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Backpatching

In order to compile code for branch instructions, it is necessary to know the address in memory (relative to the location counter) of the destination of the branch. Branch instructions may be PC-relative or have absolute addresses. For forward branches the destination address will not be known when the branch is encountered. There are two approaches to this problem:

  1. Two passes. The first pass through the code generator computes addresses but does not generate code. The second pass generates code and uses the addresses determined in the first pass. This approach takes longer.

  2. Backpatching. The generated code itself is used to hold a linked list of places where forward reference addresses are needed. When the address is found, it is patched back into the generated code.