# Trivially swappable

This is a recycling of a reply I just made to Mingxin Wang and Gašper Ažman over at std-proposals. TLDR I think “trivially swappable” is a straightforward corollary to “trivially relocatable”, but its cost-of-specification might be a bit higher, and its benefit-in-performance is essentially zero as far as any use-case I can think of.

In my C++Now 2018 talk on “trivially relocatable” (which is fully captioned and I highly recommend you watch it!), I promised in the outline to talk about the relationship between “trivially relocatable” and “trivially swappable,” and then did not actually do so. Sorry! Let’s do that now.

Essentially, if a type is trivially relocatable then it intuitively ought to be considered trivially swappable. However, there are two minor caveats that I can think of off the top of my head (and the reason I didn’t talk about it at C++Now is that I haven’t thought about it much, and the reason for that is that I don’t have a motivating use-case).

Caveat (A): Trivial relocation can be optimized into memcpy or memmove. Trivial swap cannot be optimized into mem-anything, because there is no libc primitive for swapping arrays of bytes. We could certainly propose to add a __builtin_memswap() that would perform the swap “in-place” in cache-line-sized blocks, but I’m not aware of any proposals nor prior art in that area.

Caveat (B): Notice that whereas “relocate” means “move-construct, then destroy”, we might say that “swap” means “move-construct, then move-assign, then move-assign, then destroy.” (This being the operation done by the unconstrained std::swap template.) This involves a relationship among 3 operations, which might be a little scarier than relocate’s relationship among 2 operations, which is scarier than the current Standard Library’s “trivially X” traits which all involve only a single operation.

Caveat (C): For small types like unique_ptr, __builtin_memswap() will not be any faster than the unconstrained std::swap template. The point of optimizing into mem-anything is to get speedups on large arrays, such as during std::vector reallocation. std::vector swapping is already fast, and cannot be made faster by __builtin_memswap().

Now, std::array swapping could be made faster! Consider:

std::array<std::unique_ptr<int>, 10000> a;
std::array<std::unique_ptr<int>, 10000> b;
a.swap(b);  // could probably get a factor-of-2 speedup on this operation by using __builtin_memswap


But this is not an operation that happens often enough in real programs for anyone to get really motivated about.

So, “trivially swappable” seems like a straightforward corollary to “trivially relocatable”, but its cost-of-specification might be a bit higher, and its benefit-in-performance is essentially zero as far as any use-case I can think of.

Posted 2018-06-29