Re: [PATCH] Optimize json_lex_string by batching character copying

Hi,

On 2022-07-06 15:58:44 +0700, John Naylor wrote:
> From 82e13b6bebd85a152ededcfd75495c0c0f642354 Mon Sep 17 00:00:00 2001
> From: John Naylor <john(dot)naylor(at)postgresql(dot)org>
> Date: Wed, 6 Jul 2022 15:50:09 +0700
> Subject: [PATCH v4 4/4] Use vectorized lookahead in json_lex_string on x86
>
> ---
> src/common/jsonapi.c | 48 ++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 48 insertions(+)
>
> diff --git a/src/common/jsonapi.c b/src/common/jsonapi.c
> index 81e176ad8d..44e8ed2b2f 100644
> --- a/src/common/jsonapi.c
> +++ b/src/common/jsonapi.c
> @@ -24,6 +24,12 @@
> #include "miscadmin.h"
> #endif
>
> +/* WIP: put somewhere sensible and consider removing CRC from the names */
> +#if (defined (__x86_64__) || defined(_M_AMD64)) && (defined(USE_SSE42_CRC32C) || defined(USE_SSE42_CRC32C_WITH_RUNTIME_CHECK))
> +#include <nmmintrin.h>
> +#define USE_SSE2
> +#endif
> +
> /*
> * The context of the parser is maintained by the recursive descent
> * mechanism, but is passed explicitly to the error reporting routine
> @@ -842,12 +848,54 @@ json_lex_string(JsonLexContext *lex)
> }
> else
> {
> +#ifdef USE_SSE2
> + __m128i block,
> + has_backslash,
> + has_doublequote,
> + control,
> + has_control,
> + error_cum = _mm_setzero_si128();
> + const __m128i backslash = _mm_set1_epi8('\\');
> + const __m128i doublequote = _mm_set1_epi8('"');
> + const __m128i max_control = _mm_set1_epi8(0x1F);
> +#endif
> /* start lookahead at current byte */
> char *p = s;
>
> if (hi_surrogate != -1)
> return JSON_UNICODE_LOW_SURROGATE;
>
> +#ifdef USE_SSE2
> + while (p < end - sizeof(__m128i))
> + {
> + block = _mm_loadu_si128((const __m128i *) p);
> +
> + /* direct comparison to quotes and backslashes */
> + has_backslash = _mm_cmpeq_epi8(block, backslash);
> + has_doublequote = _mm_cmpeq_epi8(block, doublequote);
> +
> + /*
> + * use saturation arithmetic to check for <= highest control
> + * char
> + */
> + control = _mm_subs_epu8(block, max_control);
> + has_control = _mm_cmpeq_epi8(control, _mm_setzero_si128());
> +
> + /*
> + * set bits in error_cum where the corresponding lanes in has_*
> + * are set
> + */
> + error_cum = _mm_or_si128(error_cum, has_backslash);
> + error_cum = _mm_or_si128(error_cum, has_doublequote);
> + error_cum = _mm_or_si128(error_cum, has_control);
> +
> + if (_mm_movemask_epi8(error_cum))
> + break;
> +
> + p += sizeof(__m128i);
> + }
> +#endif /* USE_SSE2 */
> +
> while (p < end)
> {
> if (*p == '\\' || *p == '"')
> --
> 2.36.1
>

I wonder if we can't abstract this at least a bit better. If we go that route
a bit further, then add another arch, this code will be pretty much
unreadable.

Greetings,

Andres Freund