Commit ca481d3c9a introduced mul_var_short(), which is used by
mul_var() whenever the shorter input has 1-4 NBASE digits and the
exact product is requested. As speculated on in that commit, it can be
extended to work for more digits in the shorter input. This commit
extends it up to 6 NBASE digits (up to 24 decimal digits), for which
it also gives a significant speedup. This covers more cases likely to
occur in real-world queries, for which using base-NBASE^2 arithmetic
provides little benefit.
To avoid code bloat and duplication, refactor it a bit using macros
and exploiting the fact that some portions of the code are shared
between the different cases.
Dean Rasheed, reviewed by Joel Jacobson.
Discussion: https://postgr.es/m/9d8a4a42-c354-41f3-bbf3-199e1957db97%40app.fastmail.com
c4e44224cf