forked from OSchip/llvm-project
[ASTImporter] Add development internals docs
Reviewers: a_sidorin, shafik, teemperor, gamesh411, balazske, dkrupp, a.sidorin Subscribers: rnkovacs, Szelethus, cfe-commits Tags: #clang Differential Revision: https://reviews.llvm.org/D66336 llvm-svn: 371839
This commit is contained in:
Gabor Marton
parent
b7b7f26220
commit
9eaa981e8e
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@ -1447,6 +1447,495 @@ or by simply a pointer to the canonical declaration (if the declarations
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are not ``Redeclarable`` -- in that case, a ``Mergeable`` base class is used
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instead).
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The ASTImporter
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---------------
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The ``ASTImporter`` class imports nodes of an ``ASTContext`` into another
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``ASTContext``. Please refer to the document :doc:`ASTImporter: Merging Clang
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ASTs <LibASTImporter>` for an introduction. And please read through the
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high-level `description of the import algorithm
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<LibASTImporter.html#algorithm-of-the-import>`_, this is essential for
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understanding further implementation details of the importer.
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.. _templated:
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Abstract Syntax Graph
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^^^^^^^^^^^^^^^^^^^^^
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Despite the name, the Clang AST is not a tree. It is a directed graph with
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cycles. One example of a cycle is the connection between a
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``ClassTemplateDecl`` and its "templated" ``CXXRecordDecl``. The *templated*
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``CXXRecordDecl`` represents all the fields and methods inside the class
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template, while the ``ClassTemplateDecl`` holds the information which is
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related to being a template, i.e. template arguments, etc. We can get the
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*templated* class (the ``CXXRecordDecl``) of a ``ClassTemplateDecl`` with
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``ClassTemplateDecl::getTemplatedDecl()``. And we can get back a pointer of the
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"described" class template from the *templated* class:
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``CXXRecordDecl::getDescribedTemplate()``. So, this is a cycle between two
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nodes: between the *templated* and the *described* node. There may be various
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other kinds of cycles in the AST especially in case of declarations.
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.. _structural-eq:
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Structural Equivalency
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^^^^^^^^^^^^^^^^^^^^^^
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Importing one AST node copies that node into the destination ``ASTContext``. To
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copy one node means that we create a new node in the "to" context then we set
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its properties to be equal to the properties of the source node. Before the
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copy, we make sure that the source node is not *structurally equivalent* to any
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existing node in the destination context. If it happens to be equivalent then
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we skip the copy.
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The informal definition of structural equivalency is the following:
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Two nodes are **structurally equivalent** if they are
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- builtin types and refer to the same type, e.g. ``int`` and ``int`` are
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structurally equivalent,
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- function types and all their parameters have structurally equivalent types,
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- record types and all their fields in order of their definition have the same
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identifier names and structurally equivalent types,
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- variable or function declarations and they have the same identifier name and
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their types are structurally equivalent.
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In C, two types are structurally equivalent if they are *compatible types*. For
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a formal definition of *compatible types*, please refer to 6.2.7/1 in the C11
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standard. However, there is no definition for *compatible types* in the C++
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standard. Still, we extend the definition of structural equivalency to
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templates and their instantiations similarly: besides checking the previously
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mentioned properties, we have to check for equivalent template
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parameters/arguments, etc.
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The structural equivalent check can be and is used independently from the
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ASTImporter, e.g. the ``clang::Sema`` class uses it also.
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The equivalence of nodes may depend on the equivalency of other pairs of nodes.
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Thus, the check is implemented as a parallel graph traversal. We traverse
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through the nodes of both graphs at the same time. The actual implementation is
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similar to breadth-first-search. Let's say we start the traverse with the <A,B>
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pair of nodes. Whenever the traversal reaches a pair <X,Y> then the following
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statements are true:
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- A and X are nodes from the same ASTContext.
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- B and Y are nodes from the same ASTContext.
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- A and B may or may not be from the same ASTContext.
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- if A == X (pointer equivalency) then (there is a cycle during the traverse)
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- A and B are structurally equivalent if and only if
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- B and Y are part of the same redeclaration chain,
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- All dependent nodes on the path from <A,B> to <X,Y> are structurally
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equivalent.
