is designed around the idea of speculative execution. A 2003 PhD thesis made GHC support a kind of speculative execution with an abortion mechanism to back out in case of a bad choice called optimistic execution. It was deemed too complicated. Security vulnerabilities Starting in 2017, a series of security vulnerabilities were found in the implementations of speculative execution on common processor architectures which effectively enabled an elevation of privileges. These include: Foreshadow Meltdown Microarchitectural Data Sampling Spectre SPOILER Pacman See also Branch predictor Out-of-order execution Slipstream (computer science) Speculative multithreading Hardware security bug Transient execution CPU vulnerabilities References Instruction processing Concurrency (computer science)