Difference between Dart interpreter and Dart JIT compiler?

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Background

In AOT compiling Dart for iOS Android (Dart Developer Summit 2016):

iOS restriction: Can't JIT

And also, from reading an article from the dart team: Flutter: Don’t Fear the Garbage Collector, I read:

In debug mode, most of Dart’s plumbing is shipped to the device: the Dart runtime, the just-in-time compiler/interpreter (JIT for Android and interpreter for iOS), debugging and profiling services.

Question

I wonder what the differences are between these 2 concepts, for dart specifically. Why can't iOS support JIT compilation but it can support a Dart interpreter?

Non-question

This is not about AOT vs JIT compilation, which is the more common question. You can find out about that here.

I also already know the difference between an Interpreter and a Compiler: Interpreter executes the code step by step in a higher level form, instead of compiling it to machine code, like JIT and AOT execution.

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1 Answers

There aren't any technical reasons for iOS not to support the Dart JIT compiler, it's due to a security policy by Apple. Also this limitation is not Dart specific, it affects every JIT compiler out there.

A JIT compiler needs to create new machine code on the fly and write it to executable pages, but on iOS that can only be done by apps with the "dynamic-codesigning" entitlement. This entitlement is granted for example to 1st party apps using JavaScriptCore, but not 3rd party apps.

This means App Store apps cannot write to executable memory, and so a JIT cannot be embedded in them. You can run JIT accelerated JavaScript using a WKWebView (which runs in a separate process with 1st party entitlements) but you cannot run your own JIT compiler on iOS.

An interpreter w/o a JIT on the other hand never generates new machine code so it can run without problems on iOS (though the App Store review process still imposes restrictions on what scripts it is allowed to run.)

Over the years there have been several exploits to get a JIT working, allowing for example to switch pages from +w to +x and back, but never both at the same time, but it seems currently none of those work and using a JIT in a 3rd party app is simply not possible. Even if exploits are still available, the Dart team wouldn't rely on iOS vulnerabilities that could be fixed at any time, or worse, cause apps to be flagged as malicious.

Note you'll find articles online talking about how iOS 14 allows 3rd-party apps to use a JIT compiler, but that seems to be limited to apps sideloaded via developer tools so it's not relevant for Dart / Flutter, which target devs who want to distribute their apps via the App Store.

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