Swift at Apple: Migrating the TrueType hinting interpreter

https://www.swift.org/blog/migrating-truetype-hinting-to-swift/

by DASD • 1 month ago

241 128 1 month ago

pjmlp8• 1 month ago

During the State of Platform keynote, on the subject of Swift adoption across macOS, several examples were given, not only TrueType engine.

RIS is happening across all OS levels, if the keynote is to be believed.

DASD2• 1 month ago

Curious the direction of Webkit as there was a nebulous mention of select portions being rewritten from C++ to Swift. And yet, the new ECMAScript module (ESM) loader for Safari 27 is implemented in C++ (https://webkit.org/blog/17967/news-from-wwdc26-webkit-in-saf...).

pjmlp8• 1 month ago

No idea, maybe the private parts of the code, Safari isn't open source, or is coming later.

In any case I would have liked to have more info during the deep dive sessions.

As it is, Meet with Apple on security (a 5h long event) had much more information.

hirvi74• 1 month ago

What does RIS stand for?

gyomu• 1 month ago

Rewrite in Swift

MBCook3• 1 month ago

They’ve been doing it for years. I don’t remember how we first knew, but I know they’ve been using Swift in kernels for at least some of the other chips like the Secure Enclave or whatever.

I’m not sure exactly which. I assume it’s some of the code and not all. But it’s not new in the abstract.

That said I don’t think I’ve heard of it in the kernel of MacOS on the main processor. That may be new.

Either way this is certainly the most concrete announcement I remember them ever giving on this stuff.

commandersaki• 1 month ago

I know internally they use an IPsec implementation written by Rust (I think in the iCloud infra). Heard this from an ex-Apple engineer Ben (forgot his last name) that did a wonderful presentation of Rust from first principles. He said that it was hard to get people in on Rust when most would argue for Swift.

Edit: This is the guy: https://rustcurious.com/course/

pjmlp8• 1 month ago

Some stuff was discussed at Meet with Apple security event a few months ago, and the talks on FoundationDB rewrite, or why Swift Embedded subset came to be.

However I miss them actually having had one of those 15 - 30m WWDC sessions, where they could have gone a bit deeper into the keynote examples

JimDabell• 1 month ago

Alexandre Colucci has published a series of articles analysing the use of Swift in iOS and macOS by Apple here:

https://blog.timac.org/categories/reverse-engineering/

And frequently discussed on Hacker News:

https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...

pjmlp8• 1 month ago

Thanks!

troupo• 1 month ago

I think these are the types of things Apple should've focused on instead of half-heartedly barging ahead with SwiftUI and breaking the language in the process

saagarjha2• 1 month ago

I mean they’re doing both

saagarjha2• 1 month ago

Interesting that this is published under the MIT, rather than Apple’s more favorite Apache 2, license

JumpCrisscross• 1 month ago

Why is it interesting?

drob518• 1 month ago

Presumably because MIT is even more permissive and it’s a change in Apple’s behavior.

mrpippy• 1 month ago

The author discussed this a bit on Mastodon as well:

https://xoxo.zone/@numist/116716469017975106

numist4• 1 month ago

I'm also here :)

atdt• 1 month ago

Excellent write-up!

weinzierl• 1 month ago

Back in 2023 there was talks about Microsoft rewriting the font stuff in Rust for similar reasons Apple is now doing the Swift move.

I'm not sure what became of it and if it ever shipped. If anyone knows I'd be curious.

DASD2• 1 month ago

Russinovitch (Azure's CTO/CISO) gave a speech at RustConf 2025 and mentions it(DirectWriteCore) took 2 engineers 6 months resulting in 154K LOC and 5-15 percent performance increase for font shaping. https://www.youtube.com/watch?v=uDtMuS7BExE&list=PL2b0df3jKK...

LoganDark3• 1 month ago

I'm surprised the code has visible LLM smells. Though, I shouldn't be surprised. I hope the important bits are still human-controlled (and the same for Apple's many operating systems that absolutely deserve to remain stable and understood).

dgellow• 1 month ago

From what I got Apple is using claude code A LOT internally

wahnfrieden5• 1 month ago

Yes they are using Claude Code - not the Xcode agents.

It worries me. I hope Codex adoption picks up there.

