Editing CMU Common Lisp (section)

==Compiler and other code execution units==
* CMUCL features an interpreter that is mainly used for the [[REPL]], but can be used for faster loading of Lisp files that don't need compilation.
* A machine to interpret compact bytecode (which can be emitted from the compiler). This is rarely used now, but was popular in early CMUCL releases because image sizes were drastically reduced at a time where download bandwidth on the Internet was low.
* A native code [[compiler]] named "Python" (not to be confused with [[Python (programming language)|the Python programming language]]). If Common Lisp source code has been written with appropriate declarations and is organized with speed in mind the Python compiler generates code that is almost free from overhead compared to code compiled from languages like C++.  Some inefficiencies such as function call interfaces and lack of pointer-free arrays of user-defined data types are dictated by the Common Lisp standard and still need to be worked around (e.g. by inlining more and using macros to build constructs that look like user-defined structures but are actually accessing fields in preallocated specialized arrays). The Python compiler also features powerful [[type inference]]s, helping the programmer in writing overhead-free code by either inferring types automatically or issuing hints about missed optimization opportunities.