data matrix code generator c# Virtual Functions and Double Thunking in C#

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Virtual Functions and Double Thunking
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The double-thunking problem described in the context of native function pointers can also occur when managed virtual functions are called from managed code. Figure 9-9 shows an example.
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Figure 9-9. Calling managed virtual functions from native code In this sample, both main and the virtual functions of SampleClass are compiled to managed code. Since native callers could also call the virtual functions, the vtable entries in the SampleClass vtable are function pointers to unmanaged-to-managed thunks. When managed code calls such a virtual function, it first picks the corresponding virtual function pointer from the vtable. To call the virtual function via the function pointer, a managed-to-unmanaged thunk is necessary because the vtable entry refers to the unmanaged-to-managed thunk, which is an unmanaged function.
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CHAPTER 9 MANAGED-UNMANAGED TRANSITIONS
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In this scenario, managed code calls the managed function SampleClass::F2 with two transitions, not directly. This is another variant of the double-thunking problem. Since in C++ code, virtual functions are used more often than function pointers, double-thunking occurs more often in this context. To solve this problem, virtual functions of native classes can be defined with the managed calling convention __clrcall. By using this calling convention, you close the door for native callers managed callers, on the other hand, do not face the double-thunking problem. However, you should be aware that you can only avoid double thunking if you use __clrcall when you introduce the virtual function. It is not possible to overwrite a virtual function that has a native calling convention with a __clrcall function. For example, if a native class library defines the virtual function void __thiscall f(), you cannot provide the override void __clrcall f().
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Performance of Thunks
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So far, I have only mentioned that calls via thunks are slower than method calls that do not cross a managed-unmanaged boundary. To decide whether the overhead of a transition is acceptable or not, you will likely need more precise information. The next sections explain the overhead of the different transitions. I will especially examine transitions based on interoperability vtables, P/Invoke metadata, and the CALLI instruction. Furthermore, I will discuss how to use a special optimization option for P/Invoke metadata and certain internal aspects of managed-unmanaged thunks that can affect the way you write code. To discuss the overhead of transitions, I have written an application that measures different interoperability scenarios. The code for this application is provided in Appendix B. You can also download it from the Source Code/Download section of the Apress web site (www.apress. com/) so that you can reproduce the performance tests on your machine or adapt them to your own special scenarios. To determine the performance of thunks for C functions, the application uses a native and a managed global function defined in the application itself (fManagedLocal and fNativeLocal), as well as a native and a managed function imported from an external DLL (fManagedFromDLL and fNativeFromDLL). The application measures how long it takes to call each of these methods directly, as well as via function pointers from native and managed code. For each measurement, 100 million calls are done. Table 9-2 shows the measured results on my machine (Pentium M 780, 2.26 GHz).
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Table 9-2. Performance of Thunks for Global Functions
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Direct Method Invocations (Not Via Function Pointers)
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Caller Callee Managed Function from Same Assembly (fManagedLocal) Managed Code Native Code
Indirect Calls (Via Function Pointers)
Managed Code Via __stdcall*: 2.30s Native Code
0.32s M M
2.12s U M
M U M Via __clrcall*: 0.30s M M
2.07s U M
Native Function From Same Assembly (fNativeLocal) Managed Function From Imported DLL (fManagedFromDLL) Native Function from Imported DLL (fNativeFromDLL)
0.63s M U
0.41s U U
0.63s
0.41s U U
3.54s M U M 1.97s M U
2.12s U M 0.41s U U
2.39s M U M 0.63s M U
2.07s U M 0.41s U U
In addition to C functions, the application measures calls to virtual and nonvirtual functions of C++ classes defined in the application itself, as well as in an external DLL. The results are shown in Table 9-3. Again, all functions are called from managed code as well as from native code. Table 9-3. Performance of Thunks for Member Functions of Native Classes
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