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buffer: avoid unnecessary copy in Buffer.concat for single FastBuffer #58266

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buffer: avoid unnecessary copy in Buffer.concat for single FastBuffer #58266

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2ab0573
buffer: avoid unnecessary copy in Buffer.concat for single FastBuffer
mertcanaltin May 10, 2025
44d56f1
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mertcanaltin May 10, 2025
62de85d
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mertcanaltin May 10, 2025
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9 changes: 9 additions & 0 deletions lib/buffer.js
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Original file line number Diff line number Diff line change
Expand Up @@ -593,6 +593,15 @@ Buffer.concat = function concat(list, length) {
validateOffset(length, 'length');
}

// If there is only one FastBuffer, rotate without copying
if (
list.length === 1 &&
list[0] instanceof FastBuffer &&
list[0].length === length
) {
return Buffer.from(list[0]);
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@jasnell jasnell May 12, 2025

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If list[0] is a TypedArray (like another Buffer) then this still copies.

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@mertcanaltin mertcanaltin May 12, 2025

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I’m wondering if it makes sense to return list[0] directly. I understand this avoids copying, so I guess I should update the test accordingly?
https://github.com/nodejs/node/blob/main/test/parallel/test-buffer-concat.js#L43

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@legendecas legendecas May 12, 2025

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Buffer.concat should always return a new buffer object:

node/doc/api/buffer.md

Lines 1033 to 1034 in 91d2400

Returns a new `Buffer` which is the result of concatenating all the `Buffer`
instances in the `list` together.

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@mertcanaltin mertcanaltin May 12, 2025

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As I understand it, at the end of the day we should always create a buffer.

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@mertcanaltin mertcanaltin May 12, 2025

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I will try an optimization in the inner regions, I am now closing this pr, thank you very much for your comments

}

const buffer = Buffer.allocUnsafe(length);
let pos = 0;
for (let i = 0; i < list.length; i++) {
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12 changes: 12 additions & 0 deletions test/parallel/test-buffer-concat.js
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Expand Up @@ -44,6 +44,18 @@ assert.notStrictEqual(flatOne, one[0]);
assert.strictEqual(flatLong.toString(), check);
assert.strictEqual(flatLongLen.toString(), check);

{
const original = Buffer.from('fast');
const result = Buffer.concat([original], original.length);

const msgRef =
'Buffer.concat should return a new Buffer instance even with single FastBuffer';
const msgEq = 'Buffer.concat output should equal original content';

assert.notStrictEqual(result, original, msgRef);
assert.deepStrictEqual(result, original, msgEq);
}

[undefined, null, Buffer.from('hello')].forEach((value) => {
assert.throws(() => {
Buffer.concat(value);
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