src: use simdutf for one-byte string UTF-8 write in stringBytes by mertcanaltin · Pull Request #61696 · nodejs/node

mertcanaltin · 2026-02-05T19:11:13Z

I use simdutf::convert_latin1_to_utf8_safe for one-byte strings in StringBytes::Write UTF-8 path instead of V8's
WriteUtf8V2.

buffer.write(str, 'utf8') 577% faster

benchmark results:

➜  node git:(mert/simdutf-write-utf8) ✗ node-benchmark-compare ./result.csv
                                                                                          confidence improvement accuracy (*)    (**)   (***)
buffers/buffer-from.js n=800000 len=100 source='array'                                                    0.57 %       ±1.29%  ±1.76%  ±2.40%
buffers/buffer-from.js n=800000 len=100 source='arraybuffer-middle'                                       0.40 %       ±3.23%  ±4.42%  ±6.03%
buffers/buffer-from.js n=800000 len=100 source='arraybuffer'                                             -0.22 %       ±2.94%  ±4.09%  ±5.70%
buffers/buffer-from.js n=800000 len=100 source='buffer'                                                  -0.20 %       ±2.55%  ±3.51%  ±4.84%
buffers/buffer-from.js n=800000 len=100 source='object'                                                  -1.26 %       ±3.31%  ±4.65%  ±6.60%
buffers/buffer-from.js n=800000 len=100 source='string-base64'                                           -0.62 %       ±1.37%  ±1.88%  ±2.57%
buffers/buffer-from.js n=800000 len=100 source='string-utf8'                                              1.73 %       ±2.03%  ±2.86%  ±4.08%
buffers/buffer-from.js n=800000 len=100 source='string'                                                   1.13 %       ±1.52%  ±2.09%  ±2.86%
buffers/buffer-from.js n=800000 len=100 source='uint16array'                                              0.54 %       ±1.54%  ±2.12%  ±2.89%
buffers/buffer-from.js n=800000 len=100 source='uint8array'                                              -0.18 %       ±1.62%  ±2.23%  ±3.06%
buffers/buffer-from.js n=800000 len=2048 source='array'                                                   0.86 %       ±3.64%  ±5.03%  ±6.96%
buffers/buffer-from.js n=800000 len=2048 source='arraybuffer-middle'                                      0.41 %       ±3.35%  ±4.62%  ±6.36%
buffers/buffer-from.js n=800000 len=2048 source='arraybuffer'                                            -5.09 %      ±10.56% ±15.15% ±22.24%
buffers/buffer-from.js n=800000 len=2048 source='buffer'                                          **      1.53 %       ±0.93%  ±1.28%  ±1.75%
buffers/buffer-from.js n=800000 len=2048 source='object'                                                  0.34 %       ±1.59%  ±2.19%  ±3.01%
buffers/buffer-from.js n=800000 len=2048 source='string-base64'                                          -0.41 %       ±0.88%  ±1.21%  ±1.65%
buffers/buffer-from.js n=800000 len=2048 source='string-utf8'                                    ***      5.89 %       ±1.13%  ±1.57%  ±2.17%
buffers/buffer-from.js n=800000 len=2048 source='string'                                         ***      6.34 %       ±1.27%  ±1.74%  ±2.38%
buffers/buffer-from.js n=800000 len=2048 source='uint16array'                                             0.48 %       ±1.03%  ±1.41%  ±1.93%
buffers/buffer-from.js n=800000 len=2048 source='uint8array'                                              0.78 %       ±1.56%  ±2.16%  ±2.98%
buffers/buffer-write-string-short.js n=1000000 len=1 encoding='ascii'                                     0.85 %       ±1.38%  ±1.89%  ±2.58%
buffers/buffer-write-string-short.js n=1000000 len=1 encoding='latin1'                             *      1.83 %       ±1.66%  ±2.30%  ±3.17%
buffers/buffer-write-string-short.js n=1000000 len=1 encoding='utf8'                               *      1.88 %       ±1.60%  ±2.21%  ±3.04%
