instances. We - humans - know it because we analyse the entire class, and see the overall concept of storing the elements in a vector. We know the pointer must point to FooOpen instance only, but compiler fails to see it - it only sees a pointer which can point anywhere (vector allocates uninitialized chunk of memory and its interpretation is a part of vector's logic, also the pointer is modified outside the scope of pop_back()). Without knowing the entire concept of vector<> i don't think of how it can be deduced (without analysing the entire class) that it won't point to any descendant of FooOpen which can be defined in other translation units.

FooFinal doesn't have this problem because it already guarantees no other class can inherit from it so devirtualisation is safe for objects pointed by FooFinal* or FooFinal&.

For what i've found devirtualisation can occur for non-final classes (Take this snippet https://godbolt.org/z/3a1bvax4o ), as long as there is no pointer arithmetic involved.

Answer 2

Yes, this is a missed optimisation.

Remember that a compiler is a software project, where features have to be written to exist. It may be that the relative overhead of virtual destruction in cases like this is low enough that adding this in hasn't been a priority for the gcc team so far.

It is an open-source project, so you could submit a patch that adds this in.

Answer 3

It feels a lot like § 11.4.7 (14) gives some insight into this. As of latest working draft (N4910 Post-Winter 2022 C++ working draft, Mar. 2022):

After executing the body of the destructor and destroying any objects with automatic storage duration allocated within the body, a destructor for class X calls the destructors for X's direct non-variant non-static data members, the destructors for X's non-virtual direct base classes and, if X is the most derived class (11.9.3), its destructor calls the destructors for X's virtual base classes. All destructors are called as if they were referenced with a qualified name, that is, ignoring any possible virtual overriding destructors in more derived classes. Bases and members are destroyed in the reverse order of the completion of their constructor (see 11.9.3). [Note 4: A return statement (8.7.4) in a destructor might not directly return to the caller; before transferring control to the caller, the destructors for the members and bases are called. — end note] Destructors for elements of an array are called in reverse order of their construction (see 11.9).

Also interesting for this topic, § 11.4.6, (17):

In an explicit destructor call, the destructor is specified by a ~ followed by a type-name or decltype-specifier that denotes the destructor's class type. The invocation of a destructor is subject to the usual rules for member functions (11.4.2); that is, if the object is not of the destructor's class type and not of a class derived from the destructor's class type (including when the destructor is invoked via a null pointer value), the program has undefined behavior.

So, as far as the standard cares, the invocation of a destructor is subject to the usual rules for member functions.

This, to me, sounds a lot like destructor calls do so much that compilers are likely unable to determine, at compile-time , that a destructor call does "nothing" - as it also calls destructors of members, and std::vector doesn't know this.

Answer 4

If a class 'Trivial' is extended from FooOpen, and an object of it is inserted into vector<FooOpen> , the inserted of member would be a instance of 'FooOpen' but not of 'Trivial'. That's the problem you get! Simply using vector<FooOpen*> or vector<unqiue_ptr<FooOpen>> instead of vector<FooOpen> to resolve the problem.