What is Speculative Execution?

by Joel Hruska

As discussion of the Spectre and Meltdown bugs dominates the tech news cycle, there’s been repeated reference to a specific feature of high-end CPUs: speculative execution. It’s a key capability of higher-end ARM products, Apple’s custom ARM cores, IBM’s POWER family, and the vast majority of the x86 processors produced by Intel and AMD. Here’s what speculative execution is and how it relates to other key capabilities of modern microprocessors, and how the recent Meltdown bug targets Intel CPUs in particular.

Speculative execution is a technique CPU designers use to improve CPU performance. It’s one of three components of out-of-order execution, also known as dynamic execution. Along with multiple branch prediction (used to predict the instructions most likely to be needed in the near future) and dataflow analysis (used to align instructions for optimal execution, as opposed to executing them in the order they came in), speculative execution delivered a dramatic performance improvement over previous Intel processors.

Here’s how it works. Modern CPUs are all pipelined, which means they’re capable of executing multiple instructions in parallel, as shown in the diagram below.