buffers/buffer-write-string-short.js n=1000000 len=16 encoding='ascii'                                    0.18 %       ±1.45%  ±1.99%  ±2.74%
buffers/buffer-write-string-short.js n=1000000 len=16 encoding='latin1'                                   7.48 %      ±15.46% ±22.20% ±32.63%
buffers/buffer-write-string-short.js n=1000000 len=16 encoding='utf8'                                     4.34 %       ±7.11% ±10.18% ±14.89%
buffers/buffer-write-string-short.js n=1000000 len=32 encoding='ascii'                                    1.44 %       ±1.82%  ±2.50%  ±3.42%
buffers/buffer-write-string-short.js n=1000000 len=32 encoding='latin1'                                   0.81 %       ±1.28%  ±1.76%  ±2.42%
buffers/buffer-write-string-short.js n=1000000 len=32 encoding='utf8'                                     0.43 %       ±2.93%  ±4.03%  ±5.50%
buffers/buffer-write-string-short.js n=1000000 len=8 encoding='ascii'                                     0.22 %       ±1.10%  ±1.51%  ±2.07%
buffers/buffer-write-string-short.js n=1000000 len=8 encoding='latin1'                                    0.92 %       ±1.86%  ±2.60%  ±3.67%
buffers/buffer-write-string-short.js n=1000000 len=8 encoding='utf8'                                      0.41 %       ±2.33%  ±3.20%  ±4.36%
buffers/buffer-write-string.js n=1000000 len=2048 args='' encoding=''                            ***    577.07 %       ±6.13%  ±8.80% ±12.93%
buffers/buffer-write-string.js n=1000000 len=2048 args='' encoding='ascii'                                1.27 %       ±1.34%  ±1.83%  ±2.50%
buffers/buffer-write-string.js n=1000000 len=2048 args='' encoding='hex'                                 -0.36 %       ±0.59%  ±0.83%  ±1.19%
buffers/buffer-write-string.js n=1000000 len=2048 args='' encoding='latin1'                               0.90 %       ±1.22%  ±1.67%  ±2.30%
buffers/buffer-write-string.js n=1000000 len=2048 args='' encoding='utf16le'                       *     -1.33 %       ±1.09%  ±1.50%  ±2.05%
buffers/buffer-write-string.js n=1000000 len=2048 args='' encoding='utf8'                        ***    573.78 %       ±5.59%  ±7.94% ±11.44%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset' encoding=''                      ***     12.86 %       ±1.85%  ±2.57%  ±3.59%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset' encoding='ascii'                         -2.36 %       ±4.06%  ±5.77%  ±8.35%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset' encoding='hex'                           -0.33 %       ±0.64%  ±0.88%  ±1.21%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset' encoding='latin1'                         0.67 %       ±1.75%  ±2.40%  ±3.27%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset' encoding='utf16le'                       -0.39 %       ±0.97%  ±1.33%  ±1.81%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset' encoding='utf8'                           0.12 %      ±10.16% ±14.56% ±21.33%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset+length' encoding=''               ***      9.47 %       ±1.17%  ±1.60%  ±2.18%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset+length' encoding='ascii'           **      1.49 %       ±1.01%  ±1.38%  ±1.88%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset+length' encoding='hex'                     0.52 %       ±1.73%  ±2.47%  ±3.57%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset+length' encoding='latin1'          **      1.91 %       ±1.15%  ±1.60%  ±2.22%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset+length' encoding='utf16le'                 0.45 %       ±1.07%  ±1.47%  ±2.03%
buffers/buffer-write-string.js n=1000000 len=2048 args='offset+length' encoding='utf8'           ***      8.09 %       ±1.04%  ±1.43%  ±1.96%

