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WebAssembly and Jiterpreter: The Dynamic Duo for Faster Runtime

In technology, performance is a key player that can make or break an application. As developers, we are constantly on the lookout for tools and techniques that can help us improve the speed and efficiency of our applications. Enter the Jiterpreter, a groundbreaking tool that promises to revolutionize runtime performance. This article explores how the Jiterpreter works, its applications, and how it holds the potential to significantly accelerate runtime, thereby leading to faster, more efficient applications.

WebAssembly and Jiterpreter: The Dynamic Duo for Faster Runtime

Table of Contents:
Understanding Jiterpreter
Benefits of Jiterpreter
Limitations of Jiterpreter

What is Jiterpreter?

The Jiterpreter is a new runtime feature in .NET 8 that enables partial Just-in-Time (JIT) compilation support when running on WebAssembly to achieve improved runtime performance. It is a hybrid approach that combines the benefits of both AOT (Ahead-of-Time) and JIT compilation.

AOT compilation compiles the entire program into machine code before it is run. This can make the program run faster, but it can also make it larger and slower to start up. JIT compilation, on the other hand, compiles the program code piece by piece at runtime. This can make the program start up faster, but it can also make it run slower.

Understanding Jiterpreter

The Jiterpreter is a hybrid execution strategy that combines aspects of both interpretation and Just-In-Time (JIT) compilation. It operates on the Common Intermediate Language (CIL) bytecode, which is a low-level representation of your program that is compatible with any .NET language.

Here’s a more detailed explanation of how it works:

  1. Interpreting the CIL bytecode: Like a standard interpreter, the Jiterpreter reads and executes the CIL bytecode instructions one at a time. This means it translates each instruction into a sequence of one or more machine code instructions that the CPU can understand, and then immediately executes those instructions.

  2. Partial JIT support: The Jiterpreter introduces the concept of partial JIT support in the .NET IL interpreter. This means that while most of the code is interpreted as described above, certain sections of the code that are frequently executed (often referred to as “hot” sections) are compiled using the JIT compiler.

  3. JIT Compilation: The JIT compiler translates the entire section of bytecode into machine code all at once, before execution. This machine code is then cached so that if the same section of code is executed again, the precompiled machine code can be used instead. This can significantly improve the performance of these “hot” sections of code.

Here’s the provided example code explained:

public void Execute(Program program)
    foreach (var instruction in program.Instructions)
        if (IsHot(instruction))
            // Compile and execute the instruction
            var compiled = JitCompiler.Compile(instruction);
            // Interpret the instruction

In this code:

  • Execute(Program program): This method is responsible for executing a given program.

  • foreach (var instruction in program.Instructions): This loop iterates over each instruction in the program.

  • if (IsHot(instruction)): This checks if the current instruction is frequently executed (“hot”). If it is, the instruction is compiled and executed.

  • var compiled = JitCompiler.Compile(instruction); compiled.Execute();: This compiles the “hot” instruction into machine code using the JIT compiler and then executes it.

  • else { Interpret(instruction); }: If the instruction is not “hot”, it is simply interpreted and executed.

This approach allows the Jiterpreter to balance the flexibility of interpretation with the performance benefits of JIT compilation.

The Jiterpreter takes advantage of both AOT and JIT compilation by compiling frequently used code paths ahead of time, and then JIT compiling less frequently used code paths at runtime. This can help to improve the performance of programs that run on WebAssembly, without sacrificing startup time or program size.

Benefits of Jiterpreter:

Here are some of the benefits of using the Jiterpreter:

  • Improved runtime performance: The Jiterpreter can improve the runtime performance of programs that run on WebAssembly.

  • Faster startup times: The Jiterpreter can help to reduce the startup time of programs that run on WebAssembly.

  • Smaller program size: The Jiterpreter can help to reduce the size of programs that run on WebAssembly.

Limitations of Jiterpreter:

Here are some of the limitations of using the Jiterpreter:

  • Slower Execution Speed: Interpreted code can be slower to execute than compiled code because it is translated and executed line by line.

  • Dependencies Required: Anyone who wants to execute the shared source code needs to have an interpreter installed in their system.

  • Error Detection: While interpreters can help users find errors easily as they execute code line by line, this can also be a disadvantage as it might halt the execution midway upon encountering an error.

  • Security Risks: Interpreted languages can be more vulnerable to security risks.

Difference between JIT and Jiterpreter

The Just-In-Time (JIT) compiler and the interpreter, including the Jiterpreter, are both components used in the execution of programs, but they operate differently:

  1. Just-In-Time (JIT) Compiler: A JIT compiler is a feature of the runtime execution environment that improves the performance of Java programs by compiling bytecodes into native machine code at runtime. It translates bytecode into machine code while the program is running, which results in improved bytecode execution speed. This process requires CPU time and memory.

  2. Interpreter (including Jiterpreter): An interpreter is a software that converts the source code into native machine code line by line. It checks one line at a time, which makes the scanning time lower, but the overall execution time of the program is higher. Interpreted programs have ease of testing.

In summary, the main difference between a JIT compiler and an interpreter like Jiterpreter lies in when and how they translate code. A JIT compiler translates bytecode into machine code during runtime, improving execution speed. On the other hand, an interpreter translates source code into machine code line by line, which can make the program easier to test but may result in slower execution

Jiterpreter and Blazor WebAssembly Apps

One of the key areas where the Jiterpreter can have a significant impact is in the execution of Blazor WebAssembly apps. These apps run .NET code directly in the browser using a WebAssembly-based .NET runtime. The Jiterpreter can play a crucial role in this process by providing an efficient mechanism for executing .NET code in the browser environment.

The C# code you provided is an example of how one might use the Jiterpreter to run a Blazor app:

public void RunBlazorApp(BlazorApp app)
    var jiterpreter = new Jiterpreter();

In this code:

  • A new instance of the Jiterpreter is created.

  • The Execute method of the Jiterpreter instance is then called with app.Program as the argument. This begins the execution of the .NET code associated with the BlazorApp in the browser environment.


The Jiterpreter is a valuable addition to Blazor WebAssembly, offering a dynamic approach to performance optimization without the drawbacks of full AOT compilation. It provides a balance between improved runtime performance, faster startup times, and smaller download sizes. By understanding its benefits and limitations, developers can leverage the Jiterpreter effectively to enhance the performance and responsiveness of their Blazor WebAssembly applications.

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