The purpose of the compiler macro facility is to permit selective source code transformations as optimization advice to the compiler. When a compound form is being processed (as by the compiler), if the operator names a compiler macro then the compiler macro function may be invoked on the form, and the resulting expansion recursively processed in preference to performing the usual processing on the original form according to its normal interpretation as a function form or macro form.
A compiler macro function, like a macro function, is a function of two arguments: the entire call form and the environment. Unlike an ordinary macro function, a compiler macro function can decline to provide an expansion merely by returning a value that is the same as the original form. The consequences are undefined if a compiler macro function destructively modifies any part of its form argument.
The form passed to the compiler macro function can either be a list whose car is the function name, or a list whose car is funcall and whose cadr is a list (function name); note that this affects destructuring of the form argument by the compiler macro function. define-compiler-macro arranges for destructuring of arguments to be performed correctly for both possible formats.
When compile-file chooses to expand a top level form that is a compiler macro form, the expansion is also treated as a top level form for the purposes of eval-when processing; see Section 18.104.22.168 (Processing of Top Level Forms).