Non-nil means collect call-graph information when compiling.
This records which functions were called and from where.
If the value is t, compilation displays the call graph when it finishes.
If the value is neither t nor nil, compilation asks you whether to display
The call tree only lists functions called, not macros used. Those functions
which the byte-code interpreter knows about directly (eq, cons, etc.) are
The call tree also lists those functions which are not known to be called
(that is, to which no calls have been compiled). Functions which can be
invoked interactively are excluded from this list.