Caracas288• 1 month ago

It would be interesting to see their internal guidance on LLM use. It’s a massive amount of new power that has to be wielded carefully. That kind of guidance might mean the survival or downfall of some big corps in the next few years.

airspeedswift2• 1 month ago

I assure you, every inch of the interpreter code has been stared at by humans, a lot. TBH even the assembly generated by it has.

ahartmetz• 1 month ago

All 150 kloc in six months by two people? Actually, it sounds like way too much code for the task unless 70+% of it is tests.

airstrike4• 1 month ago

As much as I enjoyed Swift, one can only wonder what the world would look like if they had gone with Rust as their default language instead.

jadengeller• 1 month ago

Modern Swift borrows a lot from Rust! And it also has its own benefits, both ergonomic and also supporting eg generic in dynamic libraries

airstrike4• 1 month ago

These days I mainly write Rust but I did write a semi complex iOS app and enjoyed Swift. I just didn't love how slow the type checker was and how it got lost. I recall having to break things into smaller bits to help the compiler, and there were some oddities about the language.

The gap between the two languages is quite small, it just makes me wish Apple was also all-in on Rust

vardump• 1 month ago

Does it borrow borrow checker?

ecshafer• 1 month ago

Swift and Rust were developed at similar times. I think of them more as having similar influences than borrowing from each other.

AceJohnny2• 1 month ago

Rust doesn't have an ABI [1]. Swift needed one to be a useable application language:

https://faultlore.com/blah/swift-abi/ (written by a core Rust developer)

[1] apart from the basic/universal C one, which prevents exposing any useful Rust semantics over the interface

afavour2• 1 month ago

One of the genius things about Swift is its interop with Objective C. Made the switch over considerably easier for developers. I’m not sure what that looks like in a Rust world.

Rust is also just a more complex language. I’m not convinced the benefits would have been worth it.

inkyoto2• 1 month ago

Not just interoperability with Objective C but with C (full) and C++ (increasingly better but not full) as well.

Swift is also interoperable with different versions of itself courtesy of the Swift stable ABI (Application Binary Interface)[0], which they invested a significant amount of time into at the expense of adding other new features to the language, which have come along later.

Rust offers a different approach: recompile everything and static linking.

[0] https://faultlore.com/blah/swift-abi/

pjmlp8• 1 month ago

Swift takes the right approach in language ergonomics, many of its use cases would be much harder with explicit .clone() all over the place, lack of back references, no standard ABI for binary libraries, interop with Objective-C and C++, no standard concurrency runtime, or error handling types.

Rust 2026 roadmap has language ergonomics on it for a reason.

That said, outside Apple ecosystem you probably better with Rust, or if one has no GC issues, OCaml, Haskell, F#, Scala, C#.

jounker• 1 month ago

Apple has a runtime system that looks a lot like smalltalk. Everything at that layer is dynamic. They needed a language that could seamlessly interact with that system.

raphlinus2• 1 month ago

Welcome to the club of doing high performance text in a memory safe language!

masiulis• 1 month ago

There are dozens of us! Dozens!

Vello has been a big inspiration and source of knowledge for my own webgpu text renderer, thank you for that!

WalterGR2• 1 month ago

high performance text

Just strings or rendering strings?

If the latter, who are the other members of the club?

raphlinus2• 1 month ago

Yeah, probably should have made a more substantive comment. I'm mostly thinking of the trio of Skrifa, Harfrust, and Parley, but there is other open source Rust work that qualifies as well.

There's also, very relevantly, the DWriteCore work from Microsoft. My understanding is that there's been some talk of open sourcing that, but it's still proprietary.

There are other things that count as high performance text in memory safe languages, for example implementations in Go, but in general those are not a good candidate to replace C and C++ in environments like operating systems and browsers.

wg0• 1 month ago

No mention of AI? Hand written code?

numist4• 1 month ago

There's mention at the end. The models (and Swift itself!) have evolved a lot since this project started, so the early code is largely hand-rolled and the later changes were mostly authored by centaurs (to steal a term from chess).

But I personally reviewed every line that shipped and was absolutely insufferable about testing.