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When we compare two classes or enums and one of them is incomplete or has
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unloaded external lexical declarations then we cannot descend to compare their
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contained declarations. So in these cases they are considered equal if they
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have the same names. This is the way how we compare forward declarations with
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definitions.
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.. TODO Should we elaborate the actual implementation of the graph traversal,
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.. which is a very weird BFS traversal?
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Redeclaration Chains
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^^^^^^^^^^^^^^^^^^^^
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The early version of the ``ASTImporter``'s merge mechanism squashed the
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declarations, i.e. it aimed to have only one declaration instead of maintaining
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a whole redeclaration chain. This early approach simply skipped importing a
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function prototype, but it imported a definition. To demonstrate the problem
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with this approach let's consider an empty "to" context and the following
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``virtual`` function declarations of ``f`` in the "from" context:
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.. code-block:: c++
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struct B { virtual void f(); };
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void B::f() {} // <-- let's import this definition
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If we imported the definition with the "squashing" approach then we would
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end-up having one declaration which is indeed a definition, but ``isVirtual()``
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returns ``false`` for it. The reason is that the definition is indeed not
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virtual, it is the property of the prototype!
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Consequently, we must either set the virtual flag for the definition (but then
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we create a malformed AST which the parser would never create), or we import
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the whole redeclaration chain of the function. The most recent version of the
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``ASTImporter`` uses the latter mechanism. We do import all function
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declarations - regardless if they are definitions or prototypes - in the order
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as they appear in the "from" context.
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.. Structural eq requires proper redecl chains
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Another reason why we must maintain and import redeclaration chains properly is
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that the :ref:`Structural Equivalency <structural-eq>` check would report false
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positive in-equivalencies otherwise. We must not allow having two (or more)
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independent redeclaration chains of structurally equivalent declarations.
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Structural equivalency identifies the chains with the canonical declaration,
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that becomes different for independent chains.
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.. One definition
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If we have an existing definition in the "to" context, then we cannot import
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another definition, we will use the existing definition. However, we can import
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prototype(s): we chain the newly imported prototype(s) to the existing
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definition. Whenever we import a new prototype from a third context, that will
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be added to the end of the redeclaration chain. This may result in long
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redeclaration chains in certain cases, e.g. if we import from several
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translation units which include the same header with the prototype.
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.. Squashing prototypes
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To mitigate the problem of long redeclaration chains of free functions, we
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could compare prototypes to see if they have the same properties and if yes
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then we could merge these prototypes. The implementation of squashing of
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prototypes for free functions is future work.
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.. Exception: Cannot have more than 1 prototype in-class
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Chaining functions this way ensures that we do copy all information from the
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source AST. Nonetheless, there is a problem with member functions: While we can
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have many prototypes for free functions, we must have only one prototype for a
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member function.
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.. code-block:: c++
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void f(); // OK
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void f(); // OK
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struct X {
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void f(); // OK
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void f(); // ERROR
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};
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void X::f() {} // OK
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Thus, prototypes of member functions must be squashed, we cannot just simply
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attach a new prototype to the existing in-class prototype. Consider the
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following contexts:
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.. code-block:: c++
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// "to" context
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struct X {
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void f(); // D0
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};
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.. code-block:: c++
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// "from" context
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struct X {
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void f(); // D1
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};
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void X::f() {} // D2
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When we import the prototype and the definition of ``f`` from the "from"
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context, then the resulting redecl chain will look like this ``D0 -> D2'``,
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where ``D2'`` is the copy of ``D2`` in the "to" context.
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.. Redecl chains of other declarations
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Generally speaking, when we import declarations (like enums and classes) we do
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attach the newly imported declaration to the existing redeclaration chain (if
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there is structural equivalency). We do not import, however, the whole
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redeclaration chain as we do in case of functions. Up till now, we haven't
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found any essential property of forward declarations which is similar to the
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case of the virtual flag in a member function prototype. In the future, this
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may change, though.
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Traversal during the Import
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The node specific import mechanisms are implemented in
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``ASTNodeImporter::VisitNode()`` functions, e.g. ``VisitFunctionDecl()``.
