Which figure of speech is used in this text?
Hello darkness, my old friend
I've come to talk with you again
Because a vision softly creeping
Left its seeds while I was sleeping
—Paul Simon, "The Sound of Silence"

 - apostrophe
 - anaphora
The text uses apostrophe, a direct address to an absent person or a nonhuman entity.
Hello darkness, my old friend is a direct address to the darkness, a nonhuman entity.