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CHAPTER 5

Structured programming

Before IBM released FORTRAN in 1957, almost all programming was done in assembly

language. Part of the reason for this is that nobody knew how to design a good high-level lan-

guage, nor did they know how to write a compiler to generate efﬁcient code. Early attempts at

high-level languages resulted in languages that were not well structured, difﬁcult to read, and

difﬁcult to debug. The ﬁrst release of FORTRAN was not a particularly elegant language by

today’s standards, but it did generate efﬁcient machine code.

In the 1960s, a new paradigm for designing high-level languages emerged. This new paradigm

emphasized grouping program statements into blocks of code that execute from beginning

to end. These basic blocks have only one entry point and one exit point. Control of which

basic blocks are executed, and in what order, is accomplished with highly structured ﬂow con-

trol statements. The structured program theorem provides the theoretical basis of structured

programming. It states that there are three ways of combining basic blocks: sequencing, selec-

tion, and iteration. These three mechanisms are sufﬁcient to express any computable function.

It has been proven that all programs can be written using only basic blocks, the pre-test loop,

and if-then-else structure. Although most high level languages provide additional ﬂow con-

trol statements for the convenience of the programmer, they are just “syntactical sugar.” Other

structured programming concepts include well-formed functions and procedures, pass-by-

reference and pass-by-value, separate compilation, and information hiding.

These structured programming languages enabled programmers to become much more pro-

ductive. Well-written programs that adhere to structured programming principles are much

easier to write, understand, debug, and maintain. Most successful high-level languages are de-

signed to enforce, or at least facilitate, good programming techniques. This is not generally

true for assembly language. The burden of writing well-structured code lies with the program-

mer, and not the language.

The best assembly programmers rely heavily on structured programming concepts. Failure to

do so results in code that contains unnecessary branch instructions and, in the worst cases, re-

sults in something called spaghetti code. Consider a code listing where a line has been drawn

from each branch instruction to its destination. If the result looks like someone spilled a plate

of spaghetti on the page, then the listing is spaghetti code. If a program is spaghetti code, then

the ﬂow of control is difﬁcult to follow. Spaghetti code is much more likely to have bugs, and

ARM 64-Bit Assembly Language

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CHAPTER 5

Structured programming

在1957年IBM发布FORTRAN之前，几乎所有的编程都是用汇编语言完成的。部分原因

在于当时没有人知道如何设计一个好的高级语言，也不知道如何编写一个能够生成高

效代码的编译器。早期的高级语言尝试导致了结构不清晰、难以阅读和调试的语言。F

ORTRAN的首次发布按照今天的标准来看并不是一种特别优雅的语言，但它 did 生成

高效的机器代码。

在1960年代，出现了一种设计高级语言的新范式。这种新范式强调将程序语句分组到

从头到尾执行的代码块中。这些basic blocks只有一个入口点和一个出口点。通过高度

结构化的ﬂow con- trol语句来控制哪些基本块被执行，以及执行的顺序。结构化程序定

理提供了结构化编程的理论基础。它指出，组合基本块有三种方式：顺序、选择和迭

代。这三种机制足以表达任何可计算函数。已经证明，所有程序都可以仅使用基本块

、pre-test loop和if-then-else结构来编写。尽管大多数高级语言为程序员的便利提供了额

外的流程控制语句，但它们只是“语法糖”。其他结构化编程概念包括结构良好的函

数和过程、按引用传递和按值传递、单独编译和信息隐藏。

这些结构化编程语言使程序员的生产力大大提高。遵循结构化编程原则编写良好的程

序更容易编写、理解、调试和维护。大多数成功的高级语言都旨在强制执行或至少促

进良好的编程技术。对于汇编语言来说，这通常不是真的。编写结构良好的代码的责

任在于程序员，而不是语言。

最好的汇编程序员严重依赖结构化编程概念。不这样做会导致代码中包含不必要的分

支指令，在最坏的情况下，会导致所谓的spaghetti code。考虑一个代码列表，其中从

每个分支指令到其目的地都画了一条线。如果结果看起来像是有人在页面上洒了一盘

意大利面，那么这个列表就是意大利面代码。如果一个程序是意大利面代码，那么控

制流就很难跟踪。意大利面代码更容易出现错误，

ARM 64-Bit Assembly Language

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Structured programming 115

Listing 5.2 if statement in AArch64 assembly.

2 if: cmp x0, x1 // perform test

3 blt endif // if (x < y) goto endif

// if statement body

5 endif:

clarity, to show where the if statement begins, and the second label, endif: is the end of the

if statement. The condition in assembly is reversed from the C code. The C code checks if x

>= y to know if the if statement should be taken. In AArch64 assembly, the opposite check is

used with the instruction blt.Ifx<y, then the if statement is not executed. Instead, control

branches past the if statement.

5.2.2 If-then-else statement

Most high-level languages provide an extension to the basic if statement, which allows the

programmer to select one of two possible blocks of code to execute. The same behavior can

be achieved by using two sequential if statements, with logically opposite tests. However

the if-then-else construct results in code that is more efﬁcient as well as easier to read,

understand, and debug.

5.2.2.1 Using branch instructions

Listing 5.3 shows a typical if-then-else statement in C. Listing 5.4 shows the AArch64

code equivalent, using branch instructions. Note that this method requires a conditional

branch, an unconditional branch, and two labels. This is the most common method of writ-

ing the bodies of the then and else selection constructs.

5.2.2.2 Using conditional selection

Branching is not the only way to write an if statement. AArch64 has selection instructions

that are able to choose between two source registers based on a condition code. These instruc-

tions can also optionally increment, set, or negate one of the choices.

Listing 5.5 shows the AArch64 code equivalent to Listing 5.3, using conditional operations.

Section 4.2.10 covers the eleven conditional selection instructions in AArch64. While they

cannot always be used to replace branch-based if statements, in certain examples such as

Listing 5.5, they can be used to accomplish the same task without using any branch instruc-

tions. In this and similar cases, the code is more efﬁcient than using a branch.

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