Re: Optimize UUID parse using SIMD

On Fri, Aug 14, 2026 at 4:33 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Thu, Aug 6, 2026 at 8:07 PM Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
> >
> >
> >
> > > On Aug 7, 2026, at 00:09, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > On Wed, Aug 5, 2026 at 8:40 PM Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com> wrote:
> > >>
> > >>
> > >>
> > >>> On Aug 6, 2026, at 08:19, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >>>
> > >>> On Tue, Jun 30, 2026 at 11:03 AM Haibo Yan <tristan(dot)yim(at)gmail(dot)com> wrote:
> > >>>>
> > >>>> On Tue, Jun 30, 2026 at 10:53 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >>>>>
> > >>>>> On Mon, Jun 29, 2026 at 5:53 PM Haibo Yan <tristan(dot)yim(at)gmail(dot)com> wrote:
> > >>>>>>
> > >>>>>> On Mon, Jun 29, 2026 at 2:55 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >>>>>>>
> > >>>>>>> On Sun, Jun 28, 2026 at 7:20 PM Haibo Yan <tristan(dot)yim(at)gmail(dot)com> wrote:
> > >>>>>>>>
> > >>>>>>>> On Thu, Jun 25, 2026 at 3:16 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >>>>>>>>>
> > >>>>>>>>> On Thu, Jun 25, 2026 at 2:31 PM Haibo Yan <tristan(dot)yim(at)gmail(dot)com> wrote:
> > >>>>>>>>>>
> > >>>>>>>>>>
> > >>>>>>>>>>
> > >>>>>>>>>> On Thu, Jun 25, 2026 at 11:28 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >>>>>>>>>>>
> > >>>>>>>>>>> Hi all,
> > >>>>>>>>>>>
> > >>>>>>>>>>> I'd like to propose the $subject.
> > >>>>>>>>>>>
> > >>>>>>>>>>> Since commit ec8719ccbfcd made hex_decode_safe() SIMD-aware, decoding
> > >>>>>>>>>>> a run of hex digits is now fast. The attached patch reuses
> > >>>>>>>>>>> hex_decode_safe() in the UUID input function to speed up parsing.
> > >>>>>>>>>>>
> > >>>>>>>>>>> We accept several textual forms of a UUID[1]. The fast path handles
> > >>>>>>>>>>> the common ones: 32 hex digits, the canonical 8x-4x-4x-4x-12x form
> > >>>>>>>>>>> (where "nx" means n hex digits), and either of those wrapped in
> > >>>>>>>>>>> braces. Otherwise, it falls back to the ordinary scalar UUID parse.
> > >>>>>>>>>>>
> > >>>>>>>>>>> I've benchmarked the parse speed using the following query:
> > >>>>>>>>>>>
> > >>>>>>>>>>> CREATE TEMP TABLE u AS SELECT gen_random_uuid()::text AS t FROM
> > >>>>>>>>>>> generate_series(1, 1000000);
> > >>>>>>>>>>> EXPLAIN (ANALYZE, TIMING OFF) SELECT t::uuid FROM u;
> > >>>>>>>>>>>
> > >>>>>>>>>>> I compared the execution time of the second query, which measures
> > >>>>>>>>>>> uuid_in() alone, with/without SIMD optimization. Here are results (the
> > >>>>>>>>>>> median of 5 runs):
> > >>>>>>>>>>>
> > >>>>>>>>>>> HEAD: 208.879 ms
> > >>>>>>>>>>> Patched: 40.983 ms
> > >>>>>>>>>>>
> > >>>>>>>>>>> The improvements look promising to me. But in a realistic pipeline the
> > >>>>>>>>>>> parse is a small fraction of the work, so end-to-end gains could be
> > >>>>>>>>>>> much smaller.
> > >>>>>>>>>>>
> > >>>>>>>>>>> Feedback is very welcome.
> > >>>>>>>>>>>
> > >>>>>>>>>> I may be missing something, but I wonder whether the fast path is relying on
> > >>>>>>>>>> slightly different input semantics from the existing UUID parser.
> > >>>>>>>>>>
> > >>>>>>>>>> In particular, hex_decode_safe() is not a strict “32 hex characters only”
> > >>>>>>>>>> decoder. It skips whitespace, which is fine for its existing callers, but I
> > >>>>>>>>>> don’t think UUID input should treat whitespace inside the UUID body as
> > >>>>>>>>>> ignorable.
> > >>>>>>>>>
> > >>>>>>>>> Good catch! hex_decode_safe() skips whitespaces so the patch accepts
> > >>>>>>>>> the following UUID value, which is bad:
> > >>>>>>>>>
> > >>>>>>>>> select '019f00b5-7f8a-722f-b707-59f0ed25cd '::uuid;
> > >>>>>>>>> uuid
> > >>>>>>>>> --------------------------------------
> > >>>>>>>>> 019f00b5-7f8a-722f-b707-59f0ed25cd00
> > >>>>>>>>> (1 row)
> > >>>>>>>>>
> > >>>>>>>>>> Also, since hex_decode_safe() returns void, the UUID fast path
> > >>>>>>>>>> cannot verify that exactly UUID_LEN bytes were produced.
