JEP 544 Introduces Ahead-of-Time Code Compilation for HotSpot JVM

Improved Startup and Warmup

JEP 544 introduces Ahead-of-Time (AOT) code compilation to the HotSpot Java Virtual Machine. The primary goal is to improve the startup and warmup phases of Java applications by making optimized native code immediately available when the JVM starts. This approach aims to achieve peak performance more quickly for applications.

Hybrid Compilation Approach

The AOT compilation process involves compiling application code to native code during a training run. This native code is then stored in an AOT cache for use in subsequent production runs. If the workload changes during production, the system can dynamically regenerate native code, combining the benefits of both AOT and Just-In-Time (JIT) compilation to sustain peak performance.

Integration and Compatibility

The proposal emphasizes that no changes are required for application code, libraries, or frameworks. It also does not necessitate changes to HotSpot configuration beyond requesting the use of the AOT cache. The feature will continue to support existing garbage collectors, including Serial, Parallel, G1, and ZGC, and will be invisible to applications when transitioning between AOT-compiled and JIT-compiled code. Initial support is planned for AArch64 and x64 processor architectures.

Motivation for AOT

Java applications typically go through startup, warmup, and peak performance phases. During startup, the HotSpot JVM initially runs code via a slow bytecode interpreter. It then profiles behavior and compiles frequently invoked methods to native code using the C1 compiler, which offers modest optimization. AOT compilation aims to bypass some of this initial interpretation and basic compilation, providing more optimized code earlier in the application lifecycle.