MSVC 2013: crash at addpd xmm6,xmmword ptr [rax+rbx*8] - c++

Question

Since some days I am using MSVC 2013 and my application crashes when executing the following code (sparse matrix multiplied by vector, pseudo code: A = this * pVector):
complex<double> x = (A.getValue(lRow) + (mValues[lIdx] * pVectorB->getValue(lCol)));
Before I used MSVC 2005 and the application runs well.
The exception (First-chance exception at 0x000000014075D1D2 in psc64.exe: 0xC0000005: Access violation reading location 0xFFFFFFFFFFFFFFFF.) was thrown.
I track the assembly to:
addpd xmm6, xmmword ptr [rax+rbx*8]
It crash only with optimization /O2 (maximize speed) but not with no optimization /Od.
I can also avoid the crash when adding code (cout<<"bla bla") into the method pVectorB->getValue(lCol).
I believe it could be some problem with not initialized variables. But I could not find any. Therefore I look into the disassembly.
I check XMM6 and ptr [rax+rbx*8]. They are the same without crash (with cout<<"bla bla") and with crash.
Is there any thing more I should look for other then XMM6 and the value of ptr [rax+rbx*8]?
I am looking for the problem since quite some time but could not find any hint to track down the problem to the line of code I have to correct.
Any help is highly appreciated. Thank you.
The code for getValue:
template <class T> class Vector
{ const T& getValue(const int pIdx) const
{
if(false == checkBounds(pIdx)){
throw MathException(__FILE__, __LINE__, "T& Vector<class T>::getValue(const pIdx): checkBounds fails pIdx = %i", pIdx);
}
return mVal[pIdx];
}
bool checkBounds(const int pIdx)const
{
bool ret = true;
if(pIdx >= mMaxSize){
DBG_SEVERE2("pIdx >= mMaxSize, pIdx = %i, mMaxSize = %i", pIdx, mMaxSize);
ret = false;
}
if(pIdx < 0){
DBG_SEVERE1("pIdx < 0, pIdx = %i", pIdx);
ret = false;
}
return ret;
}
}
The allocation of mVal:
void* lTmp= calloc((4 * sizeof(complex<double>))+4, 1);
((char*)lTmp)[0] = 0xC;
((char*)lTmp)[1] = 0xC;
((char*)lTmp)[(4 * sizeof(complex<double>)) + 2] = 0xC;
((char*)lTmp)[(4 * sizeof(complex<double>)) + 3] = 0xC;
mVal= (void*)(((char*)lTmp) + 2)
SOLUTION:
As suggested it works without the 2 byte in front and behind the desired array (mVal). But it also works with multiple of 16byte before and after the array.

Answer 1

When using memory operands to SSE instructions like addpd xmm6, xmmword ptr [rax+rbx*8] the memory operand must be aligned. There are unaligned load instructions: You could movdqu from [rax+rbx*8] and then operate on the register. But if you use the memory form, the alignment is important. The optimization flags probably changed the alignment of your array. Or it may have folded the load into a memory operand (which are faster in some cases) and caused the problem that way.