What are the advantages of just-in-time compilation versus ahead-of-time compilation?

Question

I\'ve been thinking about it lately, and it seems to me that most advantages given to JIT compilation should more or less be attributed to the intermediate

Answer 1

1. Better reflection support. This could be done in principle in an ahead-of-time compiled program, but it almost never seems to happen in practice.

2. Optimizations that can often only be figured out by observing the program dynamically. For example, inlining virtual functions, escape analysis to turn stack allocations into heap allocations, and lock coarsening.

Answer 2

1. Greater portability: The deliverable (byte-code) stays portable

2. At the same time, more platform-specific: Because the JIT-compilation takes place on the same system that the code runs, it can be very, very fine-tuned for that particular system. If you do ahead-of-time compilation (and still want to ship the same package to everyone), you have to compromise.

3. Improvements in compiler technology can have an impact on existing programs. A better C compiler does not help you at all with programs already deployed. A better JIT-compiler will improve the performance of existing programs. The Java code you wrote ten years ago will run faster today.

4. Adapting to run-time metrics. A JIT-compiler can not only look at the code and the target system, but also at how the code is used. It can instrument the running code, and make decisions about how to optimize according to, for example, what values the method parameters usually happen to have.

You are right that JIT adds to start-up cost, and so there is a time-constraint for it, whereas ahead-of-time compilation can take all the time that it wants. This makes it more appropriate for server-type applications, where start-up time is not so important and a "warm-up phase" before the code gets really fast is acceptable.

I suppose it would be possible to store the result of a JIT compilation somewhere, so that it could be re-used the next time. That would give you "ahead-of-time" compilation for the second program run. Maybe the clever folks at Sun and Microsoft are of the opinion that a fresh JIT is already good enough and the extra complexity is not worth the trouble.

Answer 3

I have been trying to understand this as well because I saw that Google is moving towards replacing their Dalvik Virtual Machine (essentially another Java Virtual Machine like HotSpot) with Android Run Time (ART), which is a AOT compiler, but Java usually uses HotSpot, which is a JIT compiler. Apparently, ARM is ~ 2x faster than Dalvik... so I thought to myself "why doesn't Java use AOT as well?". Anyways, from what I can gather, the main difference is that JIT uses adaptive optimization during run time, which (for example) allows ONLY those parts of the bytecode that are being executed frequently to be compiled into native code; whereas AOT compiles the entire source code into native code, and code of a lesser amount runs faster than code of a greater amount.
I have to imagine that most Android apps are composed of a small amount of code, so on average it makes more sense to compile the entire source code to native code AOT and avoid the overhead associated from interpretation / optimization.

Answer 4

Maybe it has to do with the modern approach to programming. You know, many years ago you would write your program on a sheet of paper, some other people would transform it into a stack of punched cards and feed into THE computer, and tomorrow morning you would get a crash dump on a roll of paper weighing half a pound. All that forced you to think a lot before writing the first line of code.

Those days are long gone. When using a scripting language such as PHP or JavaScript, you can test any change immediately. That's not the case with Java, though appservers give you hot deployment. So it is just very handy that Java programs can be compiled fast, as bytecode compilers are pretty straightforward.

But, there is no such thing as JIT-only languages. Ahead-of-time compilers have been available for Java for quite some time, and more recently Mono introduced it to CLR. In fact, MonoTouch is possible at all because of AOT compilation, as non-native apps are prohibited in Apple's app store.

Answer 5

The difference between platform-browser-dynamic and platform-browser is the way your angular app will be compiled. Using the dynamic platform makes angular sending the Just-in-Time compiler to the front-end as well as your application. Which means your application is being compiled on client-side. On the other hand, using platform-browser leads to an Ahead-of-Time pre-compiled version of your application being sent to the browser. Which usually means a significantly smaller package being sent to the browser. The angular2-documentation for bootstrapping at https://angular.io/docs/ts/latest/guide/ngmodule.html#!#bootstrap explains it in more detail.

Answer 6

It seems that this idea has been implemented in Dart language:

https://hackernoon.com/why-flutter-uses-dart-dd635a054ebf

JIT compilation is used during development, using a compiler that is especially fast. Then, when an app is ready for release, it is compiled AOT. Consequently, with the help of advanced tooling and compilers, Dart can deliver the best of both worlds: extremely fast development cycles, and fast execution and startup times.