For simple programs such as hello.c, we can rely on the compilation system to produce correct and efficient machine code. However, there are some important reasons why programmers need to understand how compilation systems work:
- Optimizing program performance. Modern compilers are sophisticated tools that usually produce good code. As programmers, we do not need to know the inner workings of the compiler in order to write efficient code. However, in order to make good coding decisions in our C programs, we do need a basic understanding of assembly language and how the compiler translates different C statements into assembly language. For example, is a switch statement always more efficient than a sequence of if then-else statements? Just how expensive is a function call? Is a while loop more efficient than a do loop? Are pointer references more efficient than array indexes? Why does our loop run so much faster if we sum into a local variable instead of an argument that is passed by reference Why do two functionally equivalent loops have such different running times? In above posts we will introduce the Intel IA32 machine language and describe how compilers translate different C constructs into that language. In above posts we will learn how to tune the performance of our C programs by making simple transformations to the C code that help the compiler do its job. And also , we will learn about the hierarchical nature of the memory system, how C compilers store data arrays in memory, and how our C programs can exploit this knowledge to run more efficiently.
- Understanding link-time errors. In our experience, some of the most perplexing programming errors are related to the operation of the linker, especially when are trying to build large software systems. For example, what does it mean when the linker reports that it cannot resolve a reference? What is the difference between a static variable and a global variable? What happens if we define two global variables in different C files with the same name? What is the difference between a static library and a dynamic library? Why does it matter what order we list libraries on the command line? And scariest of all, why do some linker-related errors not appear until run-time? We will learn the answers to these kinds of questions in above posts.
- Avoiding security holes. For many years now, buffer overflow bugs have accounted for the majority of security holes in network and Internet servers. These bugs exist because too many programmers are ignorant of the stack discipline that compilers use to generate code for functions. We will describe the stack discipline and buffer overflow bugs in above posts, as part of our study of assembly language.