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When we import a declaration then first we import everything which is needed to
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call the constructor of that declaration node. Everything which can be set
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later is set after the node is created. For example, in case of a
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``FunctionDecl`` we first import the declaration context in which the function
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is declared, then we create the ``FunctionDecl`` and only then we import the
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body of the function. This means there are implicit dependencies between AST
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nodes. These dependencies determine the order in which we visit nodes in the
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"from" context. As with the regular graph traversal algorithms like DFS, we
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keep track which nodes we have already visited in
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``ASTImporter::ImportedDecls``. Whenever we create a node then we immediately
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add that to the ``ImportedDecls``. We must not start the import of any other
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declarations before we keep track of the newly created one. This is essential,
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otherwise, we would not be able to handle circular dependencies. To enforce
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this, we wrap all constructor calls of all AST nodes in
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``GetImportedOrCreateDecl()``. This wrapper ensures that all newly created
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declarations are immediately marked as imported; also, if a declaration is
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already marked as imported then we just return its counterpart in the "to"
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context. Consequently, calling a declaration's ``::Create()`` function directly
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would lead to errors, please don't do that!
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Even with the use of ``GetImportedOrCreateDecl()`` there is still a
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probability of having an infinite import recursion if things are imported from
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each other in wrong way. Imagine that during the import of ``A``, the import of
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``B`` is requested before we could create the node for ``A`` (the constructor
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needs a reference to ``B``). And the same could be true for the import of ``B``
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(``A`` is requested to be imported before we could create the node for ``B``).
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In case of the :ref:`templated-described swing <templated>` we take
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extra attention to break the cyclical dependency: we import and set the
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described template only after the ``CXXRecordDecl`` is created. As a best
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practice, before creating the node in the "to" context, avoid importing of
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other nodes which are not needed for the constructor of node ``A``.
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Error Handling
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^^^^^^^^^^^^^^
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Every import function returns with either an ``llvm::Error`` or an
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``llvm::Expected<T>`` object. This enforces to check the return value of the
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import functions. If there was an error during one import then we return with
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that error. (Exception: when we import the members of a class, we collect the
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individual errors with each member and we concatenate them in one Error
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object.) We cache these errors in cases of declarations. During the next import
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call if there is an existing error we just return with that. So, clients of the
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library receive an Error object, which they must check.
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During import of a specific declaration, it may happen that some AST nodes had
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already been created before we recognize an error. In this case, we signal back
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the error to the caller, but the "to" context remains polluted with those nodes
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which had been created. Ideally, those nodes should not had been created, but
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that time we did not know about the error, the error happened later. Since the
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AST is immutable (most of the cases we can't remove existing nodes) we choose
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to mark these nodes as erroneous.
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We cache the errors associated with declarations in the "from" context in
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``ASTImporter::ImportDeclErrors`` and the ones which are associated with the
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"to" context in ``ASTImporterSharedState::ImportErrors``. Note that, there may
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be several ASTImporter objects which import into the same "to" context but from
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different "from" contexts; in this case, they have to share the associated
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errors of the "to" context.
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When an error happens, that propagates through the call stack, through all the
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dependant nodes. However, in case of dependency cycles, this is not enough,
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because we strive to mark the erroneous nodes so clients can act upon. In those
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cases, we have to keep track of the errors for those nodes which are
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intermediate nodes of a cycle.
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An **import path** is the list of the AST nodes which we visit during an Import
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call. If node ``A`` depends on node ``B`` then the path contains an ``A->B``
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edge. From the call stack of the import functions, we can read the very same
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path.
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Now imagine the following AST, where the ``->`` represents dependency in terms
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of the import (all nodes are declarations).
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.. code-block:: text
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A->B->C->D
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`->E
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We would like to import A.
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The import behaves like a DFS, so we will visit the nodes in this order: ABCDE.
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During the visitation we will have the following import paths:
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.. code-block:: text
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A
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AB
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ABC
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ABCD
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ABC
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AB
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ABE
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AB
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A
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If during the visit of E there is an error then we set an error for E, then as
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the call stack shrinks for B, then for A:
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.. code-block:: text
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A
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AB
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ABC
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ABCD
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ABC
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AB
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ABE // Error! Set an error to E
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AB // Set an error to B
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A // Set an error to A
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However, during the import we could import C and D without any error and they
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are independent of A,B and E. We must not set up an error for C and D. So, at
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the end of the import we have an entry in ``ImportDeclErrors`` for A,B,E but
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not for C,D.
