The Swift Runtime: Your Silent Partner

Fun and games with the Swift source code

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My wife thought I looked so good after taking this photo, she asked me to wear the tape all day

The Swift Runtime, a.k.a libswiftCore, is a C++ library that runs alongside all Swift programs. The runtime library is dynamically linked to your app binary at launch and facilitates core language features such as:

• Memory management

• Error handling

• Dynamic dispatch

• Types, generics, & protocols

That is the standard elevator pitch, but there’s just one problem: What the h*ck do I mean by “runs alongside” and “facilitates core language features”!?

Let’s find out with a quest into the Swift source code.

Here there be dragons (well, wyverns). Many a brave explorer has been lost, or lost their mind, trying to comprehend the labyrinthine code paths of its forbidden texts (more commonly referred to as C++).

To keep us tethered to reality, we will start with some familiar Swift code you’ll know and love. I’ll demonstrate a core Swift language feature in the most basic way possible.

We’ll convert this into Swift Intermediate Language, and follow how these instructions compile all the way down to calls into the Swift Runtime ABI. This will ultimately show exactly how the runtime program runs alongside your own code.

Finally, we will look inside libswiftCore — the runtime library code itself — to understand how core language features are implemented.

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