In
computer programming
Computer programming or coding is the composition of sequences of instructions, called computer program, programs, that computers can follow to perform tasks. It involves designing and implementing algorithms, step-by-step specifications of proc ...
, a dead store is a
local variable
In computer science, a local variable is a variable that is given ''local scope''. A local variable reference in the function or block in which it is declared overrides the same variable name in the larger scope. In programming languages with ...
that is assigned a
value but is read by no following
instruction. Dead stores waste
processor time and
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 may be detected through the use of
static program analysis
In computer science, static program analysis (also known as static analysis or static simulation) is the analysis of computer programs performed without executing them, in contrast with dynamic program analysis, which is performed on programs duri ...
, and removed by an
optimizing compiler
An optimizing compiler is a compiler designed to generate code that is optimized in aspects such as minimizing program execution time, memory usage, storage size, and power consumption. Optimization is generally implemented as a sequence of op ...
.
If the purpose of a store is intentionally to overwrite data, for example when a
password
A password, sometimes called a passcode, is secret data, typically a string of characters, usually used to confirm a user's identity. Traditionally, passwords were expected to be memorized, but the large number of password-protected services t ...
is being removed from memory, dead store
optimizations can cause the write not to happen, leading to a security issue. Some system
libraries
A library is a collection of Book, books, and possibly other Document, materials and Media (communication), media, that is accessible for use by its members and members of allied institutions. Libraries provide physical (hard copies) or electron ...
have specific functions designed to avoid such dangerous optimizations, e.g.
explicit_bzero on
OpenBSD
OpenBSD is a security-focused operating system, security-focused, free software, Unix-like operating system based on the Berkeley Software Distribution (BSD). Theo de Raadt created OpenBSD in 1995 by fork (software development), forking NetBSD ...
.
Examples
Java
Dead store example in
Java
Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...
:
// DeadStoreExample.java
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class DeadStoreExample
In the above code an
ArrayList object was instantiated but never used. Instead, in the next line the variable which references it is set to point to a different object. The
ArrayList which was created when
list was declared will now need to be de-allocated, for instance by a
garbage collector
A waste collector, also known as a garbage man, garbage collector, trashman (in the U.S), binman or dustman (in the UK), is a person employed by a public or private enterprise to collect and dispose of municipal solid waste (refuse) and recycla ...
.
JavaScript
Dead store example in
JavaScript
JavaScript (), often abbreviated as JS, is a programming language and core technology of the World Wide Web, alongside HTML and CSS. Ninety-nine percent of websites use JavaScript on the client side for webpage behavior.
Web browsers have ...
:
function func(a, b)
The code in the loop repeatedly overwrites the same variable, so it can be reduced to only one call.
See also
*
Dead code
The term dead code has multiple definitions. Some use the term to refer to code (i.e. instructions in memory) which can never be executed at run-time.
In some areas of computer programming, dead code is a section in the source code of a program whi ...
*
Unreachable code
In computer programming, unreachable code is part of the source code of a program which can never be executed because there exists no control flow path to the code from the rest of the program.
Unreachable code is sometimes also called ''dead code ...
References
{{Compiler optimizations
Compiler optimizations
Software anomalies
Programming language comparisons
Articles with example Java code
Articles with example JavaScript code