buffer: add SIMD Neon optimization for byteLength

Co-authored-by: Keyhan Vakil <kvakil@sylph.kvakil.me>
Co-authored-by: Daniel Lemire <daniel@lemire.me>

node.gyp

@@ -121,6 +121,7 @@
       'src/node_report_utils.cc',
       'src/node_sea.cc',
       'src/node_serdes.cc',
+      'src/node_simd.cc',
       'src/node_shadow_realm.cc',
       'src/node_snapshotable.cc',
       'src/node_sockaddr.cc',

src/node_buffer.cc

@@ -26,6 +26,7 @@
 #include "node_external_reference.h"
 #include "node_i18n.h"
 #include "node_internals.h"
+#include "node_simd.h"
 
 #include "env-inl.h"
 #include "simdutf.h"
@@ -743,14 +744,8 @@ void SlowByteLengthUtf8(const FunctionCallbackInfo<Value>& args) {
 
 uint32_t FastByteLengthUtf8(Local<Value> receiver,
                             const v8::FastOneByteString& source) {
-  uint32_t result = 0;
-  uint32_t length = source.length;
-  const uint8_t* data = reinterpret_cast<const uint8_t*>(source.data);
-  for (uint32_t i = 0; i < length; ++i) {
-    result += (data[i] >> 7);
-  }
-  result += length;
-  return result;
+  return node::simd::utf8_byte_length(
+      reinterpret_cast<const uint8_t*>(source.data), source.length);
 }
 
 static v8::CFunction fast_byte_length_utf8(

src/node_simd.cc

@@ -0,0 +1,56 @@
+#include "node_simd.h"
+
+#if NODE_HAS_SIMD_NEON
+#include <arm_neon.h>
+#endif
+
+namespace node {
+namespace simd {
+
+#if NODE_HAS_SIMD_NEON
+uint32_t utf8_byte_length(const uint8_t* data, size_t length) {
+  uint32_t result{0};
+  const int lanes = sizeof(uint8x16_t);
+  uint8_t remaining = length % lanes;
+  const auto* simd_end = data + (length / lanes) * lanes;
+  const auto threshold = vdupq_n_u8(0x80);
+
+  for (; data < simd_end; data += lanes) {
+    // load 16 bytes
+    uint8x16_t input = vld1q_u8(data);
+
+    // compare to threshold (0x80)
+    uint8x16_t with_highbit = vcgeq_u8(input, threshold);
+
+    // nshift and narrow
+    uint8x8_t highbits = vshrn_n_u16(vreinterpretq_u16_u8(with_highbit), 4);
+
+    // we have 0, 4 or 8 bits per byte
+    uint8x8_t bitsperbyte = vcnt_u8(highbits);
+
+    // sum the bytes vertically to uint32_t
+    result += vaddlv_u8(bitsperbyte);
+  }
+
+  // we overcounted by a factor of 4
+  result /= 4;
+
+  for (uint8_t j = 0; j < remaining; j++) {
+    result += (simd_end[j] >> 7);
+  }
+
+  return result + length;
+}
+#else
+uint32_t utf8_byte_length(const uint8_t* data, size_t length) {
+  uint32_t result = 0;
+  for (uint32_t i = 0; i < length; ++i) {
+    result += (data[i] >> 7);
+  }
+  result += length;
+  return result;
+}
+#endif
+
+}  // namespace simd
+}  // namespace node

src/node_simd.h

@@ -0,0 +1,22 @@
+#ifndef SRC_NODE_SIMD_H_
+#define SRC_NODE_SIMD_H_
+
+#if defined(__aarch64__) || defined(_M_ARM64)
+#define NODE_HAS_SIMD_NEON 1
+#endif
+
+#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#include <stdlib.h>
+
+namespace node {
+namespace simd {
+
+uint32_t utf8_byte_length(const uint8_t* input, size_t length);
+
+}  // namespace simd
+}  // namespace node
+
+#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS
+
+#endif  // SRC_NODE_SIMD_H_