Chapter 7: Control Flow

Overview

This chapter focuses on the translation and organization of control structures within a program—such as loops, conditionals, and branching—into intermediate code that preserves correct execution order.

You’ll learn how basic blocks and control flow graphs (CFGs) are constructed to simplify and analyze code execution paths. Each basic block is a straight-line sequence of statements with a single entry and exit point. These blocks are then connected into a CFG, enabling the compiler to reason about flow-of-control, optimize performance, and generate efficient target code.

The chapter explores key control flow constructs: if, if-else, while, do-while, and for. You'll see how each of these is translated into intermediate code using labels, conditional jumps, and backpatching techniques to manage unknown targets during code generation.

Finally, this chapter lays the foundation for later optimizations by segmenting code into flow regions that can be analyzed independently. Understanding control flow is essential for implementing data flow analysis and code optimization techniques.

🔹 Key Concepts from pp. 398–426

Basic Block
A sequence of consecutive statements with no branches in except to the first and no branches out except from the last.
Control Flow Graph (CFG)
A directed graph where each node represents a basic block and edges represent possible transfers of control.
Flow of Control Constructs
Includes if, if-else, while, do-while, and for; each has specific code generation patterns.
Backpatching
A method for generating jump addresses when the target location is unknown at the time of instruction creation.
Label Management
Labels are used as jump destinations in control structures. Managing them is essential for correct branching.
Flow Graph Construction
The process of identifying leaders and forming basic blocks to create a CFG for a given program fragment.

🧠 Key Terms to Know (pp. 398–426)

Basic Block: A straight-line code sequence with no internal jumps; treated as a unit for optimization.
Leader: The first instruction of a basic block, often marked by control flow change or jump target.
Control Flow Graph (CFG): A graph structure showing flow of execution between basic blocks.
Backpatching: Technique for handling forward jumps in generated code by deferring label resolution.
Conditional Jump: An intermediate code instruction that transfers control based on a boolean condition.
Label: A symbolic name for an address or line in intermediate code, used in jump and branch instructions.
Jump Instruction: Transfers control to another part of the code; may be conditional or unconditional.

📖 Required Reading

Read: Chapter 7 – Control Flow (pp. 398–426)
From: Compilers: Principles, Techniques, and Tools (2nd ed.) by Aho, Lam, Sethi, & Ullman (Pearson Education, 2006)