Be aware that when doing many comparisons the risk of a false-positive result increases.
In this case, there are 50 comparisons, you can thus expect the following amount of false-positive results:
  2.50 false positives, when considering a   5% risk acceptance (*, **, ***),
  0.50 false positives, when considering a   1% risk acceptance (**, ***),
  0.05 false positives, when considering a 0.1% risk acceptance (***)

mertcanaltin · 2026-02-05T19:16:58Z

@nodejs/performance ❤️

anonrig · 2026-02-05T19:18:39Z

Triggered a benchmark CI run: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1794/

nodejs-github-bot · 2026-02-05T19:31:12Z

CI: https://ci.nodejs.org/job/node-test-pull-request/71211/

ChALkeR

non-blocking, can go into a follow-up: this logic likely can be improved further or even unified with #61496

gurgunday

lgtm

codecov · 2026-02-05T20:12:24Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (102cdfe) to head (a6e8753).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61696      +/-   ##
==========================================
- Coverage   89.76%   89.74%   -0.02%     
==========================================
  Files         674      674              
  Lines      204424   204431       +7     
  Branches    39285    39288       +3     
==========================================
- Hits       183504   183470      -34     
- Misses      13220    13264      +44     
+ Partials     7700     7697       -3

Files with missing lines	Coverage Δ
src/string_bytes.cc	`69.66% <100.00%> (+0.65%)`	⬆️

... and 45 files with indirect coverage changes

🚀 New features to boost your workflow:

❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

anonrig · 2026-02-06T17:03:00Z

@mertcanaltin Please don't click "merge to main branch" or whatever that button title is, it doesn't work with commit-queue. You have to remove your last commit and keep only the existing commit. Since you also pushed something new, we have to go through the whole process again and rebuild your pull-request.

mertcanaltin · 2026-02-06T17:38:04Z

sorry. I mistakenly synced the branch. It should be solved now.

Renegade334 · 2026-02-07T01:44:40Z

This would probably be superseded by #61712? (Although the StringBytes change would likely be easier to backport.)

mertcanaltin · 2026-02-07T08:22:09Z

Thanks! @Renegade334 I agree with your comment that this optimization solves the issue more effectively at the V8 layer. That said, this PR could still be useful as an easier backport option for older release lines. I'm waiting for other maintainers' comments.

src/string_bytes.cc

addaleax

I wouldn't mind merging this (with the intention of backporting it) and then later reverting it on main once #61712 is available

nodejs-github-bot · 2026-02-07T13:32:20Z

CI: https://ci.nodejs.org/job/node-test-pull-request/71229/

gurgunday · 2026-02-07T13:58:50Z

I like the backport plan

nodejs-github-bot · 2026-02-07T19:14:26Z

Commit Queue failed

- Loading data for nodejs/node/pull/61696
✔  Done loading data for nodejs/node/pull/61696
----------------------------------- PR info ------------------------------------
Title      src: use simdutf for one-byte string UTF-8 write in stringBytes (#61696)
Author     Mert Can Altin <mertgold60@gmail.com> (@mertcanaltin)
Branch     mertcanaltin:mert/simdutf-write-utf8 -> nodejs:main
Labels     buffer, c++, author ready, needs-ci
Commits    1
 - src: use simdutf for one-byte string UTF-8 write in stringBytes
Committers 1
 - Mert Can Altin <mertgold60@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/61696
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Daniel Lemire <daniel@lemire.me>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/61696
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Daniel Lemire <daniel@lemire.me>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Thu, 05 Feb 2026 19:11:13 GMT
   ✔  Approvals: 8
   ✔  - Yagiz Nizipli (@anonrig) (TSC): https://github.com/nodejs/node/pull/61696#pullrequestreview-3764386119
   ✔  - Stephen Belanger (@Qard): https://github.com/nodejs/node/pull/61696#pullrequestreview-3758974723
   ✔  - Daniel Lemire (@lemire): https://github.com/nodejs/node/pull/61696#pullrequestreview-3759154723
   ✔  - Vinícius Lourenço Claro Cardoso (@H4ad): https://github.com/nodejs/node/pull/61696#pullrequestreview-3759180179
   ✔  - Сковорода Никита Андреевич (@ChALkeR): https://github.com/nodejs/node/pull/61696#pullrequestreview-3759185771
   ✔  - Gürgün Dayıoğlu (@gurgunday): https://github.com/nodejs/node/pull/61696#pullrequestreview-3759188442
   ✔  - Anna Henningsen (@addaleax): https://github.com/nodejs/node/pull/61696#pullrequestreview-3767062000
   ✔  - René (@Renegade334): https://github.com/nodejs/node/pull/61696#pullrequestreview-3767064889
   ✘  Last GitHub CI failed
   ℹ  Last Benchmark CI on 2026-02-05T19:18:39Z: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1794/
   ℹ  Last Full PR CI on 2026-02-07T13:32:20Z: https://ci.nodejs.org/job/node-test-pull-request/71229/
- Querying data for job/node-test-pull-request/71229/
✔  Build data downloaded
   ✔  Last Jenkins CI successful
--------------------------------------------------------------------------------
   ✔  Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/21785408042

