Gagallium

Instructors of “Introduction to programming” courses know that students are willing to blame the failures of their programs on anything. Sorting routine discards half of the data? “That might be a Windows virus!” Binary search always fails? “The Java compiler is acting funny today!” More experienced programmers know very well that the bug is generally in their code: occasionally in third-party libraries; very rarely in system libraries; exceedingly rarely in the compiler; and never in the processor. That’s what I thought too, until recently. Here is how I ran into a bug in Intel Skylake processors while trying to debug mysterious OCaml failures.