perf: speed up UTF-16 source copies

src/lexer.js

@@ -1,6 +1,7 @@
 let wasm;
 
 const isLE = new Uint8Array(new Uint16Array([1]).buffer)[0] === 1;
+const hasBuffer = typeof Buffer !== 'undefined';
 
 export function parse (source, name = '@') {
   if (!wasm)
@@ -14,7 +15,11 @@ export function parse (source, name = '@') {
     wasm.memory.grow(Math.ceil(extraMem / 65536));
 
   const addr = wasm.sa(len);
-  (isLE ? copyLE : copyBE)(source, new Uint16Array(wasm.memory.buffer, addr, len));
+  // Buffer setup is slower than the loop for short sources.
+  if (source.length >= 64 && hasBuffer)
+    Buffer.from(wasm.memory.buffer, addr, (len - 1) * 2).write(source, 'utf16le');
+  else
+    (isLE ? copyLE : copyBE)(source, new Uint16Array(wasm.memory.buffer, addr, len));
 
   const err_code = wasm.parseCJS(addr, source.length, 0, 0, 0);
 

test/_unit.js

@@ -758,6 +758,51 @@ suite('Lexer', () => {
     });
   });
 
+  test('browser UTF-16 source copy', async () => {
+    const originalBuffer = global.Buffer;
+    global.Buffer = undefined;
+    let browserLexer;
+    try {
+      browserLexer = await import('../dist/lexer.mjs?browser-utf16-copy');
+    }
+    finally {
+      global.Buffer = originalBuffer;
+    }
+    browserLexer.initSync();
+    const source = `const value = '${'𓀀'.repeat(40000)}'; exports.value = value`;
+    const { exports, reexports } = browserLexer.parse(source);
+    assert.deepStrictEqual(exports, ['value']);
+    assert.deepStrictEqual(reexports, []);
+  });
+
+  test('Node.js UTF-16 source copy cutoff', async function testNodeUtf16CopyCutoff () {
+    const nodeLexer = await import('../dist/lexer.mjs?node-utf16-copy-cutoff');
+    nodeLexer.initSync();
+
+    const originalBuffer = global.Buffer;
+    let copies = 0;
+    global.Buffer = {
+      /**
+       * @param {ArrayBuffer} arrayBuffer
+       * @param {number} byteOffset
+       * @param {number} length
+       */
+      from (arrayBuffer, byteOffset, length) {
+        copies++;
+        return originalBuffer.from(arrayBuffer, byteOffset, length);
+      }
+    };
+    try {
+      nodeLexer.parse(' '.repeat(63));
+      assert.strictEqual(copies, 0);
+      nodeLexer.parse(' '.repeat(64));
+      assert.strictEqual(copies, 1);
+    }
+    finally {
+      global.Buffer = originalBuffer;
+    }
+  });
+
   test('Simple import', () => {
     const source = `
       import test from "test";