On Mon, Feb 8, 2021 at 6:17 AM Heikki Linnakangas <hlinn...@iki.fi> wrote: > > I also tested the fallback implementation from the simdjson library > (included in the patch, if you uncomment it in simdjson-glue.c): > > mixed | ascii > -------+------- > 447 | 46 > (1 row) > > I think we should at least try to adopt that. At a high level, it looks > pretty similar your patch: you load the data 8 bytes at a time, check if > there are all ASCII. If there are any non-ASCII chars, you check the > bytes one by one, otherwise you load the next 8 bytes. Your patch should > be able to achieve the same performance, if done right. I don't think > the simdjson code forbids \0 bytes, so that will add a few cycles, but > still.
Attached is a patch that does roughly what simdjson fallback did, except I use straight tests on the bytes and only calculate code points in assertion builds. In the course of doing this, I found that my earlier concerns about putting the ascii check in a static inline function were due to my suboptimal loop implementation. I had assumed that if the chunked ascii check failed, it had to check all those bytes one at a time. As it turns out, that's a waste of the branch predictor. In the v2 patch, we do the chunked ascii check every time we loop. With that, I can also confirm the claim in the Lemire paper that it's better to do the check on 16-byte chunks: (MacOS, Clang 10) master: chinese | mixed | ascii ---------+-------+------- 1081 | 761 | 366 v2 patch, with 16-byte stride: chinese | mixed | ascii ---------+-------+------- 806 | 474 | 83 patch but with 8-byte stride: chinese | mixed | ascii ---------+-------+------- 792 | 490 | 105 I also included the fast path in all other multibyte encodings, and that is also pretty good performance-wise. It regresses from master on pure multibyte input, but that case is still faster than PG13, which I simulated by reverting 6c5576075b0f9 and b80e10638e3: ~PG13: chinese | mixed | ascii ---------+-------+------- 1565 | 848 | 365 ascii fast-path plus pg_*_verifychar(): chinese | mixed | ascii ---------+-------+------- 1279 | 656 | 94 v2 has a rough start to having multiple implementations in src/backend/port. Next steps are: 1. Add more tests for utf-8 coverage (in addition to the ones to be added by the noError argument patch) 2. Add SSE4 validator -- it turns out the demo I referred to earlier doesn't match the algorithm in the paper. I plan to only copy the lookup tables from simdjson verbatim, but the code will basically be written from scratch, using simdjson as a hint. 3. Adjust configure.ac -- John Naylor EDB: http://www.enterprisedb.com
v2-add-portability-stub-and-new-fallback.patch
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