in FBRetainCycleDetector/Layout/Classes/FBClassStrongLayout.mm [141:155]
static NSUInteger FBGetMinimumIvarIndex(__unsafe_unretained Class aCls) {
NSUInteger minimumIndex = 1;
unsigned int count;
Ivar *ivars = class_copyIvarList(aCls, &count);
if (count > 0) {
Ivar ivar = ivars[0];
ptrdiff_t offset = ivar_getOffset(ivar);
minimumIndex = offset / (sizeof(void *));
}
free(ivars);
return minimumIndex;
}