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Backpatching1
1. Backpatching
Backpatching is the process of filling up unspecified
information of labels using appropriate semantic
actions during the code generation process.
• Need for Backpatching: During the generation of 3-address
code in single pass is that may not know the address(label)
where program control should go
• A number of jumping statements is generated with the target
labels is temporarily left unspecified.
2. Cont.
• Backpatching can be used to generate a program for boolean expressions
and the flow of control statements in one pass.
• While generating three address codes for the given expression, it can
specify the address of the Label in goto statements. It is very difficult to
assign locations of these label statements in one pass so, two passes are
used.
• In the first pass, it can leave these addresses unspecified & in the next pass,
and it can fill these addresses. Therefore filling of incomplete
transformation is called Backpatching.
3. Backpatching Algorithm
Backpatching technique is incorporated using three functions.
• Makelist (i) :This is used to create a new list containing three-address location i,
and it returns a pointer to the list. This is the first function which is created to form
a true / false list.
• Merge (p1, p2) – This function concatenates lists pointed to by p1 and p2, returns a
pointer to the concatenated list. This is used to assign the same true / false labels to
more than one address.
• Backpatch (p, i) – This function is used to insert ‘i' as the target label for each of the
statements in the list pointed to by p. Using the information provided by this
function labels are attached to all the statements.
