As net package has one of the biggest init time in standard library, I have tried to improve performance by doing two things in net/addrselect.go:
1. Precompute slice with RFC rules. Currently the rules are computed and sorted in init() function. We could save the time and allocations by using prepopulated values in sorted manner. The rules haven't changed since 2015. To be extra safe we could move order validation as test case. It should slightly speed up startup of each binary with "net" package and go dns resolver. It also saves 38 allocations, ~50% of allocations in init phase of `net` module.
2. Replace internal net.IP usage with netip.Addr in `sortByRFC6724` function. It results in ~40% performance improvement on samples from tests.

The only risk is the difference between net.IP and netip.Addr behaviour.

Init benchmark:
Init-8               1.89µs ± 2%    0.12µs ± 3%  -93.79%  (p=0.000 n=5+5)

name               old alloc/op   new alloc/op   delta
Init-8               1.05kB ± 0%    0.38kB ± 0%     ~     (zero variance)

name               old allocs/op  new allocs/op  delta
Init-8                 39.0 ± 0%       1.0 ± 0%     ~     (zero variance)

Whole sortByRFC6724 function benchmark:
name               old time/op    new time/op    delta
SortByRFC6724/0-8     463ns ± 3%     303ns ± 4%  -34.72%  (p=0.000 n=5+5)
SortByRFC6724/1-8     481ns ± 8%     306ns ± 1%  -36.46%  (p=0.000 n=5+5)
SortByRFC6724/2-8     470ns ± 4%     307ns ± 4%  -34.77%  (p=0.000 n=5+5)
SortByRFC6724/3-8     567ns ± 3%     367ns ± 3%  -35.28%  (p=0.000 n=5+5)
SortByRFC6724/4-8     918ns ± 3%     560ns ± 2%  -38.93%  (p=0.000 n=5+5)

Updates #54032

Change-Id: Ic18df1ea73805cb184c6ceb73470ca7f0b922032
Reviewed-on: https://go-review.googlesource.com/c/go/+/419356
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>