Compilers: Principles, Techniques, and Tools, Updated 2nd Edition by Pearson

CHAPTER 13

Undefined Behaviour Semantics

Contributed by: Sorav Bansal, Indian Institute of Technology Delhi

13.1 Introduction

What is Undefined Behaviour or UB, and why do we need to discuss UB semantics? Let’s look at a few examples to understand this better. Consider a source expression in a high-level language like C, “x = y/z”. Now consider two machines with different ISAs (Instruction Set Architectures) and let’s assume that “x = divide y, z” is a common instruction on both ISAs that implements “x = y/z” in the assembly code. There is a subtle difference between the two ISAs however, in how they handle division-by-zero, i.e., the case when “z” evaluates to zero: on one machine, a division by zero causes a hardware exception (eventually ...