[base64] Add toBase64 path for SharedArrayBuffers

rmahdav · V8 LUCI CQ · commit e3cddbedb205 · 2025-04-23T14:05:34.000-07:00
This CL, adds a path to make sure we have an atomic read when the TypedArray is backed with a SharedArrayBuffer. Bug: 42204568 Change-Id: Ic5d5e3fb4000a037611e4ccaaa6400c370956f17 Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/6449193 Commit-Queue: Rezvan Mahdavi Hezaveh <rezvan@chromium.org> Reviewed-by: Shu-yu Guo <syg@chromium.org> Cr-Commit-Position: refs/heads/main@{#99877}
diff --git a/BUILD.bazel b/BUILD.bazel
@@ -4360,6 +4360,12 @@ cc_library(
   name = "simdutf",
   srcs = ["third_party/simdutf/simdutf.cpp"],
   hdrs = ["third_party/simdutf/simdutf.h"],
+  copts = select({
+        "@v8//bazel/config:is_clang": ["-std=c++20"],
+        "@v8//bazel/config:is_gcc": ["-std=gnu++2a"],
+        "@v8//bazel/config:is_windows": ["/std:c++20"],
+        "//conditions:default": [],
+    }),
 )
 
 v8_library(
diff --git a/src/builtins/builtins-typed-array.cc b/src/builtins/builtins-typed-array.cc
@@ -818,11 +818,16 @@ BUILTIN(Uint8ArrayPrototypeToBase64) {
     //    is false.
     // 11. Return CodePointsToString(outAscii).
 
-    // TODO(rezvan): Make sure to add a path for SharedArrayBuffers when
-    // simdutf library got updated. Also, add a test for it.
-    size_t simd_result_size = simdutf::binary_to_base64(
-        std::bit_cast<const char*>(uint8array->GetBuffer()->backing_store()),
-        length, reinterpret_cast<char*>(output->GetChars(no_gc)), alphabet);
+    size_t simd_result_size;
+    if (uint8array->buffer()->is_shared()) {
+      simd_result_size = simdutf::atomic_binary_to_base64(
+          std::bit_cast<const char*>(uint8array->GetBuffer()->backing_store()),
+          length, reinterpret_cast<char*>(output->GetChars(no_gc)), alphabet);
+    } else {
+      simd_result_size = simdutf::binary_to_base64(
+          std::bit_cast<const char*>(uint8array->GetBuffer()->backing_store()),
+          length, reinterpret_cast<char*>(output->GetChars(no_gc)), alphabet);
+    }
     DCHECK_EQ(simd_result_size, output_length);
     USE(simd_result_size);
   }
diff --git a/test/mjsunit/harmony/uint8-array-to-base64-on-shared-array-buffer.js b/test/mjsunit/harmony/uint8-array-to-base64-on-shared-array-buffer.js
@@ -0,0 +1,56 @@
+// Copyright 2025 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Flags: --js-base-64 --allow-natives-syntax
+
+const workerScript = `
+  onmessage = function(event) {
+    const sab = event.data.buffer;
+    const uint8Array = new Uint8Array(sab);
+
+    const dataToWrite = [102, 111, 111, 98, 97, 114];
+
+    for (let i = 0; i < dataToWrite.length; ++i) {
+        uint8Array[i] = dataToWrite[i];
+    }
+
+    postMessage("started");
+
+    while (true) {
+      for (let i = 0; i < dataToWrite.length; ++i) {
+        uint8Array[i] = dataToWrite[i];
+      }
+    }
+  };
+`;
+
+function testConcurrentSharedArrayBufferUint8ArrayToBase64() {
+  const sab = new SharedArrayBuffer(6);
+  const uint8ArrayMain = new Uint8Array(sab);
+
+  // Create a worker
+  const worker = new Worker(workerScript, {type: 'string'});
+
+  // Send the SharedArrayBuffer
+  worker.postMessage({buffer: sab});
+  assertEquals('started', worker.getMessage());
+
+  // Give the worker a little time to write
+  for (let i = 0; i < 10000; ++i) {
+  }
+
+  // Call toBase64 on the main thread's view of the SAB
+  for (let i=0; i < 100; i++) {
+    const base64String = uint8ArrayMain.toBase64();
+    assertEquals(
+        'Zm9vYmFy', base64String,
+        'toBase64 result mismatch with concurrent writes');
+  }
+
+  // Terminate the worker (now it should exit its loop)
+  worker.terminate();
+}
+
+// Run the test function
+testConcurrentSharedArrayBufferUint8ArrayToBase64();
