JITenabled

JITenabled is a term used to describe a runtime environment or configuration in which Just-In-Time (JIT) compilation is active. In JITenabled mode, code is initially interpreted or compiled to an intermediate form, but frequently executed sections are compiled to native machine code at runtime to improve performance. This contrasts with purely interpreted execution or with ahead-of-time (AOT) compilation where all code is compiled before deployment.

Implementation typically involves profiling hot paths, generating optimized native code, and caching it for repeated executions. Modern JITs may employ tiered compilation, starting with quick, lightweight translations and progressively replacing them with more optimized versions. They may also perform deoptimization when assumptions made during optimization prove invalid.

Advantages include faster execution of hot code, improved throughput, and adaptive optimization that responds to actual workloads. Disadvantages can include increased startup time, higher memory usage, and JIT-related complexity that can complicate debugging and security considerations, as executable code is generated at runtime.

JITenabled status is often controlled by a runtime flag or environment setting. It may be enabled by default on desktop or server platforms but disabled by default on constrained devices or in safety-critical deployments. Some environments support both JIT and AOT modes, allowing developers to balance performance and predictability.

Related concepts include just-in-time compilation in virtual machines such as the JVM, .NET CLR, and JavaScript engines, as well as ahead-of-time compilation and interpreter-based execution.