diff --git a/runtime/base/bit_utils.h b/runtime/base/bit_utils.h
index 9c78ee5..d6a44f7 100644
--- a/runtime/base/bit_utils.h
+++ b/runtime/base/bit_utils.h
@@ -53,6 +53,7 @@
           : __builtin_ctzll(x);
 }
 
+// Return the number of 1-bits in `x`.
 template<typename T>
 static constexpr int POPCOUNT(T x) {
   return (sizeof(T) == sizeof(uint32_t))