Code generation

From Academic Kids

In computer science, code generation is the process by which a compiler converts a syntactically-correct program into a series of instructions that could be executed by a machine. The input to the code generation stage typically consists of a parse tree, abstract syntax tree, or intermediate language code; the target machine may be a physical machine such as a microprocessor, or an abstract machine such as a virtual machine or an intermediate language. Sophisticated compilers may use several cascaded code generation stages to fully compile code; this is due to the fact that algorithms for code optimization are more readily applicable in an intermediate code form, and also facilitates a single compiler that can target multiple architectures as only the final code generation stage (the backend) would need to change from target to target.

In a more general sense, code generation is used to produce programs in some automatic manner, reducing the need for human programmers to write code manually. Code generations can be done either at runtime, including load time, or compiler time. Just-in-time compilers are an example of a code generator that produce native or nearly native code from byte-code or the like when programs are loaded onto the compilers. On the other hand, a compiler-compiler, (yacc, for example) almost always generates code at compiler time. A preprocessor is an example of the simplest code generator, which produces target code from the source code by replacing predefined keywords.

When code generation occurs at runtime, it is important that it is efficient in space and time. For example, when regular expressions are interpreted and used to generate code at runtime, a non-determistic FSA instead of deterministic one is often generated because usually the former can be created more quickly and occupies less memory space than the latter. Despite it generally generating less efficient code, a code generation at runtime can take the advantage of being done at runtime. Some people cites this fact to note that a JIT compiler can generate more efficient code than a compiler invoked before runtime, since it is more knowledgeable about the context and the execution path of the program than when the compiler generates code at compile time.

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