Re: Speed up JSON escape processing with SIMD plus other optimisations

Thanks for having a look.

On Wed, 12 Jun 2024 at 00:08, Melih Mutlu <m(dot)melihmutlu(at)gmail(dot)com> wrote:
> In 0001 patch, I see that there are some escape_json() calls with NUL-terminated strings and gets the length by calling strlen(), like below:
>
>> - escape_json(&buf, "timestamp");
>> + escape_json(&buf, "timestamp", strlen("timestamp"));
>
> Wouldn't using escape_json_cstring() be better instead? IIUC there isn't much difference between escape_json() and escape_json_cstring(), right? We would avoid strlen() with escape_json_cstring().

It maybe would be better, but not for this reason. Most compilers will
be able to perform constant folding to transform the
strlen("timestamp") into 9. You can see that's being done by both gcc
and clang in [1].

It might be better to use escape_json_cstring() regardless of that as
the SIMD only kicks in when there are >= 16 chars, so there might be a
few more instructions calling the SIMD version for such a short
string. Probably, if we're worried about performance here we could
just not bother passing the string through the escape function to
search for something we know isn't there and just
appendBinaryStringInfo \""timestamp\":" directly.

I don't really have a preference as to which of these we use. I doubt
the JSON escaping rules would ever change sufficiently that the latter
of these methods would be a bad idea. I just doubt it's worth the
debate as I imagine the performance won't matter that much.

David

[1] https://godbolt.org/z/xqj4rKara