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Now, what happens if there is a cycle in the import path? Let's consider this
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AST:
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.. code-block:: text
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A->B->C->A
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`->E
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During the visitation, we will have the below import paths and if during the
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visit of E there is an error then we will set up an error for E,B,A. But what's
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up with C?
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.. code-block:: text
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A
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AB
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ABC
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ABCA
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ABC
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AB
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ABE // Error! Set an error to E
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AB // Set an error to B
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A // Set an error to A
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This time we know that both B and C are dependent on A. This means we must set
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up an error for C too. As the call stack reverses back we get to A and we must
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set up an error to all nodes which depend on A (this includes C). But C is no
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longer on the import path, it just had been previously. Such a situation can
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happen only if during the visitation we had a cycle. If we didn't have any
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cycle, then the normal way of passing an Error object through the call stack
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could handle the situation. This is why we must track cycles during the import
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process for each visited declaration.
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Lookup Problems
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^^^^^^^^^^^^^^^
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When we import a declaration from the source context then we check whether we
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already have a structurally equivalent node with the same name in the "to"
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context. If the "from" node is a definition and the found one is also a
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definition, then we do not create a new node, instead, we mark the found node
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as the imported node. If the found definition and the one we want to import
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have the same name but they are structurally in-equivalent, then we have an ODR
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violation in case of C++. If the "from" node is not a definition then we add
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that to the redeclaration chain of the found node. This behaviour is essential
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when we merge ASTs from different translation units which include the same
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header file(s). For example, we want to have only one definition for the class
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template ``std::vector``, even if we included ``<vector>`` in several
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translation units.
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To find a structurally equivalent node we can use the regular C/C++ lookup
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functions: ``DeclContext::noload_lookup()`` and
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``DeclContext::localUncachedLookup()``. These functions do respect the C/C++
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name hiding rules, thus you cannot find certain declarations in a given
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declaration context. For instance, unnamed declarations (anonymous structs),
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non-first ``friend`` declarations and template specializations are hidden. This
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is a problem, because if we use the regular C/C++ lookup then we create
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redundant AST nodes during the merge! Also, having two instances of the same
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node could result in false :ref:`structural in-equivalencies <structural-eq>`
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of other nodes which depend on the duplicated node. Because of these reasons,
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we created a lookup class which has the sole purpose to register all
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declarations, so later they can be looked up by subsequent import requests.
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This is the ``ASTImporterLookupTable`` class. This lookup table should be
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shared amongst the different ``ASTImporter`` instances if they happen to import
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to the very same "to" context. This is why we can use the importer specific
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lookup only via the ``ASTImporterSharedState`` class.
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ExternalASTSource
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~~~~~~~~~~~~~~~~~
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The ``ExternalASTSource`` is an abstract interface associated with the
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``ASTContext`` class. It provides the ability to read the declarations stored
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within a declaration context either for iteration or for name lookup. A
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declaration context with an external AST source may load its declarations
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on-demand. This means that the list of declarations (represented as a linked
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list, the head is ``DeclContext::FirstDecl``) could be empty. However, member
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functions like ``DeclContext::lookup()`` may initiate a load.
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Usually, external sources are associated with precompiled headers. For example,
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when we load a class from a PCH then the members are loaded only if we do want
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to look up something in the class' context.
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In case of LLDB, an implementation of the ``ExternalASTSource`` interface is
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attached to the AST context which is related to the parsed expression. This
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implementation of the ``ExternalASTSource`` interface is realized with the help
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of the ``ASTImporter`` class. This way, LLDB can reuse Clang's parsing
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machinery while synthesizing the underlying AST from the debug data (e.g. from
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DWARF). From the view of the ``ASTImporter`` this means both the "to" and the
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"from" context may have declaration contexts with external lexical storage. If
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a ``DeclContext`` in the "to" AST context has external lexical storage then we
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must take extra attention to work only with the already loaded declarations!
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Otherwise, we would end up with an uncontrolled import process. For instance,
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if we used the regular ``DeclContext::lookup()`` to find the existing
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declarations in the "to" context then the ``lookup()`` call itself would
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initiate a new import while we are in the middle of importing a declaration!
