This change adds local 'end' and 'pos' variables for the main loop inmstead of using the ConstructTransaction variables directly.
We observed that not all vector initialization and resize operations got properly vectorized, i.e., (partially) unrolled into XMM stores for floats.
For example, vector<int32_t> v(n, 1) gets vectorized, but vector<float> v(n, 1). It looks like the compiler assumes the state is leaked / aliased in the latter case (unclear how/why for float, but not for int32), and because of this fails to see vectorization optimization?
By using a local __new_end_ (fixed), and local __pos (copied into tx.pos_ per iteration), we offer the compiler a clean loop for unrolling.
A demonstration can be seen in the isolated logic in https://gcc.godbolt.org/z/KoCNWv