By default, when compiling with /clr (not /clr:pure), the definition of a managed function causes the compiler to generate a managed entry point and a native entry point. This allows the managed function to be called from native and managed call sites. However, when a native entry point exists, it can be the entry point for all calls to the function. If a calling function is managed, the native entry point will then call the managed entry point. In effect, two calls are required to invoke the function (hence, double thunking). For example, virtual functions are always called through a native entry point.

One resolution is to tell the compiler not to generate a native entry point for a managed function, that the function will only be called from a managed context, by using the __clrcall calling convention.

Similarly, if you export (dllexport, dllimport) a managed function, a native entry point is generated and any function that imports and calls that function will call through the native entry point. To avoid double thunking in this situation, do not use native export/import semantics; simply reference the metadata via #using (see The #using Directive).

In Visual C++ 2005 the compiler was updated to reduce unnecessary double thunking. For example, any function with a managed type in the signature (including return type) will implicitly be marked as __clrcall. For more information on double thunk elimination, see http://msdn.microsoft.com/msdnmag/issues/05/01/COptimizations/default.aspx.

Description

Code

// double_thunking.cpp
// compile with: /clr
#include <stdio.h>
struct T {
   T() {
      puts(__FUNCSIG__);
   }

   T(const T&) {
      puts(__FUNCSIG__);
   }

   ~T() {
      puts(__FUNCSIG__);
   }

   T& operator=(const T&) {
      puts(__FUNCSIG__);
      return *this;
   }
};

struct S {
   virtual void /* __clrcall */ f(T t) {};
} s;

int main() {
   S* pS = &s;
   T t;

   printf("calling struct S\n");
   pS->f(t);
   printf("after calling struct S\n");
}

Sample Output

__thiscall T::T(void)
calling struct S
__thiscall T::T(const struct T &)
__thiscall T::T(const struct T &)
__thiscall T::~T(void)
__thiscall T::~T(void)
after calling struct S
__thiscall T::~T(void)

Description

Code

// double_thunking_2.cpp
// compile with: /clr
#include <time.h>
#include <stdio.h> 

#pragma unmanaged
struct T {
   T() {}
   T(const T&) {}
   ~T() {}
   T& operator=(const T&) { return *this; }
};

struct S {
   virtual void /* __clrcall */ f(T t) {};
} s;

int main() {
   S* pS = &s;
   T t;
   clock_t start, finish;
   double  duration;
   start = clock();

   for ( int i = 0 ; i < 1000000 ; i++ )
      pS->f(t);

   finish = clock();
   duration = (double)(finish - start) / (CLOCKS_PER_SEC);
   printf( "%2.1f seconds\n", duration );
   printf("after calling struct S\n");
}

Sample Output

4.2 seconds
after calling struct S

Concepts