None of your buffers are on stack in both examples. As those are
dynamic arrays you only get pointer + length as value and data
itself resides on heap in some unknown location. It can be in
cache too, of course, if it has been used actively, but it can't
be verified based on this simple snippet.
To get best cache friendliness one may use dynamic-size structs
akin to C but, of course, this does not go well with necessity to
grow the buffer dynamically for given instance.
Also note that your first snippet uses ~= operator which will
cause new heap allocations invalidating any old cache.