> > >>>>>>>>>
> > >>>>>>>>> IIUC hex_decode_safe() does return the output length in bytes. So I
> > >>>>>>>>> think we can fallback to the scalar UUID parser if
> > >>>>>>>>> esctx.error_occurred is true or if the returned value is not 16.
> > >>>>>>>>>
> > >>>>>>>>
> > >>>>>>>> You’re right, I misread that part. Checking both esctx.error_occurred and
> > >>>>>>>> the returned length sounds good to me.
> > >>>>>>>>
> > >>>>>>>>>>
> > >>>>>>>>>> So I think it would be safer either to pre-validate that the 32 source
> > >>>>>>>>>> characters are all hex digits before calling hex_decode_safe(), or to use a
> > >>>>>>>>>> UUID-specific strict hex decoder for this path. After that, a comment
> > >>>>>>>>>> explaining why hex_decode_safe() is safe here would make the invariant much
> > >>>>>>>>>> clearer.
> > >>>>>>>>>
> > >>>>>>>>> IIUC hex_decode_simd_helper() accepts only hex digits so we could
> > >>>>>>>>> re-use it for UUID parsing. Let me check if the above idea of using
> > >>>>>>>>> the return value works for us first.
> > >>>>>>>>>
> > >>>>>>>>
> > >>>>>>>> That sounds reasonable. My main concern was to keep the fast path’s accepted
> > >>>>>>>> input set identical to the scalar UUID parser. Falling back when the decoded
> > >>>>>>>> length is not UUID_LEN, together with regression tests for whitespace cases,
> > >>>>>>>> should address that.
> > >>>>>>>>
> > >>>>>>>>>>
> > >>>>>>>>>> Could you also add a few regression tests for invalid inputs that contain
> > >>>>>>>>>> whitespace inside otherwise fast-path-looking UUID strings? For example:
> > >>>>>>>>>>
> > >>>>>>>>>> ---------------------------------------------------------------
> > >>>>>>>>>>
> > >>>>>>>>>> SELECT 'a0eebc99 9c0b4ef8bb6d6bb9bd380a11'::uuid;
> > >>>>>>>>>> SELECT 'a0eebc999c0b4ef8bb6d6bb9bd380a1 '::uuid;
> > >>>>>>>>>> SELECT '{a0eebc999c0b4ef8bb6d6bb9bd380a1 }'::uuid;
> > >>>>>>>>>> SELECT 'a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a1 '::uuid;
> > >>>>>>>>>> ---------------------------------------------------------------
> > >>>>>>>>>>
> > >>>>>>>>>> These should continue to be rejected in the same way as the scalar parser.
> > >>>>>>>>>> Regards,
> > >>>>>>>>>
> > >>>>>>>>> Agreed.
> > >>>>>>>>>
> > >>>>>>>
> > >>>>>>> I've attached the updated patch.
> > >>>>>>>
> > >>>>>>> Regards,
> > >>>>>>>
> > >>>>>>> --
> > >>>>>>> Masahiko Sawada
> > >>>>>>> Amazon Web Services: https://aws.amazon.com
> > >>>>>>
> > >>>>>> I noticed a few typos in the comments:
> > >>>>>>
> > >>>>>> src/backend/utils/adt/uuid.c
> > >>>>>> line 56: “scalar implmentation” -> “scalar implementation”
> > >>>>>> line 109: “swalled” -> “swallowed”
> > >>>>>> line 110: “kepping” -> “keeping”
> > >>>>>> line 118: “grammer” -> “grammar”
> > >>>>>> line 119: “whitespaces” -> “whitespace”
> > >>>>>>
> > >>>>>> Could you fix them ?
> > >>>>>
> > >>>>> Oops, I fixed them and rechecked other places.
> > >>>>>
> > >>>>> I've attached the updated patch.
> > >>>>>
> > >>>>> Regards,
> > >>>>>
> > >>>>> --
> > >>>>> Masahiko Sawada
> > >>>>> Amazon Web Services: https://aws.amazon.com
> > >>>>
> > >>>> The code looks good to me now. I only noticed one small typo in the
> > >>>> commit trailer: Reviwed-by should be Reviewed-by.
> > >>>>
> > >>>> Otherwise, it looks good. Thank you for fixing these issues.
> > >>>>
> > >>>
> > >>> After spending more time on this patch, I find out two things:
> > >>>
> > >>> 1. USE_NO_SIMD doesn't work in uuid.c without including port/simd.h.
> > >>> But including port/simd.h seems wrong as it doesn't use any SIMD
> > >>> support functions.
