Do Expressive Programming Languages Always Have Undefined Behavior?

In the Hacker News comments on one of my previous posts about undefined behavior, someone said this: AFAIK Gödel’s incompleteness theorems imply that _any_ language will have at least some undefined behaviour. Let’s take a quick look at this statement, keeping in mind that incompleteness and undecidability can be remarkably tricky topics. Some years ago […]

Undefined Behavior != Unsafe Programming

Undefined behavior (UB) in C and C++ is a clear and present danger to developers, especially when they are writing code that will execute near a trust boundary. A less well-known kind of undefined behavior exists in the intermediate representation (IR) for most optimizing, ahead-of-time compilers. For example, LLVM IR has undef and poison in […]

Detecting Strict Aliasing Violations in the Wild

Type-based alias analysis, where pointers to different types are assumed to point to distinct objects, gives compilers a simple and effective way to disambiguate memory references in order to generate better code. Unfortunately, C and C++ make it easy for programmers to violate the assumptions upon which type-based alias analysis is built. “Strict aliasing” refers […]

Principles for Undefined Behavior in Programming Language Design

I’ve had a post with this title on the back burner for years but I was never quite convinced that it would say anything I haven’t said before. Last night I watched Chandler Carruth’s talk about undefined behavior at CppCon 2016 and it is good material and he says it better than I think I […]

Advanced Compilers Weeks 3-5

This continues a previous post. We went through the lattice theory and introduction to dataflow analysis parts of SPA. I consider this extremely good and important material, but I’m afraid that the students looked pretty bored. It may be the case that this material is best approached by first looking at practical aspects and only […]

Solutions to Integer Overflow

Humans are typically not very good at reasoning about integers with limited range, whereas computers fundamentally work with limited-range numbers. This impedance mismatch has been the source of a lot of bugs over the last 50 years. The solution comes in multiple parts. In most programming languages, the default integer type should be a bignum: […]