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127 lines
5.6 KiB
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=============================
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Introduction to the Clang AST
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=============================
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This document gives a gentle introduction to the mysteries of the Clang
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AST. It is targeted at developers who either want to contribute to
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Clang, or use tools that work based on Clang's AST, like the AST
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matchers.
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.. raw:: html
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<center><iframe width="560" height="315" src="https://www.youtube.com/embed/VqCkCDFLSsc?vq=hd720" frameborder="0" allowfullscreen></iframe></center>
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`Slides <https://llvm.org/devmtg/2013-04/klimek-slides.pdf>`_
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Introduction
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============
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Clang's AST is different from ASTs produced by some other compilers in
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that it closely resembles both the written C++ code and the C++
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standard. For example, parenthesis expressions and compile time
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constants are available in an unreduced form in the AST. This makes
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Clang's AST a good fit for refactoring tools.
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Documentation for all Clang AST nodes is available via the generated
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`Doxygen <https://clang.llvm.org/doxygen>`_. The doxygen online
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documentation is also indexed by your favorite search engine, which will
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make a search for clang and the AST node's class name usually turn up
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the doxygen of the class you're looking for (for example, search for:
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clang ParenExpr).
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Examining the AST
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=================
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A good way to familiarize yourself with the Clang AST is to actually look
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at it on some simple example code. Clang has a builtin AST-dump mode,
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which can be enabled with the flag ``-ast-dump``.
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Let's look at a simple example AST:
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::
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$ cat test.cc
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int f(int x) {
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int result = (x / 42);
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return result;
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}
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# Clang by default is a frontend for many tools; -Xclang is used to pass
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# options directly to the C++ frontend.
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$ clang -Xclang -ast-dump -fsyntax-only test.cc
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TranslationUnitDecl 0x5aea0d0 <<invalid sloc>>
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... cutting out internal declarations of clang ...
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`-FunctionDecl 0x5aeab50 <test.cc:1:1, line:4:1> f 'int (int)'
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|-ParmVarDecl 0x5aeaa90 <line:1:7, col:11> x 'int'
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`-CompoundStmt 0x5aead88 <col:14, line:4:1>
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|-DeclStmt 0x5aead10 <line:2:3, col:24>
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| `-VarDecl 0x5aeac10 <col:3, col:23> result 'int'
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| `-ParenExpr 0x5aeacf0 <col:16, col:23> 'int'
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| `-BinaryOperator 0x5aeacc8 <col:17, col:21> 'int' '/'
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| |-ImplicitCastExpr 0x5aeacb0 <col:17> 'int' <LValueToRValue>
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| | `-DeclRefExpr 0x5aeac68 <col:17> 'int' lvalue ParmVar 0x5aeaa90 'x' 'int'
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| `-IntegerLiteral 0x5aeac90 <col:21> 'int' 42
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`-ReturnStmt 0x5aead68 <line:3:3, col:10>
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`-ImplicitCastExpr 0x5aead50 <col:10> 'int' <LValueToRValue>
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`-DeclRefExpr 0x5aead28 <col:10> 'int' lvalue Var 0x5aeac10 'result' 'int'
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The toplevel declaration in
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a translation unit is always the `translation unit
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declaration <https://clang.llvm.org/doxygen/classclang_1_1TranslationUnitDecl.html>`_.
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In this example, our first user written declaration is the `function
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declaration <https://clang.llvm.org/doxygen/classclang_1_1FunctionDecl.html>`_
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of "``f``". The body of "``f``" is a `compound
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statement <https://clang.llvm.org/doxygen/classclang_1_1CompoundStmt.html>`_,
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whose child nodes are a `declaration
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statement <https://clang.llvm.org/doxygen/classclang_1_1DeclStmt.html>`_
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that declares our result variable, and the `return
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statement <https://clang.llvm.org/doxygen/classclang_1_1ReturnStmt.html>`_.
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AST Context
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===========
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All information about the AST for a translation unit is bundled up in
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the class
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`ASTContext <https://clang.llvm.org/doxygen/classclang_1_1ASTContext.html>`_.
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It allows traversal of the whole translation unit starting from
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`getTranslationUnitDecl <https://clang.llvm.org/doxygen/classclang_1_1ASTContext.html#abd909fb01ef10cfd0244832a67b1dd64>`_,
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or to access Clang's `table of
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identifiers <https://clang.llvm.org/doxygen/classclang_1_1ASTContext.html#a4f95adb9958e22fbe55212ae6482feb4>`_
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for the parsed translation unit.
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AST Nodes
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=========
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Clang's AST nodes are modeled on a class hierarchy that does not have a
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common ancestor. Instead, there are multiple larger hierarchies for
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basic node types like
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`Decl <https://clang.llvm.org/doxygen/classclang_1_1Decl.html>`_ and
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`Stmt <https://clang.llvm.org/doxygen/classclang_1_1Stmt.html>`_. Many
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important AST nodes derive from
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`Type <https://clang.llvm.org/doxygen/classclang_1_1Type.html>`_,
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`Decl <https://clang.llvm.org/doxygen/classclang_1_1Decl.html>`_,
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`DeclContext <https://clang.llvm.org/doxygen/classclang_1_1DeclContext.html>`_
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or `Stmt <https://clang.llvm.org/doxygen/classclang_1_1Stmt.html>`_, with
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some classes deriving from both Decl and DeclContext.
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There are also a multitude of nodes in the AST that are not part of a
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larger hierarchy, and are only reachable from specific other nodes, like
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`CXXBaseSpecifier <https://clang.llvm.org/doxygen/classclang_1_1CXXBaseSpecifier.html>`_.
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Thus, to traverse the full AST, one starts from the
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`TranslationUnitDecl <https://clang.llvm.org/doxygen/classclang_1_1TranslationUnitDecl.html>`_
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and then recursively traverses everything that can be reached from that
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node - this information has to be encoded for each specific node type.
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This algorithm is encoded in the
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`RecursiveASTVisitor <https://clang.llvm.org/doxygen/classclang_1_1RecursiveASTVisitor.html>`_.
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See the `RecursiveASTVisitor
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tutorial <https://clang.llvm.org/docs/RAVFrontendAction.html>`_.
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The two most basic nodes in the Clang AST are statements
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(`Stmt <https://clang.llvm.org/doxygen/classclang_1_1Stmt.html>`_) and
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declarations
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(`Decl <https://clang.llvm.org/doxygen/classclang_1_1Decl.html>`_). Note
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that expressions
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(`Expr <https://clang.llvm.org/doxygen/classclang_1_1Expr.html>`_) are
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also statements in Clang's AST.
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