Generating code for expressions

The easiest code to generate is straight-line code consisting of statements and expressions that execute in sequence with no control flow. As described earlier in this chapter, there are two attributes to compute for each node: the attribute for where to find an expression's value is called addr, while the intermediate code necessary to compute its value is called icode. The values to be computed for these attributes for a subset of the Jzero expression grammar are shown in the following table. The ||| operator refers to list concatenation:

Figure 9.4 – Semantic rules for expressions

The main intermediate code generation algorithm is a bottom-up post-order traversal of the syntax tree. To present it in small chunks, the traversal is broken into the main method, gencode(), and helper methods for each non-terminal. In Unicon, the gencode() method in tree.icn looks as follows:

method gencode()
  every (...