Speculative execution is an

optimization Mathematical optimization (alternatively spelled ''optimisation'') or mathematical programming is the selection of a best element, with regard to some criteria, from some set of available alternatives. It is generally divided into two subfiel ...

technique where a

computer system A computer is a machine that can be programmed to automatically carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic sets of operations known as ''programs'', wh ...

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 if extra

resources ''Resource'' refers to all the materials available in our environment which are Technology, technologically accessible, Economics, economically feasible and Culture, culturally Sustainability, sustainable and help us to satisfy our needs and want ...

are available. This approach is employed in a variety of areas, including

branch prediction In computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch (e.g., an if–then–else structure) will go before this is known definitively. The purpose of the branch predictor is to improve the flow ...

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 remembe ...

and files, and optimistic concurrency control in

database systems In computing, a database is an organized collection of Data (computing), data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, Application software, applications, and ...

Speculative multithreading Thread Level Speculation (TLS), also known as Speculative Multi-threading, or Speculative Parallelization, is a technique to speculatively execute a section of computer code that is anticipated to be executed later in parallel with the normal ex ...

is a special case of speculative execution.

Overview

Modern pipelined

microprocessor A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...

s use speculative execution to reduce the cost of

conditional branch A branch, jump or transfer 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 ''br ...

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, also called a ramus in botany, is a stem that grows off from another stem, or when structures like veins in leaves are divided into smaller veins. History and etymology In Old English, there are numerous words for branch, includ ...

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

. 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 predictor In computer architecture, a branch predictor is a digital circuit that tries to guess which way a branch (e.g., an if–then–else structure) will go before this is known definitively. The purpose of the branch predictor is to improve the flow ...

s and

memory dependence prediction Memory dependence prediction is a technique, employed by high-performance out-of-order execution microprocessors that execute memory access operations (loads and stores) out of program order, to predict true dependencies between loads and stores at ...

. A generalized form is sometimes referred to as value prediction.

Runahead

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, 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.

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 The Microarchitectural Data Sampling (MDS) vulnerabilities are a set of weaknesses in Intel x86 microprocessors that use hyper-threading, and leak data across protection boundaries that are architecturally supposed to be secure. The attacks e ...

* Spectre * SPOILER * Pacman

References

{{CPU technologies Instruction processing Concurrency (computer science)