CPFE
C/C++ Front End Documentation#
Introduction#
This document describes the function and internal structure of the Edison Design Group C/C++ Front End. More precisely, it gives the general overview necessary as an introduction to the study of the source code of the Front End. The source code comments provide more detailed information, and that information is not necessarily repeated here.
The front end accepts the C++ language as defined by the ISO/IEC 14882:2020 standard, as well as a number of the features of the ISO/IEC 14882:2024 standard. The front end can also be configured to accept earlier revisions of the C++ standard (e.g., ISO/IEC 14882:2003). With the proper command-line options, it alternatively accepts the C language of the ISO/IEC 9899:1990, ISO/IEC 9899:1999, ISO/IEC 9899:2011, and ISO/IEC 9899:2018 standards, as well as a number of features from the ISO/IEC 9899:2024 standard. Certain other dialects of C++ and C are also accepted, including Microsoft, GNU and Clang dialects. C++/CLI, the Microsoft extension to C++ in support of Microsoft’s .NET environment (and formalized through the ECMA-372 standard), is also implemented.
It is a single pass, including integrated preprocessing. (Preprocessing alone – to a textual output file – can also be done, as in traditional compilers with a separate preprocessor.) The syntax-analysis technique is recursive descent, with a precedence modification to speed up the scanning of expressions.
The front end does no optimization. Its goal is to produce a complete and clean parsed form of the source program, and to diagnose errors. It does complete error checking, produces clear error messages (including the position of the error within the source line), and avoids cascading of errors. It also tries to avoid seeming overly finicky to a knowledgeable C or C++ programmer.
The front end is written in a portable dialect of C++, using features from the ISO/IEC 14882:2011 standard (i.e., it’s written using C++11 features). Host and target computer characteristics (e.g., command-line interface, character sets, integer sizes, floating-point representation) are carefully separated from the main body of code so that they can be easily changed for rehosting or retargeting. There is extensive debugging and assertion-checking code, which can be included or excluded by conditional compilation options.
The output from the front end is a high-level tree-structured in-memory intermediate language. This intermediate language contains full information on declarations, statements, and expressions, all in tree form, and all with source-correspondence information that simplifies the generation of symbolic debugging information.
The intermediate language can, if desired, be written to a file, read back from such a file, or displayed in human-readable form.
When it translates C programs to intermediate language, the front end uses only the C subset of the intermediate language. When it translates C++ programs to intermediate language, it uses the full intermediate language. However, an optional IL lowering pass can be used translate the C++ intermediate language constructs into C intermediate language constructs to reduce the work required of back ends.
It is expected that the front end will be combined with an appropriate back end that will generate object code from the intermediate language. A simple back end is provided that generates C from the intermediate language, and another that generates C++ from the intermediate language (the latter is intended for source-to-source applications).
Also included: minimal runtime, a template prelinker, an IL display utility, a name demangler, and prototype header files.
This document covers:
The external interface of the front end – the command-line options, language dialect, intermediate language, and host/target configuration issues. (Error messages are listed in an appendix.)
The design of the front end – its philosophy and an overview of the source code.
Readers of this document should be familiar with the C++ language, with the C language, and with the basics of compiler theory.