[PATCH] Introduce a section to the programmers manual about type hierarchies, polymorphism, and virtual dispatch.
chandlerc at gmail.com
Tue Jan 27 19:06:38 PST 2015
@@ -2480,6 +2480,76 @@
the LSBit set. (Portability is relying on the fact that all known compilers
place the ``vptr`` in the first word of the instances.)
+Designing Type Hiercharies and Polymorphic Interfaces
+There are two different design patterns that tend to result in the use of
+virtual dispatch for methods in a type hierarchy in C++ programs. The first is
+a genuine type hierarchy where different types in the hierarchy model
+a specific subset of the functionality and semantics, and these types nest
+strictly within each other. Good examples of this can be seen in the ``Value``
+or ``Type`` type hierarchies.
+A second is the desire to dispatch dynamically across a collection of
+polymorphic interface implementations. This latter use case can be modeled with
+virtual dispatch and inheritance by defining an abstract interface base class
+which all implementations derive from and override. However, this
+implementation strategy forces an **"is-a"** relationship to exist that is not
+actually meaningful. There is often not some nested hierarchy of useful
+generalizations which code might interact with and move up and down. Instead,
+there is a singular interface which is dispatched across a range of
+The preferred implementation strategy for the second use case is that of
+generic programming (sometimes called "compile-time duck typing" or "static
+polymorphism"). For example, a template over some type parameter ``T`` can be
+instantiated across any particular implementation that conforms to the
+interface or *concept*. A good example here is the highly generic properties of
+any type which models a node in a directed graph. LLVM models these primarily
+through templates and generic programming. Such templates include the
+``LoopInfoBase`` and ``DominatorTreeBase``. When this type of polymorphism
+truly needs **dynamic** dispatch you can generalize it using a technique
+called *concept-based polymorphism*. This pattern emulates the interfaces and
+behaviors of templates using a very limited form of virtual dispatch for type
+erasure inside its implementation. You can find examples of this technique in
+the ``PassManager.h`` system, and there is a more detailed introduction to it
+by Sean Parent in several of his talks and papers:
+#. `Inheritance Is The Base Class of Evil
+ - The GoingNative 2013 talk describing this technique, and probably the best
+ place to start.
+#. `Value Semantics and Concepts-based Polymorphism
+ <http://www.youtube.com/watch?v=_BpMYeUFXv8>`_ - The C++Now! 2012 talk
+ describing this technique in more detail.
+#. `Sean Parent's Papers and Presentations
+ - A Github project full of links to slides, video, and sometimes code.
+When deciding between creating a type hierarchy (with either tagged or virtual
+dispatch) and using templates or concepts-based polymorphism, consider whether
+there is some refinement of an abstract base class which is a semantically
+meaningful type on an interface boundary. If anything more refined than the
+root abstract interface is meaningless to talk about as a partial extension of
+the semantic model, then your use case likely fits better with polymorphism and
+you should avoid using virtual dispatch. However, there may be some exigent
+circumstances that require one technique or the other to be used.
+If you do need to introduce a type hierarchy, we prefer to use explicitly
+closed type hierarchies with manual tagged dispatch and/or RTTI rather than the
+open inheritance model and virtual dispatch that is more common in C++ code.
+This is because LLVM rarely encourages library consumers to extend its core
+types, and leverages the closed and tag-dispatched nature of its hierarchies to
+generate significantly more efficient code. We have also found that a large
+amount of our usage of type hierarchies fits better with tag-based pattern
+matching rather than dynamic dispatch across a common interface. Within LLVM we
+have built custom helpers to facilitate this design. See this document's
+section on `isa and dyn_cast <isa>`_ and our `detailed document
+<http://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html>`_ which describes how you
+can implement this pattern for use with the LLVM helpers.
The Core LLVM Class Hierarchy Reference
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