AndriyKunitsyn6• 1 month ago

What's funny is from 2023 (I think), macOS just draws the UI unhinted. You have a 1080p display and you don't want to see the letters in the UI blurred to death? Tough luck, 1080p is incompatible with macOS, everybody needs "retina", and nobody cares that Windows and all Linux DEs look on 1080p just fine.

It looks like this hinter will be used only in rendering PDFs, because that's where they test the performance.

nomel2• 1 month ago

My last 1080p monitor was around 20 years ago. I have trouble comprehending people still use them regularly.

AndriyKunitsyn6• 1 month ago

People also use usable mice instead of touchpads, and they put the "ctrl" key where Apple thinks a useless "fn" should be. All kinds of things happen outside Apple world.

To me, it's more about what I'm used to. I have a perfectly fine several years-old monitor, so why should I throw it away?

afavour2• 1 month ago

A very quick search yielded Dell selling 1080p laptops today:

https://www.dell.com/en-us/shop/dell-laptops/dell-15-laptop/...

It is very, very common. Just not in the Mac world.

mschuster912• 1 month ago

The problem is, as soon as you are not on a Mac but Linux or Windows, you are in for an awful, truly awful lot of pain. HiDPI support is a mess because even in the rare case applications are made with HiDPI in mind they are not tested on HiDPI machines.

Other way around, most Mac software is not tested how it behaves on inferior external monitors.

yjftsjthsd-h• 1 month ago

Then let me blow your mind:) One of my daily drivers is an ex-Chromebook at 1366x768. Granted, it's also physically smallish so the DPI isn't quite as low as a macbook would be with those pixels, but still. And that's a touch cramped but it's fine.

TazeTSchnitzel• 1 month ago

macOS has been drawing unhinted text for an eternity, and for those who can tolerate it on low-DPI screens, it's a great thing: the letter shapes look the same at all sizes, and the spacing between letters is consistent at all sizes.

bschwindHN• 1 month ago

I'm a high DPI snob so I haven't used a low res monitor for work in forever, but isn't the entire point of font hinting to make the text more legible at smaller pixel grid sizes? Yes, of course the shapes are more consistent since the hinter isn't touching them, but isn't the end result just less legible text?

kbolino2• 1 month ago

I had this problem on the first Apple Silicon Mac Mini in 2020 so it's at least a little older than 2023.

jacquesgt2• 1 month ago

While hinting is disabled for most fonts, there are some fonts that require hinting to render correctly. We have to support hinting for those fonts, and it was easier to make it secure by rewriting hinting in Swift than it would have been to comprehensively identify every font created by those foundries.

carlosjobim2• 1 month ago

Retina displays were introduced more than a decade ago. Why should Apple still support outdated technology after such a long time?

If you work with text and fine UI elements, do yourself a favour and get proper tools for the job. Get an ergonomic mouse and a good keyboard while you're at it. In every other field professionals use high quality tools to do their jobs, IT shouldn't be any different.

A plumber has equipment worth tens of thousands of dollars, while IT professionals think it's outrageous to pay a few hundred for equipment which is undeniably an improvement.

jacquesgt2• 1 month ago

If you want to help improve the security of OS software through the magic of memory safe languages, the team that did this work is hiring: https://jobs.apple.com/en-us/search?search=Spear&sort=releva...

Knowledge of Swift not required. If you know your way around OS software, can reason about the security of the code you write, and are excited about writing exhaustively tested software, we’d love to talk to you.

We’re hiring for roles in kernel/systems and userspace. Like the Platforms SOTU mentioned, we’re using Swift at all layers of the software stack now. https://www.youtube.com/live/yl2jsIoMfDU

I had the pleasure of leading the effort to ship Swift in the Secure Enclave back in 2022. Now I have multiple teams working on accelerating the transition to memory safe languages. We’re showing that with good planning and a relentless focus on testing, we can improve security, performance, and functionality. And we get to have a ton of fun working with some amazing colleagues. It’s the most enjoyable and impactful work I’ve ever done in my career.

byproxy• 1 month ago

Any resources you'd recommend for learning about reasoning about the security of code?

rinon• 1 month ago

For low-level system security, I'm a fan of https://llsoftsec.github.io/llsoftsecbook/LLSoftSecBook.pdf as an overview for any systems developer, not just compiler devs. It might not be the most approachable, but it's got great info on everything memory corruption.

rfc30922• 1 month ago

How do I apply? Historically jobs.apple.com is a black hole unless you know people.

musicale2• 1 month ago

Apple (like most large tech companies) is indeed a resumé black hole, and knowing people helps, so definitely network if possible. However, applying to the right position, with a good resumé that highlights experience/skills/projects/open source contributions/education/etc. that are directly related to the position, also matters. I am aware of several people who simply applied online and got interviews.

