In compiler theory, copy propagation is the process of replacing the occurrences of targets of direct assignments with their values. [1] A direct assignment is an instruction of the form x = y
, which simply assigns the value of y
to x
.
From the following code:
y = x z = 3 + y
Copy propagation would yield:
z = 3 + x
Copy propagation often makes use of reaching definitions, use-def chains and def-use chains when computing which occurrences of the target may be safely replaced. If all upwards exposed uses of the target may be safely modified, the assignment operation may be eliminated.
Copy propagation is a useful "clean up" optimization frequently used after other compiler passes have already been run. Some optimizations—such as classical implementations of elimination of common sub expressions [1] —require that copy propagation be run afterwards in order to achieve an increase in efficiency.
An optimizing compiler is a compiler designed to generate code that is optimized in aspects such as minimizing program execution time, memory use, storage size, and power consumption. Optimization is generally implemented as a sequence of optimizing transformations, algorithms that transform code to produce semantically equivalent code optimized for some aspect. It is typically CPU and memory intensive. In practice, factors such as available memory and a programmer's willingness to wait for compilation limit the optimizations that a compiler might provide. Research indicates that some optimization problems are NP-complete, or even undecidable.
Constant folding and constant propagation are related compiler optimizations used by many modern compilers. An advanced form of constant propagation known as sparse conditional constant propagation can more accurately propagate constants and simultaneously remove dead code.
In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a type to every term. Usually the terms are various language constructs of a computer program, such as variables, expressions, functions, or modules. A type system dictates the operations that can be performed on a term. For variables, the type system determines the allowed values of that term.
In computing, inline expansion, or inlining, is a manual or compiler optimization that replaces a function call site with the body of the called function. Inline expansion is similar to macro expansion, but occurs during compilation, without changing the source code, while macro expansion occurs prior to compilation, and results in different text that is then processed by the compiler.
In computer programming, an assignment statement sets and/or re-sets the value stored in the storage location(s) denoted by a variable name; in other words, it copies a value into the variable. In most imperative programming languages, the assignment statement is a fundamental construct.
In compiler design, static single assignment form is a type of intermediate representation (IR) where each variable is assigned exactly once. SSA is used in most high-quality optimizing compilers for imperative languages, including LLVM, the GNU Compiler Collection, and many commercial compilers.
BLISS is a system programming language developed at Carnegie Mellon University (CMU) by W. A. Wulf, D. B. Russell, and A. N. Habermann around 1970. It was perhaps the best known system language until C debuted a few years later. Since then, C became popular and common, and BLISS faded into obscurity. When C was in its infancy, a few projects within Bell Labs debated the merits of BLISS vs. C.
The syntax of the C programming language is the set of rules governing writing of software in C. It is designed to allow for programs that are extremely terse, have a close relationship with the resulting object code, and yet provide relatively high-level data abstraction. C was the first widely successful high-level language for portable operating-system development.
In computing, aliasing describes a situation in which a data location in memory can be accessed through different symbolic names in the program. Thus, modifying the data through one name implicitly modifies the values associated with all aliased names, which may not be expected by the programmer. As a result, aliasing makes it particularly difficult to understand, analyze and optimize programs. Aliasing analysers intend to make and compute useful information for understanding aliasing in programs.
In computer science, a tail call is a subroutine call performed as the final action of a procedure. If the target of a tail is the same subroutine, the subroutine is said to be tail recursive, which is a special case of direct recursion. Tail recursion is particularly useful, and is often easy to optimize in implementations.
Within computer science, a use-definition chain is a data structure that consists of a use U, of a variable, and all the definitions D of that variable that can reach that use without any other intervening definitions. A UD Chain generally means the assignment of some value to a variable.
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.
In compiler theory, a reaching definition for a given instruction is an earlier instruction whose target variable can reach the given one without an intervening assignment. For example, in the following code:
d1 : y := 3 d2 : x := y
In the Java programming language, the final
keyword is used in several contexts to define an entity that can only be assigned once.
Interprocedural optimization (IPO) is a collection of compiler techniques used in computer programming to improve performance in programs containing many frequently used functions of small or medium length. IPO differs from other compiler optimizations by analyzing the entire program as opposed to a single function or block of code.
C++11 is a version of a joint technical standard, ISO/IEC 14882, by the International Organization for Standardization (ISO) and International Electrotechnical Commission (IEC), for the C++ programming language. C++11 replaced the prior version of the C++ standard, named C++03, and was later replaced by C++14. The name follows the tradition of naming language versions by the publication year of the specification, though it was formerly named C++0x because it was expected to be published before 2010.
This is an overview of Fortran 95 language features. Included are the additional features of TR-15581:Enhanced Data Type Facilities, which have been universally implemented. Old features that have been superseded by new ones are not described – few of those historic features are used in modern programs although most have been retained in the language to maintain backward compatibility. The current standard is Fortran 2023; many of its new features are still being implemented in compilers.
Value numbering is a technique of determining when two computations in a program are equivalent and eliminating one of them with a semantics-preserving optimization.
Upwards exposed uses or reachable uses, is a concept in compiler theory which occurs in the copy propagation stage of compilation.
Flix is a functional, imperative, and logic programming language developed at Aarhus University, with funding from the Independent Research Fund Denmark, and by a community of open source contributors. The Flix language supports algebraic data types, pattern matching, parametric polymorphism, currying, higher-order functions, extensible records, channel and process-based concurrency, and tail call elimination. Two notable features of Flix are its type and effect system and its support for first-class Datalog constraints.
Muchnick, Steven S. Advanced Compiler Design and Implementation. Morgan Kaufmann. 1997.