Thread: Multi-stage translators

Multi-stage translators:-

In addition to being conceptually divided into stages, translators are often divided into passes; each with several stages may be combined or interleaved. Traditionally, states receive a source program, or the conclusion of the previous passage, will make some changes, and then writes the output of intermediate files, where it can be rescanned for the next pass.

These passes may be handled by different integrated parts of a single compiler, or they may be handled by running two or more separate programs. They may communicate by using their own specialized forms of intermediate language, they may communicate by making use of internal data structures (rather than files), or they may make several passes over the same original source code.

The number of passes used depends on a variety of factors. Certain languages require at least two passes to be made if code is to be generated easily - for example, those where declaration of identifiers may occur after the first reference to the identifier, or where properties associated with an identifier cannot be readily deduced from the context in which it first appears. A multi-pass compiler can often save space. Although modern computers are usually blessed with far more memory than their predecessors of only a few years back, multiple passes may be an important consideration if one wishes to translate complicated languages within the confines of small systems. Multi-pass compilers may also allow for better provision of code optimization, error reporting and error handling. Lastly, they lend themselves to team development, with different members of the team assuming responsibility for different passes. However, multi-pass compilers are usually slower than single-pass ones and their probable need to keep track of several files make them slightly uncomfortable to write and to use. Compromises at the design stage often result in languages that are well suited to single-pass compilation.

In practice, considerable use is made of two-stage translators in which the first stage is a high-level translator that converts the source program into ASSEMBLER, or even into some other relatively high-level language for which an efficient translator already exists. The compilation process would then be depicted as in Figure (given below) - our example shows a Modula-3 program being prepared for execution on a machine that has a Modula-3 to C converter:



It is increasingly common to find compilers for high-level languages that have been implemented using C, and which themselves produce C code as output. The success of these is based on the premises that "all modern computers come equipped with a C compiler" and "source code written in C is truly portable". Neither premise is, unfortunately, completely true. However, compilers written in this way are as close to achieving the dream of themselves being portable as any that exist at the present time.