diff --git a/tachyon/base/bits.h b/tachyon/base/bits.h
index 3cd0d75e5..fa454f5a9 100644
--- a/tachyon/base/bits.h
+++ b/tachyon/base/bits.h
@@ -163,6 +163,7 @@ constexpr T LeftmostBit() {
 
 TACHYON_EXPORT uint64_t BitRev(uint64_t n);
 
+// Reverses the |bit_len| least significant bits of |x|.
 inline size_t ReverseBitsLen(size_t x, size_t bit_len) {
   return BitRev(x) >> (sizeof(size_t) * 8 - bit_len);
 }