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perf(buffer): optimize swap16/swap64 with __builtin_bswap (#26190)

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## Summary

Optimize `Buffer.swap16()` and `Buffer.swap64()` by replacing
byte-by-byte swapping loops with `__builtin_bswap16/64` compiler
intrinsics.

## Problem

`Buffer.swap16` and `Buffer.swap64` were significantly slower than
Node.js due to inefficient byte-level operations:

- **swap16**: Swapped bytes one at a time in a loop
- **swap64**: Used a nested loop with 4 byte swaps per 8-byte element

## Solution

Replace the manual byte swapping with `__builtin_bswap16/64` intrinsics,
which compile to single CPU instructions (`BSWAP` on x86, `REV` on ARM).

Use `memcpy` for loading/storing values to handle potentially unaligned
buffers safely.

## Benchmark Results (64KB buffer, Apple M4 Max)

| Operation | Bun 1.3.6 | Node.js 24 | This PR | Improvement |
|-----------|-----------|------------|---------|-------------|
| swap16    | 1.00 µs   | 0.57 µs    | 0.56 µs | **1.79x faster** |
| swap32 | 0.55 µs | 0.77 µs | 0.54 µs | (no change, already fast) |
| swap64    | 2.02 µs   | 0.58 µs    | 0.56 µs | **3.6x faster** |

Bun now matches or exceeds Node.js performance for all swap operations.

## Notes

- `swap32` was not modified as the compiler already optimizes the 4-byte
swap pattern
- All existing tests pass
This commit is contained in:
SUZUKI Sosuke
2026-01-19 06:33:04 +09:00
committed by GitHub
parent 12a45b7cbf
commit f8a049e9f2
1 changed files with 17 additions and 22 deletions
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39
src/bun.js/bindings/JSBuffer.cpp
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@@ -1649,8 +1649,8 @@ static JSC::EncodedJSValue jsBufferPrototypeFunction_swap16Body(JSC::JSGlobalObj
auto& vm = JSC::getVM(lexicalGlobalObject);
auto scope = DECLARE_THROW_SCOPE(vm);
constexpr int elemSize = 2;
int64_t length = static_cast<int64_t>(castedThis->byteLength());
constexpr size_t elemSize = 2;
size_t length = castedThis->byteLength();
if (length % elemSize != 0) {
throwNodeRangeError(lexicalGlobalObject, scope, "Buffer size must be a multiple of 16-bits"_s);
return {};
@@ -1661,14 +1661,14 @@ static JSC::EncodedJSValue jsBufferPrototypeFunction_swap16Body(JSC::JSGlobalObj
return {};
}
uint8_t* typedVector = castedThis->typedVector();
uint8_t* data = castedThis->typedVector();
size_t count = length / elemSize;
for (size_t elem = 0; elem < length; elem += elemSize) {
const size_t right = elem + 1;
uint8_t temp = typedVector[elem];
typedVector[elem] = typedVector[right];
typedVector[right] = temp;
for (size_t i = 0; i < count; i++) {
uint16_t val;
memcpy(&val, data + i * elemSize, sizeof(val));
val = __builtin_bswap16(val);
memcpy(data + i * elemSize, &val, sizeof(val));
}
return JSC::JSValue::encode(castedThis);
@@ -1715,7 +1715,7 @@ static JSC::EncodedJSValue jsBufferPrototypeFunction_swap64Body(JSC::JSGlobalObj
auto scope = DECLARE_THROW_SCOPE(vm);
constexpr size_t elemSize = 8;
int64_t length = static_cast<int64_t>(castedThis->byteLength());
size_t length = castedThis->byteLength();
if (length % elemSize != 0) {
throwNodeRangeError(lexicalGlobalObject, scope, "Buffer size must be a multiple of 64-bits"_s);
return {};
@@ -1726,19 +1726,14 @@ static JSC::EncodedJSValue jsBufferPrototypeFunction_swap64Body(JSC::JSGlobalObj
return {};
}
uint8_t* typedVector = castedThis->typedVector();
uint8_t* data = castedThis->typedVector();
size_t count = length / elemSize;
constexpr size_t swaps = elemSize / 2;
for (size_t elem = 0; elem < length; elem += elemSize) {
const size_t right = elem + elemSize - 1;
for (size_t k = 0; k < swaps; k++) {
const size_t i = right - k;
const size_t j = elem + k;
uint8_t temp = typedVector[i];
typedVector[i] = typedVector[j];
typedVector[j] = temp;
}
for (size_t i = 0; i < count; i++) {
uint64_t val;
memcpy(&val, data + i * elemSize, sizeof(val));
val = __builtin_bswap64(val);
memcpy(data + i * elemSize, &val, sizeof(val));
}
return JSC::JSValue::encode(castedThis);