第7章高水準プログラミング言語

The main job of the middle-end stage of compilation is optimization, producing what will be needed to generate the fastest and most efficiently running code as possible. The attributes which are usually optimized are execution time and space (memory usage). Figure 7.24 outlines this process.

The result created after semantic analysis when the program passes all checks is called intermediate form (IF). The nature of the IF is often chosen for machine independence and ease of optimization. IFs usually resemble the machine code for some imaginary idealized machine. An example is managed code which is targeted to run under a particular runtime environment. This is true of C# and other Microsoft.net languages. They are designed to run under its CLR, which manages the execution of Microsoft’s .NET languages. In this case, the purpose is to allow code from the different languages to work well together at execution time.

There is a growing need to process massive amounts of “big” data to gather insights in real time about the best way a business can serve their customers. This is called data analytics, the systematic computational analysis of data or statistics. Netflix uses this technique as a means with which to plan its targeted advertising. Basically, Netflix monitors which types of movies its customers typically watch and collects related data that it uses to bring in new movies and advertise them to its customers. This has led to an increased use of concurrent/parallel processing frameworks that allow concurrent programming. The popularity has caused languages like Python to get a lot of attention for big data analysis. Do some research on Python and provide a well-documented opinion on the features of Python that support this.

The back-end process is known as code generation, the transformation of the code to the target or object language. It uses the intermediate form of the code in combination with the symbol table to carry this out. As previously stated, the final produced code is not necessarily machine language. In hybrid implementations, such as in Java and others, the code will still be a type of intermediate form and it is left for another process to turn it into machine code, usually at runtime.

Pure compilation code generation generally produces this phase (and all compile phases) while also relying on the symbol table, which is the structure responsible for following identifiers within a program and tracking the work of the compiler. Once compilation is complete, the debugger may retain the symbol table.

The back-end phase of the compiler may also perform machine-independent code generation. The purpose of this is to allow a piece of source code to generate instructions that will run on different platforms without change such as Windows and Android. Java is a great example of this, as referenced in Figure 7.25.

This can then be run in any environment for which a JVM has been built.

In some languages, compilers are present, but they aren’t pure and only perform a selective compilation of pieces, or pre-processing, of the remaining source. Occasionally the compiler generates code that builds expectations around decisions that won’t be completed until runtime. If these assumptions are sound, the code runs very fast. If not, a dynamic check reverts to the interpreter.

Sometimes a program delays compilation until the last minute. In Java, a bytecode is a set of instructions for a virtual machine. The process of just-in-time (JIT) compilation is when intermediate code, which is the language translation phase that produces a code level between the source code and the object code, gets its final compilation (or usually interpretation) phase right at the start of runtime. Bytecode is the standard format for distribution of Java programs to any runtime platform such as Windows, macOS, and Linux. The bytecode is interpreted by a Java virtual machine (JVM) that has been implemented by the producer specifically for their platform. Since there is a JVM for macOS but not for Mac IOS, Java is incompatible with iPhones.