# Announcing “trivially relocatable”

Frequent users of Compiler Explorer, a.k.a. godbolt.org, may have noticed that a few days ago a new compiler appeared in its dropdown menu. “x86-64 clang (experimental P1144)” is a branch of Clang which I have patched to support the concept of “trivially relocatable types,” as described in my C++Now 2018 talk and as proposed for standardization in my upcoming paper P1144 “Object relocation in terms of move plus destroy” (coauthored with Mingxin Wang).

The implementation consists of several new language and library features, most of which are displayed in this snippet:

struct [[trivially_relocatable]] Widget {
Widget(Widget&&);
~Widget();
};

struct R {
std::optional<std::string> name;
std::unique_ptr<int> p, q;
Widget w;
};

static_assert(std::is_trivially_relocatable_v<R>);


Here we see an assertion that objects of type R are trivially relocatable — i.e., that the relocation operation for R is trivial — i.e., that the operation of “moving an R and then immediately destroying the original” is equivalent to memcpy.

The library type-trait std::is_trivially_relocatable<T> can be used to ask the compiler whether any given type T has this property or not. The compiler knows exactly which types have this property, in the same way that it knows which types have the property “trivially copyable.” The compiler tracks trivial relocatability, even for built-in types such as closures.

auto lama = [a = 1](){};
static_assert(std::is_trivially_copyable_v<decltype(lama)>);

auto lamb = [b = std::string("hello world")](){};
static_assert(std::is_trivially_relocatable_v<decltype(lamb)>);


So that’s how we ask the compiler about the trivial relocatability of a given type. What about the reverse? How do we tell the compiler that we know a given user-defined type is trivially relocatable, even though it has user-provided special member functions? For that, we provide an attribute:

struct [[trivially_relocatable]] Widget {
Widget(Widget&&);
~Widget();
};


(For those keeping score, this attribute is ignorable in exactly the same sense that [[no_unique_address]] is ignorable.)

Finally, the compiler on Godbolt is hooked up to my own branch of libc++, where we have told the compiler (using the above attribute) that all of the most popular standard types are in fact trivially relocatable. This is important, because by default the compiler cannot assume that any type is trivially relocatable unless it follows the Rule of Zero. This ensures that we don’t break any existing code. Example:

// You can rely on this.
static_assert(!std::is_trivially_relocatable_v<boost::interprocess::offset_ptr<int>>);

// This happens to be true, thanks to my patch.
static_assert(std::is_trivially_relocatable_v<std::unique_ptr<int>>);

// This happens to be false, since I did not patch Boost.
static_assert(!std::is_trivially_relocatable_v<boost::movelib::unique_ptr<int>>);


Observe that while boost::movelib::unique_ptr<int> can in fact safely be relocated via memcpy, it has not clearly communicated that fact to the compiler (and thence to consumers of the type-trait, such as vector::reserve, who could have optimized based on that fact). It would be easy to patch Boost just as I patched libc++, to add the attribute and thus enable the optimization.

By the way: In many cases the attribute must be added conditionally. The implementation takes care of this for you. Godbolt:

template<class P>
struct Dummy {
using pointer = P;
void operator()(P) {}
};

using P1 = std::unique_ptr<int, Dummy<int *>>;
using P2 = std::unique_ptr<int, Dummy<offset_ptr<int>>>;

static_assert(std::is_trivially_relocatable_v<P1>);
static_assert(!std::is_trivially_relocatable_v<P2>);


This produces about a 3x speed boost on operations that relocate ranges of trivially relocatable objects (such as happens in vector::reserve, or any time you move a fixed_capacity_vector). For library writers who want to get this optimization but don’t want to write all the tag-dispatching logic themselves, we provide a new standard algorithm, std::uninitialized_relocate, which can be used as a building block. Godbolt:

struct S {
std::any a;
std::variant<std::string, int> v;
};

void foo(S *src, int n, S *dst) {
std::uninitialized_relocate_n(src, n, dst);
}


Take a look, play around with it, let me know what you think! If you come up with compelling or thought-provoking examples, I’d like to add them to the paper.

P1144 has not yet appeared in print. You should expect to see P1144R0 in the pre-meeting mailing for San Diego. In the meantime, here is D1144R0 draft revision 7.

P.S. — Many, many thanks to Matt Godbolt for letting me put this implementation online!

Posted 2018-07-18