Optimizing COPY with SIMD

Inspired by David Rowley's work [1] on optimizing JSON escape processing
with SIMD, I noticed that the COPY code could potentially benefit from SIMD
instructions in a few places, eg:

(1) CopyAttributeOutCSV() has 2 byte-by-byte loops
(2) CopyAttributeOutText() has 1
(3) CopyReadLineText() has 1
(4) CopyReadAttributesCSV() has 1
(5) CopyReadAttributesText() has 1

Attached is a quick POC patch that uses SIMD instructions for case (1)
above. For sufficiently large attribute values, this is a significant
performance win. For small fields, performance looks to be about the same.
Results on an M1 Macbook Pro.

======
neilconway=# select count(*), avg(length(a))::int, avg(length(b))::int,
avg(length(c))::int from short_strings;
count | avg | avg | avg
--------+-----+-----+-----
524288 | 8 | 8 | 8
(1 row)

neilconway=# select count(*), avg(length(a))::int, avg(length(b))::int,
avg(length(c))::int from long_strings;
count | avg | avg | avg
-------+-----+-----+-----
65536 | 657 | 657 | 657
(1 row)

master @ 8fea1bd541:

$ for i in ~/*.sql; do hyperfine --warmup 5 "./psql -f $i"; done
Benchmark 1: ./psql -f /Users/neilconway/copy-out-bench-long-quotes.sql
Time (mean ± σ): 2.027 s ± 0.075 s [User: 0.001 s, System: 0.000
s]
Range (min … max): 1.928 s … 2.207 s 10 runs

Benchmark 1: ./psql -f /Users/neilconway/copy-out-bench-long.sql
Time (mean ± σ): 1.420 s ± 0.027 s [User: 0.001 s, System: 0.000
s]
Range (min … max): 1.379 s … 1.473 s 10 runs

Benchmark 1: ./psql -f /Users/neilconway/copy-out-bench-short.sql
Time (mean ± σ): 546.0 ms ± 9.6 ms [User: 1.4 ms, System: 0.3 ms]
Range (min … max): 539.0 ms … 572.1 ms 10 runs

master + SIMD patch:

$ for i in ~/*.sql; do hyperfine --warmup 5 "./psql -f $i"; done
Benchmark 1: ./psql -f /Users/neilconway/copy-out-bench-long-quotes.sql
Time (mean ± σ): 797.8 ms ± 19.4 ms [User: 0.9 ms, System: 0.0 ms]
Range (min … max): 770.0 ms … 828.5 ms 10 runs

Benchmark 1: ./psql -f /Users/neilconway/copy-out-bench-long.sql
Time (mean ± σ): 732.3 ms ± 20.8 ms [User: 1.2 ms, System: 0.0 ms]
Range (min … max): 701.1 ms … 763.5 ms 10 runs

Benchmark 1: ./psql -f /Users/neilconway/copy-out-bench-short.sql
Time (mean ± σ): 545.7 ms ± 13.5 ms [User: 1.3 ms, System: 0.1 ms]
Range (min … max): 533.6 ms … 580.2 ms 10 runs
======

Implementation-wise, it seems complex to use SIMD when
encoding_embeds_ascii is true (which should be uncommon). In principle, we
could probably still use SIMD here, but it would require juggling between
the SIMD chunk size and sizes returned by pg_encoding_mblen(). For now, the
POC patch falls back to the old code path when encoding_embeds_ascii is
true.

Any feedback would be very welcome.

Cheers,
Neil

[1]
https://www.postgresql.org/message-id/CAApHDvpLXwMZvbCKcdGfU9XQjGCDm7tFpRdTXuB9PVgpNUYfEQ@mail.gmail.com