# Case study: Making std::any trivially relocatable

Many library types are not trivially relocatable even though one’s initial impression is “sure, why wouldn’t they be?” For example, std::list. Another example: std::any. In my libc++ branch, I make std::any trivially relocatable under a preprocessor #ifdef. Here’s the code change:

template <class _Tp>
using _IsSmallObject = integral_constant<bool,
sizeof(_Tp) <= sizeof(_Buffer)
&& alignment_of<_Buffer>::value
% alignment_of<_Tp>::value == 0
#ifdef _LIBCPP_TRIVIALLY_RELOCATABLE_ANY
&& is_trivially_relocatable<_Tp>::value
#else
&& is_nothrow_move_constructible<_Tp>::value
#endif
>;


The reason I put this under an #ifdef in my branch is that it actually changes the semantics of std::any. Most of my library changes are non-ABI-breaking — I’m just taking the existing fact that certain types (such as std::vector) are trivially relocatable, and exposing that fact to the compiler in a machine-readable way (which is the action I call “warranting”). This requires a little bit of refactoring but does not affect the struct layout or behavior of those types.

But with std::any, making the above change to _IsSmallObject actually does break ABI!

First, consider this program, which demonstrates a way in which -D_LIBCPP_TRIVIALLY_RELOCATABLE_ANY does not break ABI.

// tu1.cpp
#include <any>
struct [[trivially_relocatable]] FooBar { FooBar(); FooBar(const FooBar&); };
std::any TU1() { return FooBar{}; }

// tu2.cpp
#include <any>
std::any TU1();
struct [[trivially_relocatable]] FooBar { FooBar(); FooBar(const FooBar&); };
FooBar TU2() { return std::any_cast<FooBar>(TU1()); }


Suppose we compile tu1.cpp with -D_LIBCPP_TRIVIALLY_RELOCATABLE_ANY, and we compile tu2.cpp without it. Then our FooBar object will be stored in the SSO buffer. But this is fine because TU2 does not expect to know where it’s stored; it just asks the any object’s “vtable” where it is. No problem.

Suppose we compile tu1.cpp without -D_LIBCPP_TRIVIALLY_RELOCATABLE_ANY, and we compile tu2.cpp with it. Then our FooBar object will not be stored in the SSO buffer. But this is fine because TU2 does not expect to know where it’s stored; it just asks the any object’s “vtable” where it is. No problem.

However!

Consider this program, which demonstrates a way in which -D_LIBCPP_TRIVIALLY_RELOCATABLE_ANY does break ABI.

// tu1.cpp
#include <any>
#include <list>
std::any TU1() { return std::list<int>{1,2,3}; }

// tu2.cpp
#include <any>
#include <list>
#include <vector>
std::any TU1();
std::list<int> TU2() {
std::vector<std::any> vec;
vec.push_back(TU1());
vec.push_back(42);  // suppose this causes reallocation
return std::any_cast<std::list<int>>(vec[0]);
}


Suppose we compile tu1.cpp with -D_LIBCPP_TRIVIALLY_RELOCATABLE_ANY, and we compile tu2.cpp without it. Then our std::list object will not be stored in the SSO buffer. But this is fine because TU2 does not expect to know where it’s stored. When it comes time to reallocate vector’s underlying buffer, it relocates each any object in the buffer by calling any’s move-constructor and then its destructor. And any’s move-constructor doesn’t expect to know where the std::list<int> object is stored; it just asks the any object’s “vtable” where it is. No problem.

Suppose we compile tu1.cpp without -D_LIBCPP_TRIVIALLY_RELOCATABLE_ANY, and we compile tu2.cpp with it. Then our std::list object will be stored in the SSO buffer. Now when it comes time to reallocate vector’s underlying buffer, it relocates each any object in the buffer by calling memcpy (because in tu2.cpp, we have warranted that std::any is trivially relocatable). So it’ll memcpy the whole object, including the SSO buffer containing a std::list object — and it’ll break that std::list object! When we go to call the copy-constructor on that broken list object, on the final line of TU2(), we’ll get an infinite loop or segfault.

So if you compile part of your program with -D_LIBCPP_TRIVIALLY_RELOCATABLE_ANY and part without, you run the risk of segfaults or worse. That’s because -D_LIBCPP_TRIVIALLY_RELOCATABLE_ANY changes the actual run-time behavior of std::any. We’re not just warranting an existing fact in a way that the compiler can understand; we’re actually changing the facts.

My proposal P1144 “Object relocation in terms of move plus destroy” specifically does not propose that any standard library types should (or should not) become trivially relocatable — I don’t want to require any vendors to change their facts. I just want them to warrant the existing facts on their implementation.

Posted 2018-08-02