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电子代写|编译器代写Compilers代考|UVMCS202 Reducing the Number of Passes

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电子代写|编译器代写Compilers代考|Reducing the Number of Passes

Since each phase is a transformation on a stream of data representing an intermediate form of the source program, the reader may wonder how several phases can be combined into one pass without the reading and writing of intermediate files. In some cases one pass produces its output with little or no memory of prior inputs. Lexical analysis is typical. In this situation, a small buffer serves as the interface between passes. In other cases, we may merge phases into one pass by means of a technique known as “backpatching,” which is discussed in Section 7.8. In general terms, if the output of a phase cannot be determined without looking at the remainder of the phase’s input, the phase can generate output with “slots” which can be filled in later, after more of the input is read.

While we cannot give an example of backpatching as it pertains to compilers until we have described in some detail what the phases do, an example from assemblers will serve as a paradigm. An assembler might have a statement like
GOTO L
which precedes a statement with label L. A two-pass assembler uses its first pass to enter into its symbol table a list of all identifiers (statement labels and data names) together with the machine address (relative to the beginning of the program), to which these identifiers correspond. Then a second pass replaces mnemonic operation codes, such as GOTO, by their machine-language equivalent and replaces uses of identifiers by their machine addresses.

A one-pass assembler, on the other hand, could generate a skeleton of the GOTO machine instruction the first time it saw GOTO L. It could then append the machine address for this instruction to a list of instructions to be backpatched once the machine address for $\mathrm{L}$ is determined. For example, when the assembler encounters a statement such as
it scans the list of statements referring to $L$ and places the machine address for statement L: ADD X in the address field of each such instruction. Subsequent assembly instructions referring to $\mathrm{L}$ can have the value for $\mathrm{L}$ substituted immediately.

电子代写|编译器代写Compilers代考|Lexical Analysis

The lexical analyzer is the interface between the source program and the compiler. The lexical analyzer reads the source program one character at a time, carving the source program into a sequence of atomic units called tokens. Each token represents a sequence of characters that can be treated as a single logical entity. Identifiers, keywords, constants, operators, and punctuation symbols such as commas and parentheses are typical tokens. For example, in the FORTRAN statement
we find the following eight tokens: IF; (; 5; .EQ.; MAX; ); GOTO; 100 .
What is called a token depends on the language at hand and, to some extent, on the discretion of the compiler designer; but in general each token is a substring of the source program that is to be treated as a single unit. For example, it is not reasonable to treat $\mathrm{M}$ or MA (of the identifier MAX above) as an independent entity.

There are two kinds of token: specific strings such as IF or a semicolon, and classes of strings such as identifiers, constants, or labels. To handle both cases, we shall treat a token as a pair consisting of two parts: a token type and a token value. For convenience, a token consisting of a specific string such as a semicolon will be treated as having a type (the string itself) but no value. A token such as the identifier MAX, above, has a type “identifier'” and a value consisting of the string MAX. Frequently, we shall refer to the type or value as the token itself. Thus, when we talk about identifier being a token, we are referring to a token type; when we talk about MAX being a token, we are referring to a token whose value is MAX.

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GOTO L

MATLAB代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。