Here’s an illustration of a rudimentary mental model that I use when thinking about advances in computing tools.

Computers are already (essentially) Turing-complete, so anything that can be done can in theory already be done using any existing technology. But what these leaps do is that they bring down the skill and resources needed to accomplish a certain task. The red arrow in the image is what such a leap does.

Generative AI is such a leap. The red dot represents not only something genuinely useful that people used to have to learn a skill or pay someone to do, but can now do with ease (removing a distracting object from a photograph), but also vectors of abuse that similarly gain the same level of ease (crafting misinformation, generating fake revenge porn). With every leap, thousands of these red dots move toward the origin, some good for humanity, and others bad.

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• #opinion

After writing the post about mergeable libraries, I thought I should write a post about how libraries work in general, and capture some recommendations on how to use them.

Let’s start at the beginning, with what happens when a source file is compiled.

A compiler creates object files from source files

A compiler takes a translation unit (a source file with any imported headers), and produces a compiled object file, containing the code and data defined in the translation unit.

Exported symbols

An object file may export symbols. Exported symbols are strings that identify data structures within the object file, which may be function implementations, constants, initialized data, whatever.

// sound.c

static int tweetCount = 0;

extern void tweet(void) {
    tweetCount++;
}

The file above compiles into an object file sounds.o which exports the symbol _tweet for the implementation of the tweet() function^{1 2}.

Some symbols are purely internal to the translation unit. tweetCount, for example, is not exported.

In addition to open/public (exported) and private/fileprivate (not exported), Swift symbols have an access control option, package, which makes the symbol visible only within a Swift package. As far as libraries are concerned, package symbols are exported.

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I was curious about what the use of mergeable libraries meant for a typical app that may have a lot of library dependencies, so I devised a test which would measure the performance of merged frameworks versus old-school dynamic frameworks.

To read more about how mergeable libraries work, read this post.

The results

Let’s start with the results. These were taken on an iPhone 14 Pro using Instruments.

And here is a close-up of the region around the origin:

As you can see, the difference is dramatic. Virtually all of the O(n) cost of dynamic library loading is gone, even when an intermediate merged framework is introduced. It’s almost like the dynamic libraries have been turned into static ones, and that is not far from the truth.

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• #programming

Mergeable Libraries is a technology introduced in WWDC23 that allows you to “absorb” and reexport dynamic libraries into a higher-level dynamic library, or directly into your app’s binary.

To see the performance implications of using mergeable libraries in your app, see this post.

How mergeable libraries work

The mergeable libraries feature allows you to copy the contents of dependent¹ dynamic libraries one level up while maintaining the illusion in code that the original dependent libraries still existed where they were, when using API such as dlopen and Bundle.

Because the contents of dependent libraries are copied into the binary that links them, unresolved symbols in that binary can be statically resolved against the library’s contents which are now stored within the same binary. This is how mergeable libraries improve load times: it allows the linker to turn load-time resolution of symbols into link-time resolution, giving you dynamic-library semantics with near-static-library performance.

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• #programming

Remember to welcome progress, no matter how small, no matter how late.

It’s easy for someone who’s been shouting warnings from the rooftops to be frustrated that people are only now beginning to listen to them, and then make such small and entirely-insufficient changes based on that “new” information.

But that is what progress looks like. People can’t pay attention to everything, because they have their entire lives to contend with. So when they start to pay attention to your message, it is vital that you don’t turn against them for doing so, but instead nurture their interest and direct their next steps.

Yes, you’ve seen it coming. You’ve been experiencing it. You know what needs to be done to fix the issue. But not everyone does. And when someone begins to show signs of acquiring a sliver of your insight, celebrate it.

That is what progress looks like.

• #opinion