Editing Program optimization (section)

==Macros==
Optimization during code development using [[Macro (computer science)|macros]] takes on different forms in different languages.

In some procedural languages, such as [[C (programming language)|C]] and [[C++]], macros are implemented using token substitution. Nowadays, [[inline function]]s can be used as a [[type safe]] alternative in many cases. In both cases, the inlined function body can then undergo further compile-time optimizations by the compiler, including [[constant folding]], which may move some computations to compile time.

In many [[functional programming]] languages, macros are implemented using parse-time substitution of parse trees/abstract syntax trees, which it is claimed makes them safer to use. Since in many cases interpretation is used, that is one way to ensure that such computations are only performed at parse-time, and sometimes the only way.

[[Lisp programming language|Lisp]] originated this style of macro,{{Citation needed|date=September 2008}} and such macros are often called "Lisp-like macros". A similar effect can be achieved by using [[template metaprogramming]] in [[C++]].

In both cases, work is moved to compile-time. The difference between [[C (programming language)|C]] macros on one side, and Lisp-like macros and [[C++]] [[template metaprogramming]] on the other side, is that the latter tools allow performing arbitrary computations at compile-time/parse-time, while expansion of [[C (programming language)|C]] macros does not perform any computation, and relies on the optimizer ability to perform it. Additionally, [[C (programming language)|C]] macros do not directly support [[recursion (computer science)|recursion]] or [[iteration]], so are not [[Turing complete]].

As with any optimization, however, it is often difficult to predict where such tools will have the most impact before a project is complete.