Is this a compiler optimisation bug, or an undefined behaviour?

This is a code optimizer bug. It inlines both getData() and SetBit(). The combination appears to be fatal, it loses track of the value of 1 << ((pos) & 7) and always produces zero.

This bug does not occur on VS2012. A workaround is to force one of the functions to not get inlined. Given the code, you probably want to do that for getData():

__declspec(noinline)
size_t getData(size_t p_value)
{ 
    // etc..
}