seemingly random changes to the this pointer [asm]

.686P
.MMX
.XMM
.MODEL FLAT, C

.CODE

Matrix3Add PROC mat1:DWORD, mat2:DWORD, mat3:DWORD
	
	mov eax, dword ptr[mat1]
	mov ebx, dword ptr[mat3]
	mov ecx, dword ptr[mat2]
	movaps xmm0, dword ptr[ecx]
	movaps xmm1, dword ptr[ecx + 10h]
	addps xmm0, dword ptr[eax]
	addps xmm1, dword ptr[eax + 10h]
	movaps [ebx], xmm0
	movaps [ebx + 10h], xmm1
	fld dword ptr [ecx+20h]
	fadd dword ptr [eax+20h]
	fstp dword ptr [ebx+20h]
	mov ecx, dword ptr[mat2]
	ret

Matrix3Add ENDP

END

extern "C" void Matrix3Add( Matrix3* mat1, Matrix3* mat2, Matrix3* mat3 );

inline Matrix3 Matrix3::operator+ ( const Matrix3& mat )
{
	Matrix3 matReturn;

	Matrix3Add((Matrix3*)&mat, this, &matReturn);

	return matReturn; // mat, this and matReturn are still valid
};

inline Matrix3(const Matrix3& mat)
{
	__asm
	{
		mov ecx, 0x9
		mov esi, dword ptr[mat]
		mov edi, this //'this' now points to some random value

		rep movsd
	};
};