[yuvi]

> You can set the pointer to any address, which includes future stack.

This is wrong. Attempting to use random pointers to outside of what your compiler and malloc allocated is undefined.

And indeed, aliasing issues like in your example will prevent gcc from optimizing out excess strlen; it can only do so if it can prove that both global memory and the buffer passed to strlen have not been modified. Local variables and return values cannot alias anything, and part of the guarantee of pure functions like strlen is that they have no side effects.