5.8.1 Inline Expansion Recording

Inline expansion requires that the source for the inline expanded function to be available when calls to the function are compiled. The compiler doesn’t remember the inline expansion for every function, since that would take an excessive about of space. Instead, the programmer must tell the compiler to record the inline expansion before the definition of the inline expanded function is compiled. This is done by globally declaring the function inline before the function is defined, by using the inline and extensions:maybe-inline (see maybe-inline-declaration) declarations.

In addition to recording the inline expansion of inline functions at the time the function is compiled, compile-file also puts the inline expansion in the output file. When the output file is loaded, the inline expansion is made available for subsequent compilations; there is no need to compile the definition again to record the inline expansion.

If a function is declared inline, but no expansion is recorded, then the compiler will give an efficiency note like:

Note: MYFUN is declared inline, but has no expansion.

When you get this note, check that the inline declaration and the definition appear before the calls that are to be inline expanded. This note will also be given if the inline expansion for a defun could not be recorded because the defun was in a non-null lexical environment.