> > >>>
> > >>> 2. hex_decode_safe() is faster than the current UUID parse
> > >>> (isxdigit()+strtoul() approach) even without SIMD. I've created a
> > >>> small benchmark test tool (attached as 0002 patch, not intended to be
> > >>> pushed into the core), and measures UUID parsing performance of three
> > >>> approaches: 'scalar' is the current string_to_uuid() that uses
> > >>> isxdigit()+strtoul()), 'simd' uses hex_decode_safe() with SIMD, and
> > >>> 'nosimd' uses hex_decode_safe() without SIMD, with different shapes of
> > >>> UUIDs. Here are results:
> > >>>
> > >>> =# select path, shape, n_inputs, best_ms::numeric(10,3) from
> > >>> uuid_parse_bench(100000, 5);
> > >>> path | shape | n_inputs | best_ms
> > >>> --------+------------------+----------+---------
> > >>> scalar | canonical | 100000 | 22.661
> > >>> simd | canonical | 100000 | 1.400
> > >>> nosimd | canonical | 100000 | 1.652
> > >>> scalar | bare32 | 100000 | 15.932
> > >>> simd | bare32 | 100000 | 0.471
> > >>> nosimd | bare32 | 100000 | 1.110
> > >>> scalar | braced_canonical | 100000 | 17.330
> > >>> simd | braced_canonical | 100000 | 1.088
> > >>> nosimd | braced_canonical | 100000 | 1.314
> > >>> scalar | braced_bare32 | 100000 | 15.942
> > >>> simd | braced_bare32 | 100000 | 0.488
> > >>> nosimd | braced_bare32 | 100000 | 1.141
> > >>> scalar | dashed4 | 100000 | 16.185
> > >>> simd | dashed4 | 100000 | 16.493
> > >>> nosimd | dashed4 | 100000 | 16.403
> > >>> scalar | invalid_hex | 100000 | 0.199
> > >>> simd | invalid_hex | 100000 | 1.150
> > >>> nosimd | invalid_hex | 100000 | 0.385
> > >>> (18 rows)
> > >>>
> > >>> Each of shape means:
> > >>> - 'canonical': 8x-4x-4x-4x-12x, what uuid_out() emits
> > >>> - 'bare32': 32 contiguous hex digits
> > >>> - 'braced_canonical': {8x-4x-4x-4x-12x}
> > >>> - 'braced_bare32': {32 hex digits}
> > >>> - 'dashed4': dash after every group of 4
> > >>> - 'invalid_hdx': canonical but with a invalid digit
> > >>>
> > >>> 'nosimd' is 10x~ faster than 'scalar' in most cases. All paths are
> > >>> mostly the same in 'dashed4' and 'invalid_hex' cases because 'simd'
> > >>> and 'nosimd' fall back to the 'scalar' case. According to these
> > >>> results, my conclusion is that we can use hex_decode_safe() for
> > >>> canonical forms and 32 contiguous hex forms anyway, and let
> > >>> hex_decode_safe() choose whether to use SIMD. We would win in either
> > >>> case. We still use the current scalar approach for uncommon UUID forms
> > >>> and error reporting purposes.
> > >>>
> > >>> Regards,
> > >>>
> > >>> --
> > >>> Masahiko Sawada
> > >>> Amazon Web Services: https://aws.amazon.com
> > >>> <v4-0001-Optimize-UUID-parse-using-SIMD.patch><v4-0002-uuid_parse_bench-module.patch>
> > >>
> > >> A few comments on v4.
> > >>
> > >> 1 - 0001
> > >> ```
> > >> +static void
> > >> +string_to_uuid(const char *source, pg_uuid_t *uuid, Node *escontext)
> > >> +{
> > >> + const char *body = source;
> > >> + size_t len = strlen(source);
> > >> ```
> > >>
> > >> I think it would be better to avoid strlen(). The old code processes at most UUID_LEN (16) byte pairs, so it does not need to scan arbitrarily far on malformed input. So, maybe we could use something like strnlen(source, 39) instead.
> > >
> > > While strnlen(source, 39) works there, 39 is a magic number and it's
> > > tied to the current format check logic. What is the benefit of using
> > > strnlen(source, 39) instead? I'm not sure it warrants having the magic
> > > number.
> >
> > It doesn't have to be exactly 39; 1024 (long enough) would also work, or perhaps something based on UUID_LEN, such as UUID_LEN * 3. I think the main point is to avoid unbounded scanning on malformed input.
> >
> > The old code did not have this issue because it only examined as much input as needed based on UUID_LEN. The new fast path starts to use strlen(), so this would be a new risk introduced by the optimization.
>
> I don't think the scan can be really unbounded. string_to_uuid()
> receives a cstring, so by the time it is called the caller has already
> walked or copied the whole string to produce it. So unless the
> unbounded scan can be reached in some path I have overlooked, I'd
> prefer to keep strlen() here. Happy to change it if you still think it
> is worth it.

After more thoughts, while I still don't think the scan can be
unbounded, using strlen() would add an extra scan just to determine we
use hex_decode_safe(). I'll change it to use strnlen() instead.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com