The discussion of phases deals with the logical organization of a compiler. In

an implementation, activities from several phases may be grouped together

into a pass that reads an input file and writes an output file. For example,

the front-end phases of lexical analysis, syntax analysis, semantic analysis, and

intermediate code generation might be grouped together into one pass. Code

optimization might be an optional pass. Then there could be a back-end pass

consisting of code generation for a particular target machine。

就不翻译了。这里的pass就像海关关口一样，一入一出的gate一样。