@anonrig I recommend trying something like this... We have three cases: one external one-byte string (easy), one non-external one byte string (we use WriteOneByte) and the general case where we go to 16-bit words, we try converting to latin1 and then we see what happens. In this scenario, there might be an error condition, currently in the code I suggest, it returns the empty string.

static void Btoa(const FunctionCallbackInfo<Value>& args) {
  CHECK_EQ(args.Length(), 1);
  Environment* env = Environment::GetCurrent(args);
  THROW_AND_RETURN_IF_NOT_STRING(env, args[0], "argument");

  Local<String> input = args[0].As<String>();
  MaybeStackBuffer<char> buffer;
  size_t written;

  if (input->IsExternalOneByte()) {  // 8-bit case
    auto ext = input->GetExternalOneByteStringResource();
    size_t expected_length = simdutf::base64_length_from_binary(ext->length());
    buffer.AllocateSufficientStorage(expected_length + 1);
    buffer.SetLengthAndZeroTerminate(expected_length);
    written = simdutf::binary_to_base64(
        ext->data(), ext->length(), buffer.out(), simdutf::base64_default);
  } if(input->IsOneByte()) {
    MaybeStackBuffer<uint8_t> stack_buf(input->Length());
        input->WriteOneByte(env->isolate(),
                           stack_buf.out(),
                           0,
                           input->Length(),
                           String::NO_NULL_TERMINATION);

    size_t expected_length = simdutf::base64_length_from_binary(input->Length());
    buffer.AllocateSufficientStorage(expected_length + 1);
    buffer.SetLengthAndZeroTerminate(expected_length);
    written = simdutf::binary_to_base64(
        reinterpret_cast<const char*>(*stack_buf), input->Length(), buffer.out());
  } else {
    String::Value value(env->isolate(), input);
    MaybeStackBuffer<char> stack_buf(value.length());
    size_t out_len = simdutf::convert_utf16_to_latin1(reinterpret_cast<const char16_t *>(*value), value.length(), stack_buf.out());
    if(out_len == 0) { // error
      return args.GetReturnValue().SetEmptyString();
    }
    size_t expected_length = simdutf::base64_length_from_binary(out_len);
    buffer.AllocateSufficientStorage(expected_length + 1);
    buffer.SetLengthAndZeroTerminate(expected_length);
    written = simdutf::binary_to_base64(
        *stack_buf, out_len, buffer.out());
  }

  auto value =
      String::NewFromOneByte(env->isolate(),
                             reinterpret_cast<const uint8_t*>(buffer.out()),
                             NewStringType::kNormal,
                             written)
          .ToLocalChecked();
  return args.GetReturnValue().Set(value);
}

This should be considered ChatGTP-quality code: I did not even try to test it... It is only as an idea.

@anonrig Lint. Lint. Lint.

@lemire I've updated the benchmark results as well :-)

@anonrig My proposed code would return the empty string on error, but we need to throw, I pushed a commit that does that. (Very minor change.)

CI: https://ci.nodejs.org/job/node-test-pull-request/58249/

CI: https://ci.nodejs.org/job/node-test-pull-request/58258/

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----------------------------------- PR info ------------------------------------
Title      buffer: improve `btoa` performance (#52427)
Author     Yagiz Nizipli  (@anonrig)
Branch     anonrig:improve-btoa -> nodejs:main
Labels     buffer, c++, needs-ci
Commits    2
 - buffer: improve `btoa` performance
 - fix: return exception
Committers 2
 - Yagiz Nizipli 
 - Daniel Lemire 
PR-URL: https://github.com/nodejs/node/pull/52427
Reviewed-By: Daniel Lemire 
Reviewed-By: James M Snell 
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/52427
Reviewed-By: Daniel Lemire 
Reviewed-By: James M Snell 
--------------------------------------------------------------------------------
   ℹ  This PR was created on Mon, 08 Apr 2024 17:24:56 GMT
   ✔  Approvals: 2
   ✔  - Daniel Lemire (@lemire): https://github.com/nodejs/node/pull/52427#pullrequestreview-1992265694
   ✔  - James M Snell (@jasnell) (TSC): https://github.com/nodejs/node/pull/52427#pullrequestreview-1993089038
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--------------------------------------------------------------------------------
[main 50c914c521] buffer: improve `btoa` performance
 Author: Yagiz Nizipli 
 Date: Mon Apr 8 13:23:56 2024 -0400
 3 files changed, 81 insertions(+), 7 deletions(-)
 create mode 100644 benchmark/buffers/buffer-btoa.js
[main de7c8dcb6a] fix: return exception
 Author: Daniel Lemire 
 Date: Wed Apr 10 09:44:11 2024 -0400
 2 files changed, 6 insertions(+), 2 deletions(-)
   ✔  Patches applied
There are 2 commits in the PR. Attempting autorebase.
Rebasing (2/4)
Executing: git node land --amend --yes

--------------------------------- New Message ----------------------------------

buffer: improve btoa performance
PR-URL: #52427

Reviewed-By: Daniel Lemire daniel@lemire.me

Reviewed-By: James M Snell jasnell@gmail.com
[detached HEAD f1e7db2933] buffer: improve btoa performance

Author: Yagiz Nizipli yagiz@nizipli.com

Date: Mon Apr 8 13:23:56 2024 -0400

3 files changed, 81 insertions(+), 7 deletions(-)

create mode 100644 benchmark/buffers/buffer-btoa.js

Rebasing (3/4)

Rebasing (4/4)
Executing: git node land --amend --yes

--------------------------------- New Message ----------------------------------

fix: return exception
PR-URL: #52427

Reviewed-By: Daniel Lemire daniel@lemire.me

Reviewed-By: James M Snell jasnell@gmail.com
[detached HEAD a285a09324] fix: return exception

Author: Daniel Lemire dlemire@lemire.me

Date: Wed Apr 10 09:44:11 2024 -0400

2 files changed, 6 insertions(+), 2 deletions(-)
Successfully rebased and updated refs/heads/main.
ℹ  Add commit-queue-squash label to land the PR as one commit, or commit-queue-rebase to land as separate commits.

Landed in 21211a3