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: […]

Isolating a Free-Range Miscompilation

If we say that a compiler is buggy, we need to be able to back up that claim with reproducible, compelling, and understandable evidence. Usually, this evidence centers on a test case that triggers the buggy behavior; we’ll say something like “given this test case, compiler A produces an executable that prints 0 whereas compiler […]

A Month of Invalid GCC Bug Reports, and How to Eliminate Some of Them

During July 2016 the GCC developers marked 38 bug reports as INVALID. Here’s the full list. They fall into these (subjective) categories: 8 bug reports stemmed from undefined behavior in the test case (71753, 71780, 71803, 71813, 71885, 71955, 71957, 71746) 1 bug report was complaining about UB exploitation in general (71892) 15 bug reports […]