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In Itanium, dynamic classes have one vtable with several different address points for dynamic base classes that can't share vtables. In the MS C++ ABI, each vbtable that can't be shared gets its own symbol, similar to how ctor vtables work in Itanium. However, instead of mangling the subobject offset into the symbol, the unique portions of the inheritance path are mangled into the symbol to make it unique. This patch implements the MSVC 2012 scheme for forming unique vbtable symbol names. MSVC 2010 use the same mangling with a different subset of the path. Implementing that mangling and possibly others is TODO. Each vbtable is an array of i32 offsets from the vbptr that points to it to another virtual base subobject. The first entry of a vbtable always points to the base of the current subobject, implying that it is the same no matter which parent class contains it. Reviewers: rjmccall Differential Revision: http://llvm-reviews.chandlerc.com/D636 llvm-svn: 184309
130 lines
5.0 KiB
C++
130 lines
5.0 KiB
C++
//===--- MicrosoftVBTables.h - Virtual Base Table Emission ----------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This class generates data about MSVC virtual base tables.
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//
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//===----------------------------------------------------------------------===//
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#include "clang/AST/BaseSubobject.h"
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#include "clang/Basic/LLVM.h"
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#include "llvm/ADT/SmallPtrSet.h"
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#include "llvm/ADT/ArrayRef.h"
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#include "llvm/IR/GlobalVariable.h"
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#include <vector>
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namespace clang {
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class ASTRecordLayout;
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namespace CodeGen {
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class CodeGenModule;
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struct VBTableInfo {
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VBTableInfo(const CXXRecordDecl *ReusingBase, BaseSubobject VBPtrSubobject,
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llvm::GlobalVariable *GV)
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: ReusingBase(ReusingBase), VBPtrSubobject(VBPtrSubobject), GV(GV) { }
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/// The vbtable will hold all of the virtual bases of ReusingBase. This may
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/// or may not be the same class as VBPtrSubobject.Base. A derived class will
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/// reuse the vbptr of the first non-virtual base subobject that has one.
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const CXXRecordDecl *ReusingBase;
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/// The vbptr is stored inside this subobject.
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BaseSubobject VBPtrSubobject;
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/// The GlobalVariable for this vbtable.
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llvm::GlobalVariable *GV;
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/// \brief Emits a definition for GV by setting it's initializer.
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void EmitVBTableDefinition(CodeGenModule &CGM, const CXXRecordDecl *RD,
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llvm::GlobalVariable::LinkageTypes Linkage) const;
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};
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// These are embedded in a DenseMap and the elements are large, so we don't want
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// SmallVector.
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typedef std::vector<VBTableInfo> VBTableVector;
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struct VBTablePath;
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typedef llvm::SmallVector<VBTablePath *, 6> VBTablePathVector;
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/// Produces MSVC-compatible vbtable data. The symbols produced by this builder
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/// match those produced by MSVC 2012, which is different from MSVC 2010.
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///
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/// Unlike Itanium, which uses only one vtable per class, MSVC uses a different
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/// symbol for every "address point" installed in base subobjects. As a result,
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/// we have to compute unique symbols for every table. Since there can be
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/// multiple non-virtual base subobjects of the same class, combining the most
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/// derived class with the base containing the vtable is insufficient. The most
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/// trivial algorithm would be to mangle in the entire path from base to most
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/// derived, but that would be too easy and would create unnecessarily large
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/// symbols. ;)
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///
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/// MSVC 2012 appears to minimize the vbtable names using the following
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/// algorithm. First, walk the class hierarchy in the usual order, depth first,
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/// left to right, to find all of the subobjects which contain a vbptr field.
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/// Visiting each class node yields a list of inheritance paths to vbptrs. Each
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/// record with a vbptr creates an initially empty path.
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///
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/// To combine paths from child nodes, the paths are compared to check for
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/// ambiguity. Paths are "ambiguous" if multiple paths have the same set of
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/// components in the same order. Each group of ambiguous paths is extended by
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/// appending the class of the base from which it came. If the current class
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/// node produced an ambiguous path, its path is extended with the current class.
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/// After extending paths, MSVC again checks for ambiguity, and extends any
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/// ambiguous path which wasn't already extended. Because each node yields an
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/// unambiguous set of paths, MSVC doesn't need to extend any path more than once
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/// to produce an unambiguous set of paths.
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///
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/// The VBTableBuilder class attempts to implement this algorithm by repeatedly
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/// bucketing paths together by sorting them.
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///
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/// TODO: Presumably vftables use the same algorithm.
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///
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/// TODO: Implement the MSVC 2010 name mangling scheme to avoid emitting
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/// duplicate vbtables with different symbols.
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class VBTableBuilder {
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public:
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VBTableBuilder(CodeGenModule &CGM, const CXXRecordDecl *MostDerived);
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void enumerateVBTables(VBTableVector &VBTables);
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private:
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bool hasVBPtr(const CXXRecordDecl *RD);
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llvm::GlobalVariable *getAddrOfVBTable(const CXXRecordDecl *ReusingBase,
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ArrayRef<const CXXRecordDecl *> BasePath);
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/// Enumerates paths to bases with vbptrs. The paths elements are compressed
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/// to contain only the classes necessary to form an unambiguous path.
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void findUnambiguousPaths(const CXXRecordDecl *ReusingBase,
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BaseSubobject CurSubobject,
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VBTablePathVector &Paths);
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void extendPath(VBTablePath *Info, bool SecondPass);
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bool rebucketPaths(VBTablePathVector &Paths, size_t PathsStart,
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bool SecondPass = false);
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CodeGenModule &CGM;
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const CXXRecordDecl *MostDerived;
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/// Caches the layout of the most derived class.
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const ASTRecordLayout &DerivedLayout;
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/// Set of vbases to avoid re-visiting the same vbases.
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llvm::SmallPtrSet<const CXXRecordDecl*, 4> VBasesSeen;
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};
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} // namespace CodeGen
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} // namespace clang
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