Optimizing Java by Chris Newland & James Gough & Benjamin J Evans

Author:Chris Newland & James Gough & Benjamin J Evans [Chris Newland]
Language: eng
Format: epub
Publisher: O'Reilly Media, Inc.
Published: 2018-04-29T21:00:00+00:00

Comparing AOT and JIT Compilation

AOT compilation has the advantage of being relatively simple to understand. Machine code is produced directly from source, and the machine code that corresponds to a compilation unit is directly available as assembly. This, in turn, also offers the possibility of code that will have straightforward performance characteristics.

Offset against this is the fact that AOT means giving up access to valuable runtime information that could help inform optimization decisions. Techniques such as link-time optimization (LTO) and a form of PGO are now starting to appear in gcc and other compilers, but they are in the early stages of development compared to their counterparts in HotSpot.

Targeting processor-specific features during AOT compilation will produce an executable that is compatible only with that processor. This can be a useful technique for low-latency or extreme performance use cases; building on the exact same hardware as the application will run on ensures that the compiler can take advantage of all available processor optimizations.

However, this technique does not scale: if you want maximum performance across a range of target architectures, you will need to produce a separate executable for each one.

By contrast, HotSpot can add optimizations for new processor features as they are released and applications will not have to recompile their classes and JARs to take advantage of them. It is not unusual to find that program performance improves measurably between new releases of the HotSpot VM as the JIT compiler is improved.

At this point, let’s address the persistent myth that “Java programs can’t be AOT-compiled.” This is simply not true: commercial VMs that offer AOT compilation of Java programs have been available for several years now and in some environments are a major route to deploying Java applications.

Finally, starting with Java 9 the HotSpot VM has begun to offer AOT compilation as an option, initially for core JDK classes. This is an initial (and quite limited) step toward producing AOT-compiled binaries from Java source, but it represents a departure from the traditional JIT environments that Java did so much to popularize.

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