Speculative execution is an optimization technique where a computer system performs some task that may not be needed. Work is done before it is known whether it is actually needed, so as to prevent a delay that would have to be incurred by doing the work after it is known that it is needed. If it turns out the work was not needed after all, most changes made by the work are reverted and the results are ignored. The objective is to provide more

concurrency Concurrent means happening at the same time. Concurrency, concurrent, or concurrence may refer to: Law * Concurrence, in jurisprudence, the need to prove both ''actus reus'' and ''mens rea'' * Concurring opinion (also called a "concurrence"), a ...

if extra resources are available. This approach is employed in a variety of areas, including branch prediction in pipelined processors, value prediction for exploiting value locality, prefetching

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remembered ...

and

files File or filing may refer to: Mechanical tools and processes * File (tool), a tool used to ''remove'' fine amounts of material from a workpiece **Filing (metalworking), a material removal process in manufacturing ** Nail file, a tool used to gent ...

, and optimistic concurrency control in database systems.Lazy and Speculative Execution
Butler Lampson Microsoft Research OPODIS, Bordeaux, France 12 December 2006 Speculative multithreading is a special case of speculative execution.

Overview

Modern pipelined

microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circu ...

s use speculative execution to reduce the cost of

conditional branch A branch is an instruction in a computer program that can cause a computer to begin executing a different instruction sequence and thus deviate from its default behavior of executing instructions in order. ''Branch'' (or ''branching'', ''branc ...

instructions using schemes that predict the execution path of a program based on the history of branch executions. In order to improve performance and utilization of computer resources, instructions can be scheduled at a time when it has not yet been determined that the instructions will need to be executed, ahead of a

branch A branch, sometimes called a ramus in botany, is a woody structural member connected to the central trunk of a tree (or sometimes a shrub). Large branches are known as boughs and small branches are known as twigs. The term '' twig'' usually ...

Variants

''Speculative computation'' was a related earlier concept.

Eager execution

Eager execution is a form of speculative execution where both sides of the conditional branch are executed; however, the results are committed only if the predicate is true. With unlimited resources, eager execution (also known as ''oracle execution'') would in theory provide the same performance as perfect branch prediction. With limited resources, eager execution should be employed carefully, since the number of resources needed grows exponentially with each level of branch executed eagerly.

Predictive execution

Predictive execution is a form of speculative execution where some outcome is predicted and execution proceeds along the predicted path until the actual result is known. If the prediction is true, the predicted execution is allowed to commit; however, if there is a misprediction, execution has to be unrolled and re-executed. Common forms of this include branch predictors and memory dependence prediction. A generalized form is sometimes referred to as value prediction.

Related concepts

Lazy execution

Lazy execution is the opposite of eager execution, and does not involve speculation. The incorporation of speculative execution into implementations of the Haskell programming language, a lazy language, is a current research topic.

Eager Haskell Haskell () is a general-purpose, statically-typed, purely functional programming language with type inference and lazy evaluation. Designed for teaching, research and industrial applications, Haskell has pioneered a number of programming lang ...

, a variant of the language, 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.https://mail.haskell.org/pipermail/haskell/2006-August/018424.html

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 Privilege may refer to: Arts and entertainment * ''Privilege'' (film), a 1967 film directed by Peter Watkins * ''Privilege'' (Ivor Cutler album), 1983 * ''Privilege'' (Television Personalities album), 1990 * ''Privilege (Abridged)'', an alb ...

. These include: * Foreshadow *

Meltdown Meltdown may refer to: Science and technology * Nuclear meltdown, a severe nuclear reactor accident * Meltdown (security vulnerability), affecting computer processors * Mutational meltdown, in population genetics Arts and entertainment Music * ...

* Microarchitectural Data Sampling * Spectre * SPOILER * Pacman

References

{{CPU technologies Instruction processing Concurrency (computer science)