Search code examples
c++gcccompiler-constructionclangclang-tidy

Is the AST, abstract syntax tree, defined by the language or by the frontend?

In the last few weeks I have been experimenting with ASTs and Clang, in particular clang-tidy.

Clang offers some classes and way to interact with the ASTs, but what I don't understand is if the clang::VarDecl I am using so often is something named and created by the creators of Clang, or by the creators of the language.

Who decided that was to be called VarDecl?

I mean, is the AST (and all its elements) something that came from the mind of the inventor of the language and the various frontends just creates classes named after a document written by him/her or every frontend potentially creates its AST of a given source code and so Clang's and GCC's are different?

Solution

  • Is the AST, abstract syntax tree, defined by the language

    Not fully. Each definition in the C++ language standard comes with a short syntax notation and there is a informative annex with grammar summary. But the annex notes https://eel.is/c++draft/gram :

    This summary of C++ grammar is intended to be an aid to comprehension. It is not an exact statement of the language. In particular, the grammar described here accepts a superset of valid C++ constructs. [...]

    There is no VarDecl in that grammar from standard, variable declaration just one interpretation of simple-declaration.

    or by the frontend?

    Internals of the compiler, if it has a frontend, or it hasn't, it has 3 or 1000 stages, are part of the compiler implementation. From the point of the language, the compiler can be implemented in any way it wants, as long as it translates valid programs correctly. Let's say generally, language specifies what should happen when, not how.

    So to answer the question, AST (if used at all in any form) is defined by the compiler.

    Who decided that was to be called VarDecl?

    I most probably suspect Chris Lattner at https://github.com/llvm/llvm-project/commit/a11999d83a8ed1a2661feb858f0af786f2b829ad .

    that came from the mind of the inventor of the language and the various frontends just creates classes named after a document written by him/her or every frontend potentially creates its AST of a given source code and so Clang's and GCC's are different?

    Surely they are influenced by what is in the standard, but every compiler has its own internals. Well, in short, Clang VarDecl and GCC VAR_DECL are different, and they are also different conceptually - let's say GCC uses switch(...) case VAR_DECL: and Clang uses classes clang::VarDecl.