buffer: speed up concat via TypedArray#set

lib/buffer.js

@@ -51,6 +51,7 @@ const {
   TypedArrayPrototypeGetLength,
   TypedArrayPrototypeSet,
   TypedArrayPrototypeSlice,
+  TypedArrayPrototypeSubarray,
   Uint8Array,
 } = primordials;
 
@@ -582,8 +583,9 @@ Buffer.concat = function concat(list, length) {
   if (length === undefined) {
     length = 0;
     for (let i = 0; i < list.length; i++) {
-      if (list[i].length) {
-        length += list[i].length;
+      const bufLength = list[i].length;
+      if (bufLength) {
+        length += bufLength;
       }
     }
   } else {
@@ -600,7 +602,22 @@ Buffer.concat = function concat(list, length) {
       throw new ERR_INVALID_ARG_TYPE(
         `list[${i}]`, ['Buffer', 'Uint8Array'], list[i]);
     }
-    pos += _copyActual(buf, buffer, pos, 0, buf.length);
+
+    const bufLength = buf.length;
+    if (bufLength === 0 || pos >= length) {
+      // Empty buf, or destination is full and subsequent copies are no-ops.
+      // We don't break to preserve compatibility and throw if isUint8Array fails.
+      continue;
+    }
+
+    const available = length - pos;
+    const toCopy = bufLength > available ? available : bufLength;
+    if (toCopy === bufLength) {
+      TypedArrayPrototypeSet(buffer, buf, pos);
+    } else {
+      TypedArrayPrototypeSet(buffer, TypedArrayPrototypeSubarray(buf, 0, toCopy), pos);
+    }
+    pos += toCopy;
   }
 
   // Note: `length` is always equal to `buffer.length` at this point

test/parallel/test-buffer-concat.js

@@ -76,6 +76,13 @@ assert.throws(() => {
 const random10 = common.hasCrypto ?
   require('crypto').randomBytes(10) :
   Buffer.alloc(10, 1);
+const derived18 = Buffer.alloc(18);
+for (let i = 0, j = 0; i < 18; i++) {
+  if (i < 10)
+    derived18[i] = random10[i];
+  else
+    derived18[i] = random10[j++];
+}
 const empty = Buffer.alloc(0);
 
 assert.notDeepStrictEqual(random10, empty);
@@ -85,6 +92,7 @@ assert.deepStrictEqual(Buffer.concat([], 100), empty);
 assert.deepStrictEqual(Buffer.concat([random10], 0), empty);
 assert.deepStrictEqual(Buffer.concat([random10], 10), random10);
 assert.deepStrictEqual(Buffer.concat([random10, random10], 10), random10);
+assert.deepStrictEqual(Buffer.concat([random10, random10], 18), derived18);
 assert.deepStrictEqual(Buffer.concat([empty, random10]), random10);
 assert.deepStrictEqual(Buffer.concat([random10, empty, empty]), random10);