One more reference:

http://msdn.microsoft.com/en-us/library/9dbwhz68(v=VS.80).aspx

gives essentially the same rules for MSVC. "The alignment of the beginning of a structure or a union is the maximum alignment of any individual member. Each member within the structure or union must be placed at its proper alignment as defined in the previous table, which may require implicit internal padding, depending on the previous member."