Inline expansion

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 (the text), while macro expansion occurs prior to compilation, and results in different text that is then processed by the compiler.

Inlining is an important optimization, but has complicated effects on performance. As a rule of thumb, some inlining will improve speed at very minor cost of space, but excess inlining will hurt speed, due to inlined code consuming too much of the instruction cache, and also cost significant space. A survey of the modest academic literature on inlining from the 1980s and 1990s is given in Jones & Marlow 1999.

Inline expansion is similar to macro expansion as the compiler places a new copy of the function in each place it is called. Inlined functions run a little faster than the normal functions as function-calling-overheads are saved, however, there is a memory penalty. If a function is inlined 10 times, there will be 10 copies of the function inserted into the code. Hence inlining is best for small functions that are called often. In C++ the member functions of a class, if defined within the class definition, are inlined by default (no need to use the inline keyword); otherwise, the keyword is needed. The compiler may ignore the programmer’s attempt to inline a function, mainly if it is particularly large.

Inline expansion is used to eliminate the time overhead (excess time) when a function is called. It is typically used for functions that execute frequently. It also has a space benefit for very small functions, and is an enabling transformation for other optimizations.

Without inline functions, the compiler decides which functions to inline. The programmer has little or no control over which functions are inlined and which are not. Giving this degree of control to the programmer allows for the use of application-specific knowledge in choosing which functions to inline.

Ordinarily, when a function is invoked, control is transferred to its definition by a branch or call instruction. With inlining, control drops through directly to the code for the function, without a branch or call instruction.

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