What is the JIT-compiler?

JIT (just-in-time) compilation converts MSIL to native code on demand at application run time, when the contents of an assembly are loaded and executed. Because the common language runtime supplies a JIT compiler for each supported CPU architecture, developers can build a set of MSIL assemblies that can be JIT-compiled and run on different computers with different machine architectures.

The JIT-compiler is part of the Common Language Runtime (CLR). It converts an assembly's MSIL to native code when individual methods defined in that assembly are called. JIT compilation takes into account the fact that some code might never get called during execution. Rather than using time and memory to convert all the MSIL in a portable executable (PE) file to native code, it converts the MSIL as needed during execution and stores the resulting native code in memory so that it is accessible for subsequent calls in the context of that process. The loader creates and attaches a stub to each method in a type when the type is loaded and initialized. When a method is called for the first time, the stub passes control to the JIT compiler, which converts the MSIL for that method into native code and modifies the stub to point directly to the generated native code. Subsequent calls to the JIT-compiled method therefore proceed directly to the native code.

Understanding .NET Just-In-Time Compilation

Source: telerik.com

Short answer

  • The JIT-compiler is part of the Common Language Runtime (CLR).
  • It converts an assembly's MSIL to native code when individual methods defined in that assembly are called.

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