COMPUTING SCIENCE 370
Programming Languages

LISP -- Storage Reclamation

Cell Allocation

• Available LISP cells are kept in a free list:

  freeList

  --->

  ·-

  --->

  ·-

  --->

  . . .

• cons and other functions obtain new cells from the free list.

Cell Replacement

Cell Replacement
Explicit	Automatic
	Reference counts	Garbage collection

• Explicit disposal

  E.g.,

  • dispose(p) in Pascal
  • DEALLOCATE(p) in Modula-2

  -	A bother for the user.
  -	Not secure.

  E.g., a Pascal programmer can gain access to the free list:
  bList := aList;

  bList	--->	|    ·		freeList	--->	·-	--->	. . .
  aList	--->
  		|   v

  dispose( aList );

  bList	--->	·	--->	·-	--->	. . .
  freeList	--->

• Reference counts

  • Each cell contains a reference count indicating the number of references to it.

    aList ---->	|    2   ·-	--->	|    1   ·-	--->	|    2   ·
    	|     ^  v     |  a     |		|  v  b		|     ^  v     |  c     |
    	|				|
    bList ---->	|    1    |		cList	--->	|    1    |
    	|  v  d				|  v  e

  • When a reference is created, the reference count is incremented.

  • When a reference is destroyed, the reference count is decremented.

  • When the reference count becomes zero, its two references (car and cdr) are destroyed and the cell is placed on the free list.

    • This may result in other cells having zero references.

  PROBLEM: cyclic structures may not be reclaimed.

  	|-------	-------------		---|
  	v			|
  aList ---->	|    2   ·-	--->	|    1   ·-	---|
  	|  v  a		|  v  b

  • The reference counts of both cells in this circular list become 1 if the binding to aList is broken, but both are inaccessible.

    	|-------	-------------		---|
    	v			|
    aList --X-->	|    1   ·-	--->	|    1   ·-	---|
    	|  v  a		|  v  b

  • This situation can occur when rplaca and rplacd are used.

• Garbage collection

  • Inaccessible cells are abandoned, and when the free list is exhausted, a garbage collection occurs to replenish the free list.

  • Two phases:

    1. Mark accessible cells.

       • preorder traversal of a forest

       for each root r, mark( r )
       
       mark( r )
          if r is not marked then
             set mark bit of r
             mark( left( r ))
             mark( right( r ))
          end if
       

    2. Sweep unmarked cells onto the free list.

       for each cell
          if cell is marked then
             reset mark bit of cell
          else
             link cell onto free list
       

  • Usually implemented using a table containing one bit for each cons cell available to the interpreter, rather than using a special bit in each cell.

  PROBLEM: long unexpected delays.

  SOLUTIONS: incremental garbage collection, generational scavenging, . . .