第7章高水準プログラミング言語

By the end of this section, you will be able to:

• Discuss how to build and run programs written in various HLLs

• Describe the work of an HLL runtime management implementation

• List and explain various HLL optimization methods applicable to programs

To implement programs that you create, you must use a process to generate machine code from source code. As previously discussed, the major methods of implementing programming languages are compilation, pure interpretation, and hybrid implementation. These are complex processes best learned in stages. There are differences between a compiler and an interpreter, as shown in Table 7.9.

The single process that takes the source code the programmer creates and transforms it into another language is called compilation. In short, it is the rendering from one language into another and includes a deconstruction of the input, and its product may be machine language. Modern compilation usually starts with preprocessing, a step in which the source code is manipulated to ready it for the compilation stage. Some of the actions the preprocessor can take are as follows:

• Removing any comments and white space

• Executing a preprocessor directive, which describes resources the program needs to compile, usually files to insert into the source code. The C/C++ preprocessor provides a fine example.

1. The #include directive of C/C++ deals with a variety of files such as header or standard files containing the definitions of objects and functions from the API that are going to be used in the program. It can also take care of user-defined files or modules.

2. The #define directive of C/C++ names a piece of code for the preprocessor to replace every time it sees the name.

3. The #ifdef directive of C/C++ tells the compiler to include a specified piece of code in compilation depending upon some condition.

• Other tasks include pre-identifying high-level constructs such as loops and functions.

The phases of compilation are grouped into three stages that are usually executed in three passes through the preprocessed source code:

• front end that is responsible for the analysis of source code

• middle end that is responsible for performing optimizations of the code

• back end (or code generator) that is responsible for the production of the target code

The front end is responsible for analyzing the source code for syntax, semantics, and other tasks. The first part of this process is lexical analysis, which converts the source code strings of characters into tokens. These are sequences of compiler recognizable atomic symbols which make it much easier for the next steps to recognize them.

The process proceeds to syntax analysis, usually referred to as parsing or reading the code to make sure it conforms with the syntax rules of the language. This is done to test conformability logically by breaking code into separate parts.

The next step is semantic analysis, which is the discovery of meaning in the code. It takes care of type checking in strongly typed languages. It also performs binding, which associates variable and function references with their declarations and definitions. The compiler runs a static semantic analysis which concerns itself only with the source code without testing inputs.

Semantic analysis cannot figure out all meaning in the code. Many languages also use dynamic binding (late binding) that defers the binding of certain elements such as objects and methods until runtime. This is particularly true of OOP languages which support inheritance and polymorphism. Some processes, such as when an array goes out of bounds, will not be known until run time, and are part of the program’s dynamic semantics.

Semantic analysis concludes with the creation and management of the symbol table, a data structure that is used to connect every symbol with necessary information such as data type, scope, and memory location.