[swift-dev] SIL: "unowned" the calling convention and "unowned" the variable ownership convention

[John McCall]

> On Jan 4, 2016, at 11:07 AM, Jordan Rose via swift-dev <swift-dev at swift.org> wrote:
> Hi, Michael. The calling convention is equivalent to the 'unowned(unsafe)' variant of 'unowned', so I don't think it's entirely unrelated.
> 
> I don't like "Immediate" because I don't know what it means. Admittedly I don't work on SIL, but when is something passed "immediate" as opposed to "guaranteed"? Is "immediate" the case where it's valid now but mutating any external memory could make it invalid?

Yes.

> What makes that "immediate”?

The name is used in SILGen when you’re going to use a value “immediately”, i.e. before any code executes that could possibly invalidate the reference.  For example, the base expression of a load of a stored class property can be emitted as a +0 immediate r-value, because the caller is going to immediately project the property and load.  That allows us to e.g. not retain after loading from a var; it’s a minor but frequently-impactful SILGen optimization.

Anyway, I agree with Jordan that that name is not particularly appropriate for a parameter convention.

This really just affects documentation and code within the compiler: it’s not actually written in textual SIL because it’s the default convention.  Still, I’m fine with changing the name from “unowned”.  Something that suggests the temporary nature of validity, maybe — “fleeting”? :)  One consideration is that this is also the convention used for trivial values, although I suppose we could split that out (to “trivial”, which would of course be the default for trivial arguments) and maybe even always require an explicit convention on non-trivial arguments.

John.