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(By the time we initiate the lookup we haven't registered yet that we already
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started to import the node of the "from" context.) This is why we use
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``DeclContext::noload_lookup()`` instead.
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Class Template Instantiations
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Different translation units may have class template instantiations with the
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same template arguments, but with a different set of instantiated
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``MethodDecls`` and ``FieldDecls``. Consider the following files:
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.. code-block:: c++
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// x.h
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template <typename T>
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struct X {
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int a{0}; // FieldDecl with InitListExpr
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X(char) : a(3) {} // (1)
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X(int) {} // (2)
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};
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|
||||
// foo.cpp
|
||||
void foo() {
|
||||
// ClassTemplateSpec with ctor (1): FieldDecl without InitlistExpr
|
||||
X<char> xc('c');
|
||||
}
|
||||
|
||||
// bar.cpp
|
||||
void bar() {
|
||||
// ClassTemplateSpec with ctor (2): FieldDecl WITH InitlistExpr
|
||||
X<char> xc(1);
|
||||
}
|
||||
|
||||
In ``foo.cpp`` we use the constructor with number ``(1)``, which explicitly
|
||||
initializes the member ``a`` to ``3``, thus the ``InitListExpr`` ``{0}`` is not
|
||||
used here and the AST node is not instantiated. However, in the case of
|
||||
``bar.cpp`` we use the constructor with number ``(2)``, which does not
|
||||
explicitly initialize the ``a`` member, so the default ``InitListExpr`` is
|
||||
needed and thus instantiated. When we merge the AST of ``foo.cpp`` and
|
||||
``bar.cpp`` we must create an AST node for the class template instantiation of
|
||||
``X<char>`` which has all the required nodes. Therefore, when we find an
|
||||
existing ``ClassTemplateSpecializationDecl`` then we merge the fields of the
|
||||
``ClassTemplateSpecializationDecl`` in the "from" context in a way that the
|
||||
``InitListExpr`` is copied if not existent yet. The same merge mechanism should
|
||||
be done in the cases of instantiated default arguments and exception
|
||||
specifications of functions.
|
||||
|
||||
.. _visibility:
|
||||
|
||||
Visibility of Declarations
|
||||
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
||||
|
||||
During import of a global variable with external visibility, the lookup will
|
||||
find variables (with the same name) but with static visibility (linkage).
|
||||
Clearly, we cannot put them into the same redeclaration chain. The same is true
|
||||
the in case of functions. Also, we have to take care of other kinds of
|
||||
declarations like enums, classes, etc. if they are in anonymous namespaces.
|
||||
Therefore, we filter the lookup results and consider only those which have the
|
||||
same visibility as the declaration we currently import.
|
||||
|
||||
We consider two declarations in two anonymous namsepaces to have the same
|
||||
visibility only if they are imported from the same AST context.
|
||||
|
||||
Strategies to Handle Conflicting Names
|
||||
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
||||
|
||||
During the import we lookup existing declarations with the same name. We filter
|
||||
the lookup results based on their :ref:`visibility <visibility>`. If any of the
|
||||
found declarations are not structurally equivalent then we bumped to a name
|
||||
conflict error (ODR violation in C++). In this case, we return with an
|
||||
``Error`` and we set up the ``Error`` object for the declaration. However, some
|
||||
clients of the ``ASTImporter`` may require a different, perhaps less
|
||||
conservative and more liberal error handling strategy.
|
||||
|
||||
E.g. static analysis clients may benefit if the node is created even if there
|
||||
is a name conflict. During the CTU analysis of certain projects, we recognized
|
||||
that there are global declarations which collide with declarations from other
|
||||
translation units, but they are not referenced outside from their translation
|
||||
unit. These declarations should be in an unnamed namespace ideally. If we treat
|
||||
these collisions liberally then CTU analysis can find more results. Note, the
|
||||
feature be able to choose between name conflict handling strategies is still an
|
||||
ongoing work.
|
||||
|
||||
.. _CFG:
|
||||
|
||||
The ``CFG`` class
|
||||
|
|
|
|||
Loading…
Reference in New Issue