Use AArch64 NEON intrinsics for histogram bucket updates

Replace the scalar ARM fallback in update_histogram_buckets with a NEON
implementation that processes two buckets per iteration, matching the
existing AVX path.  The wrapper now dispatches to the SIMD path on both
x86-64 and AArch64 and falls back to scalar code on other architectures.

src/metric.cpp

@@ -24,6 +24,8 @@
 
 #if defined(__x86_64__) || defined(__amd64__) || defined(_M_X64)
 #include <immintrin.h>
+#elif defined(__aarch64__)
+#include <arm_neon.h>
 #endif
 #include <simdutf.h>
 
@@ -1400,8 +1402,8 @@ void Gauge::set(double x) {
 Histogram::Histogram() = default;
 
 // Vectorized histogram bucket updates with mutex protection for consistency
-// AVX-optimized implementation for high performance on x86-64, scalar fallback
-// on other architectures (e.g., ARM64).
+// AVX-optimized implementation for high performance on x86-64, NEON-optimized
+// implementation on ARM64, and a scalar fallback for other architectures.
 
 #if defined(__x86_64__) || defined(__amd64__) || defined(_M_X64)
 __attribute__((target("avx"))) static void
@@ -1443,12 +1445,46 @@ update_histogram_buckets_simd(std::span<const double> thresholds,
     }
   }
 }
+#elif defined(__aarch64__)
+static void
+update_histogram_buckets_simd(std::span<const double> thresholds,
+                                std::span<uint64_t> counts, double x,
+                                size_t start_idx) {
+  const size_t size = thresholds.size();
+  size_t i = start_idx;
+
+  // Process 2 buckets at a time with 128-bit NEON vectors
+  const float64x2_t x_vec = vdupq_n_f64(x);
+  const uint64x2_t one = vdupq_n_u64(1);
+
+  for (; i + 2 <= size; i += 2) {
+    // Compare x <= thresholds per lane; true lanes are all ones.
+    float64x2_t thresholds_vec = vld1q_f64(&thresholds[i]);
+    uint64x2_t cmp_result = vcleq_f64(x_vec, thresholds_vec);
+
+    // Convert all-ones/all-zeros masks to per-lane 1/0 increments.
+    uint64x2_t increments = vandq_u64(cmp_result, one);
+
+    // Load current counts, add increments, and store back.
+    uint64x2_t current_counts = vld1q_u64(&counts[i]);
+    uint64x2_t updated_counts = vaddq_u64(current_counts, increments);
+    vst1q_u64(&counts[i], updated_counts);
+  }
+
+  // Handle remainder with scalar operations
+  for (; i < size; ++i) {
+    if (x <= thresholds[i]) {
+      counts[i]++;
+    }
+  }
+}
 #endif
 
 static void update_histogram_buckets(std::span<const double> thresholds,
                                      std::span<uint64_t> counts, double x,
                                      size_t start_idx) {
-#if defined(__x86_64__) || defined(__amd64__) || defined(_M_X64)
+#if defined(__x86_64__) || defined(__amd64__) || defined(_M_X64) || \
+    defined(__aarch64__)
   update_histogram_buckets_simd(thresholds, counts, x, start_idx);
 #else
   const size_t size = thresholds.size();