Within each procedure, the tree nodes and instructions are assigned
unique ID numbers. These numbers can be used to identify the nodes and
instructions internally and in annotations that are written to the
output files. The number method retrieves the ID numbers. It is
generally best to avoid using the numbers on tree_instr nodes,
because they are not written to the output files and the
tree_instr nodes are automatically rearranged when converting
between expression trees and flat lists of instructions. Instead, use
the ID numbers for the corresponding instructions.
The number_instrs method in the tree_proc class assigns ID
numbers to tree nodes and instructions that do yet have numbers (i.e.
those that were created since the procedure was read in). This method
is called automatically before each procedure is written out, but you
may also call it whenever ID numbers are needed for new tree nodes or
instructions. The next unused ID number is recorded in the procedure
symbol table. In the proc_symtab class, the
next_instr_num method returns the next ID number and increments
the counter; the instr_num method returns the next number but
does not increment the counter. Since you can only assign new ID
numbers with the number_instrs method, you will rarely, if ever,
need to access this counter directly.
In some circumstances, the tree node and instruction ID numbers must be
reset. The most common case is when moving code from one procedure to
another. Since the ID numbers are only unique within each procedure,
moving things between procedures is likely to create duplicate ID
numbers. To avoid this problem, the tree_node,
tree_node_list, and instruction classes provide
clear_numbers methods to reset the ID numbers. These methods
recursively descend the ASTs and expression trees to set all of the ID
numbers within the object and its children to zero.