Fooling Around

Started as Is it possible to use local component in DLL? on microsoft.public.vc.atl newsgroup. The question is to embed a custom DirectX Media Object (DMO) into executable so that it is only available to proprietary application and not to entire system and could be reused in other applications. If in particular this DMO should be available to intelligent connect, e.g. used by embedded Windows Media Player control, the solution requires that private DMO appears fully registered as full featured regular DMO.

A while ago I proposed a solution on getting COM Class/DMO embedded into executable and temporarily exposed as availble without registry registration through CoRegisterClassObject/RegisterClassObject, however in addition to COM class registration, a DMO needs to also be specifically listed as a DMO, through DMORegister API.

In order to expose private DMO to DirectShow subsystem the following steps has to be performed:

• register COM class object as a COM class (CoRegisterClassObject) to enable its instantiation by its CLSID
• use DMORegister API to list DMO in global list (warning, seems to be requiring write permission on HKEY_CLASSES_ROOT key)
• have the filter graph of interest created
• use DMOUnregister API as early as possible to unlist private DMO from global list

ATLENSURE_SUCCEEDED(__objMap_CPrivateMediaObject.RegisterClassObject(CLSCTX_INPROC_SERVER, REGCLS_MULTI_SEPARATE));
DMO_PARTIAL_MEDIATYPE pInputMediaTypes[] = { { MEDIATYPE_Video, CPrivateMediaObject::GetXxxxSubtype() } };
ATLENSURE_SUCCEEDED(DMORegister(L"Private DMO", CLSID_PrivateMediaObject, DMOCATEGORY_VIDEO_DECODER, 0, _countof(pInputMediaTypes), pInputMediaTypes, 0, NULL));
...
ATLVERIFY(SUCCEEDED(DMOUnregister(CLSID_PrivateMediaObject, DMOCATEGORY_VIDEO_DECODER)));

The sample code/project takes an .AVI file which is a XVID video file with a FOURCC compression code changed from XVID to XXXX. The file is unplayable but there is a private DMO which accepts XXXX video and copies data as is into XVID format on its output.

A Visual C++ .NET 2008 source code is available from SVN.

I just read a post on frame pointer omission (FPO) optimization, also known as /Oy compiler flag. And a comment from Vladimir Scherbina on problems getting the feature actually work. I wondered if there has been any improvement with a Visual Studio .NET 2008 SP1. latest compiler from Microsoft and briefly - the result is yes, the feature seems to be working.

This is the reference code for the feature:

const volatile INT g_nValueA = 0;
const volatile INT g_nValueB = 0;

#pragma optimize("y", off)

__declspec(dllexport) __declspec(noinline) BOOL STDMETHODCALLTYPE A(const volatile INT& nValue = g_nValueA)
{
	return nValue != 0;
}

#pragma optimize("y", on)

__declspec(dllexport) __declspec(noinline) BOOL STDMETHODCALLTYPE B(const volatile INT& nValue = g_nValueB)
{
	return nValue != 0;
}

int _tmain(int argc, _TCHAR* argv[])
{
	A();
	B();
	return 0;
}

The code is to be compiled in Release and function A is to be not optimized while B is subject for optimization and omitting the frame pointer.

Disassembly shows for A:

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