+
+/* Immediate use lists are used to directly access all uses for an SSA
+ name and get pointers to the statement for each use.
+
+ The structure ssa_use_operand_d consists of PREV and NEXT pointers
+ to maintain the list. A USE pointer, which points to address where
+ the use is located and a LOC pointer which can point to the
+ statement where the use is located, or, in the case of the root
+ node, it points to the SSA name itself.
+
+ The list is anchored by an occurrence of ssa_operand_d *in* the
+ ssa_name node itself (named 'imm_uses'). This node is uniquely
+ identified by having a NULL USE pointer. and the LOC pointer
+ pointing back to the ssa_name node itself. This node forms the
+ base for a circular list, and initially this is the only node in
+ the list.
+
+ Fast iteration allows each use to be examined, but does not allow
+ any modifications to the uses or stmts.
+
+ Normal iteration allows insertion, deletion, and modification. the
+ iterator manages this by inserting a marker node into the list
+ immediately before the node currently being examined in the list.
+ this marker node is uniquely identified by having null stmt *and* a
+ null use pointer.
+
+ When iterating to the next use, the iteration routines check to see
+ if the node after the marker has changed. if it has, then the node
+ following the marker is now the next one to be visited. if not, the
+ marker node is moved past that node in the list (visualize it as
+ bumping the marker node through the list). this continues until
+ the marker node is moved to the original anchor position. the
+ marker node is then removed from the list.
+
+ If iteration is halted early, the marker node must be removed from
+ the list before continuing. */