auto-save-list-file-prefix is a variable defined in `startup.el'.
Its value is

Prefix for generating `auto-save-list-file-name'.
This is used after reading your init file to initialize
`auto-save-list-file-name', by appending Emacs's pid and the system name,
if you have not already set `auto-save-list-file-name' yourself.
Directories in the prefix will be created if necessary.
Set this to nil if you want to prevent `auto-save-list-file-name'
from being initialized.

You can customize this variable.