Editing GNU Compiler Collection (section)

=== Front ends ===
[[File:Xxx Scanner and parser example for C.gif|thumb|right|300px|Front ends consist of [[Preprocessor|preprocessing]], [[lexical analysis]], [[Parsing|syntactic analysis]] (parsing) and semantic analysis. The goals of compiler front ends are to either accept or reject candidate programs according to the language grammar and semantics, identify errors and handle valid program representations to later compiler stages. This example shows the lexer and parser steps performed for a simple program written in [[C (programming language)|C]].]]

Each [[front end (compiler)|front end]] uses a parser to produce the [[abstract syntax tree]] of a given [[source file]]. Due to the syntax tree abstraction, source files of any of the different supported languages can be processed by the same [[back end (Compiler)|back end]]. GCC started out using [[LALR parser]]s generated with [[GNU Bison|Bison]], but gradually switched to hand-written [[Recursive descent parser|recursive-descent parsers]] for C++ in 2004,<ref>{{Cite web|url=https://gcc.gnu.org/gcc-3.4/changes.html|title=GCC 3.4 Release Series — Changes, New Features, and Fixes - GNU Project|website=gcc.gnu.org|access-date=July 25, 2016|archive-date=January 18, 2023|archive-url=https://web.archive.org/web/20230118185814/https://gcc.gnu.org/gcc-3.4/changes.html|url-status=live}}</ref> and for C and Objective-C in 2006.<ref>{{Cite web|url=https://gcc.gnu.org/gcc-4.1/changes.html|title=GCC 4.1 Release Series — Changes, New Features, and Fixes - GNU Project|website=gcc.gnu.org|access-date=July 25, 2016|archive-date=January 18, 2023|archive-url=https://web.archive.org/web/20230118185814/https://gcc.gnu.org/gcc-4.1/changes.html|url-status=live}}</ref> As of 2021 all front ends use hand-written recursive-descent parsers.

Until GCC 4.0, the tree representation of the program was not fully independent of the processor being targeted. The meaning of a tree was somewhat different for different language front ends, and front ends could provide their own tree codes. This was simplified with the introduction of GENERIC and GIMPLE, two new forms of language-independent trees that were introduced with the advent of GCC 4.0. GENERIC is more complex, based on the GCC 3.x Java front end's intermediate representation. GIMPLE is a simplified GENERIC, in which various constructs are ''[[lowering (computer science)|lowered]]'' to multiple GIMPLE instructions. The [[C (Programming Language)|C]], [[C++]], and [[Java (programming language)|Java]] front ends produce GENERIC directly in the front end. Other front ends instead have different intermediate representations after parsing and convert these to GENERIC.

In either case, the so-called "gimplifier" then converts this more complex form into the simpler [[static single-assignment form|SSA]]-based GIMPLE form that is the common language for a large number of language- and architecture-independent global (function scope) optimizations.