[swift-dev] Questions about Struct's memory allocation

John McCall rjmccall at apple.com
Tue Dec 20 11:41:38 CST 2016

> On Dec 20, 2016, at 9:14 AM, Sergo Bero via swift-dev <swift-dev at swift.org> wrote:
> Hello everyone,
> I have a question about Swift Structs, regarding memory usage...
> as WWDC talk ( Session 416 ) says, Swift uses heap memory to store large values ( contain more than 3 words as current inline buffer is 3 words ),
> has something changed in this area since WWDC?
> What are the advantages from copying it to heap?
> is current behaviour final and will it be “baked” into Swift ABI?
> is it possible to tell compiler ( or force ) to always use stack for value types ?

Structs are currently always allocated "inline" in their container.  So if the struct is the type of a struct property, the outer struct will include enough space for the inner struct; if the struct is the type of a class property, the class's instance size will include enough space for the struct; and if the struct is the type of a local variable, there will generally be that many bytes allocated in the stack frame.

There are two exceptions to that, both relating to situations where an arbitrarily-sized type needs to be fit into something fixed-size.

In generic code, we don't know how large an unconstrained type argument will take up, so if a function that's generic over some type T has a local variable of type T, we have to allocate space for that somehow.  Today, that's done by allocating an inline buffer of 3 words and then using malloc if it spills over that.  But we're exploring using dynamic stack allocations instead, like C's alloca() function; if that works out, we'll eliminate of the two cases.

The other case comes up with protocol types.  A protocol type inherently has to have *some* fixed size, so it's an unavoidable limitation that *some* types will need to be allocated out-of-line when stored into a protocol value.


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