Backpatching1

Backpatching

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.

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