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Compiling MSIL to Native Code
Before you can run Microsoft intermediate language (MSIL), it must be converted by a .NET Framework just-in-time (JIT) compiler to native code, which is CPU-specific code that runs on the same computer architecture as the JIT compiler. Because the common language runtime supplies a JIT compiler for each supported CPU architecture, developers can write a set of MSIL that can be JIT-compiled and run on computers with different architectures. However, your managed code will run only on a specific operating system if it calls platform-specific native APIs, or a platform-specific class library.
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 so that it is accessible for subsequent calls. The loader creates and attaches a stub to each of a type's methods when the type is loaded. On the initial call to the method, the stub passes control to the JIT compiler, which converts the MSIL for that method into native code and modifies the stub to direct execution to the location of the native code. Subsequent calls of the JIT-compiled method proceed directly to the native code that was previously generated, reducing the time it takes to JIT-compile and run the code.
The runtime supplies another mode of compilation called install-time code generation. The install-time code generation mode converts MSIL to native code just as the regular JIT compiler does, but it converts larger units of code at a time, storing the resulting native code for use when the assembly is subsequently loaded and run. When using install-time code generation, the entire assembly that is being installed is converted into native code, taking into account what is known about other assemblies that are already installed. The resulting file loads and starts more quickly than it would have if it were being converted to native code by the standard JIT option.
As part of compiling MSIL to native code, code must pass a verification process unless an administrator has established a security policy that allows code to bypass verification. Verification examines MSIL and metadata to find out whether the code is type safe, which means that it only accesses the memory locations it is authorized to access. Type safety helps isolate objects from each other and therefore helps protect them from inadvertent or malicious corruption. It also provides assurance that security restrictions on code can be reliably enforced.
The runtime relies on the fact that the following statements are true for code that is verifiably type safe:
A reference to a type is strictly compatible with the type being referenced.
Only appropriately defined operations are invoked on an object.
Identities are what they claim to be.
During the verification process, MSIL code is examined in an attempt to confirm that the code can access memory locations and call methods only through properly defined types. For example, code cannot allow an object's fields to be accessed in a manner that allows memory locations to be overrun. Additionally, verification inspects code to determine whether the MSIL has been correctly generated, because incorrect MSIL can lead to a violation of the type safety rules. The verification process passes a well-defined set of type-safe code, and it passes only code that is type safe. However, some type-safe code might not pass verification because of limitations of the verification process, and some languages, by design, do not produce verifiably type-safe code. If type-safe code is required by security policy and the code does not pass verification, an exception is thrown when the code is run.