I may be wrong about this, but I don't think Apple bans you from applying to multiple positions within the same year the way some companies do.

There also seems to be a decent pipeline for new graduates (though I think highlighting relevant academic, research, and open source projects can still help.) Internships can also be a path if you are currently in school.

I don't know if Apple recruits on linkedin, but that might also be an option.

Of course connecting right here on HN seems like a great idea as well.

ComputerGuru• 1 month ago

On-site only?

musicale2• 1 month ago

Great to see this happening. Personally I want an OS where everything is memory-safe by default.

Of course in an alternate universe where macOS (and iOS etc.) was based on Multics rather than Unix, it would have had essentially zero buffer overflows - which are hard to create in PL/I but hard to avoid in C. Even Apple's Pascal compilers from the 1980s had range checking...

But legacy C code can/should absolutely use things like clang's -fbounds-safety (has been in clang on macOS for years) etc. Fil-C is another option.

Degorath• 1 month ago

Do you folks also hire outside the US?

comex• 1 month ago

Beware: As of a few months ago, when I tried to use the lifetime features shown off in this post, I ran into constant compiler crashes with very simple programs, until I gave up and wrote off the features as unusable. This happened on both stable and nightly compilers. I guess they work well enough for this TrueType interpreter, but I suspect they’re using a narrow subset of what the features are supposed to support. Or maybe things have been fixed very recently.

That said, I’m looking forward to using Swift lifetimes once they actually work!

stephencanon• 1 month ago

The work discussed in this post shipped in the OS last year (fall 2025), so nothing here is dependent on very recent changes.

monster_truck• 1 month ago

Average swift experience

ks2048• 1 month ago

So, hinting only takes place at low resolutions, I believe. How often is it used, eg viewing “typical” PDFs on “typical” screens?

dcrazy5• 1 month ago

There are fonts that rely on hinting for correct shaping at all sizes: https://xoxo.zone/@numist/116716601962175503

KolmogorovComp• 1 month ago

Operations like filter and map allocate memory, but that allocation is only necessary if the value escapes. The Swift standard library provides .lazy.map and .lazy.filter, but they don’t work in every case. For logic that only iterates over the filter or map, it’s much more efficient to loop with continue (or use for … in … where) and transform elements into local variables as necessary.

It does feel like a compiler/optimiser failure to have to rewrite those cases.

Altern4tiveAcc• 1 month ago

I've always wondered if JS engines could rewrite those array functions at compile time, like this: https://github.com/SomeRanDev/Haxe-MagicArrayTools

Though, it probably wouldn't work if user code modified the Array prototype.

leecommamichael• 1 month ago

It isn't. You'd need full lifetime analysis as part of the compiler to even decide whether you _could_ skip the allocation, and even then it's 100% up to the circumstance and actual lifetime of the data to say whether it can be, e.g. stack-allocated to avoid placing it somewhere less localized.

yyyk• 1 month ago

"we used a fuzzer to minimize a corpus of 10 million PDF files down to 4,200 without any loss of code coverage"

Did they need a fuzzer for that? They could've render them all and see what's exercised?

thechao• 1 month ago

Its a set cover problem, and is NP-hard. 4100 of something probably runs nicely in your laptop in a reasonable amount of time.

numist4• 1 month ago

We did "render them all and see what's exercised" by using libFuzzer's -merge=1 option. No new PDFs were generated during corpus minimization.

dpark• 1 month ago

By the end of the project, we wrote nearly four times as many lines of test code as we wrote for the Swift interpreter itself.

This is the most interesting bit to me. Engineers consistently underestimate the amount of effort that testing demands for projects that need truly high quality, it’s nice to see this shared.

Swift at Apple: Migrating the TrueType hinting interpreter

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