nodejs-github-bot · 2026-02-07T19:53:34Z

Commit Queue failed

- Loading data for nodejs/node/pull/61696
✔  Done loading data for nodejs/node/pull/61696
----------------------------------- PR info ------------------------------------
Title      src: use simdutf for one-byte string UTF-8 write in stringBytes (#61696)
Author     Mert Can Altin <mertgold60@gmail.com> (@mertcanaltin)
Branch     mertcanaltin:mert/simdutf-write-utf8 -> nodejs:main
Labels     buffer, c++, author ready, needs-ci
Commits    1
 - src: use simdutf for one-byte string UTF-8 write in stringBytes
Committers 1
 - Mert Can Altin <mertgold60@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/61696
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Daniel Lemire <daniel@lemire.me>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/61696
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Daniel Lemire <daniel@lemire.me>
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>
--------------------------------------------------------------------------------
   ℹ  This PR was created on Thu, 05 Feb 2026 19:11:13 GMT
   ✔  Approvals: 8
   ✔  - Yagiz Nizipli (@anonrig) (TSC): https://github.com/nodejs/node/pull/61696#pullrequestreview-3764386119
   ✔  - Stephen Belanger (@Qard): https://github.com/nodejs/node/pull/61696#pullrequestreview-3758974723
   ✔  - Daniel Lemire (@lemire): https://github.com/nodejs/node/pull/61696#pullrequestreview-3759154723
   ✔  - Vinícius Lourenço Claro Cardoso (@H4ad): https://github.com/nodejs/node/pull/61696#pullrequestreview-3759180179
   ✔  - Сковорода Никита Андреевич (@ChALkeR): https://github.com/nodejs/node/pull/61696#pullrequestreview-3759185771
   ✔  - Gürgün Dayıoğlu (@gurgunday): https://github.com/nodejs/node/pull/61696#pullrequestreview-3759188442
   ✔  - Anna Henningsen (@addaleax): https://github.com/nodejs/node/pull/61696#pullrequestreview-3767062000
   ✔  - René (@Renegade334): https://github.com/nodejs/node/pull/61696#pullrequestreview-3767064889
   ✘  Last GitHub CI failed
   ℹ  Last Benchmark CI on 2026-02-07T19:14:26Z: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1794/
   ℹ  Last Full PR CI on 2026-02-07T19:14:26Z: https://ci.nodejs.org/job/node-test-pull-request/71229/
- Querying data for job/node-test-pull-request/1794/
   ✔  Last Jenkins CI successful
--------------------------------------------------------------------------------
   ✔  Aborted `git node land` session in /home/runner/work/node/node/.ncu

https://github.com/nodejs/node/actions/runs/21785918980

Renegade334 · 2026-02-07T19:57:34Z

The last set of actions were cursed by the GHA outage.

PR-URL: #61696 Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com> Reviewed-By: Stephen Belanger <admin@stephenbelanger.com> Reviewed-By: Daniel Lemire <daniel@lemire.me> Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br> Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com> Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day> Reviewed-By: Anna Henningsen <anna@addaleax.net> Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>

Renegade334 · 2026-02-07T20:05:22Z

Landed in 28979fb

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mertcanaltin force-pushed the mert/simdutf-write-utf8 branch from 8de635a to c74c73d Compare February 6, 2026 17:30

src: use simdutf for one-byte string UTF-8 write in stringBytes

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