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//===--- MicrosoftVBTables.h - Virtual Base Table Emission ----------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This class generates data about MSVC virtual base tables.
//
//===----------------------------------------------------------------------===//
#include "clang/AST/BaseSubobject.h"
#include "clang/Basic/LLVM.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/IR/GlobalVariable.h"
#include <vector>
namespace clang {
class ASTRecordLayout;
namespace CodeGen {
class CodeGenModule;
struct VBTableInfo {
VBTableInfo(const CXXRecordDecl *ReusingBase, BaseSubobject VBPtrSubobject,
llvm::GlobalVariable *GV)
: ReusingBase(ReusingBase), VBPtrSubobject(VBPtrSubobject), GV(GV) { }
/// The vbtable will hold all of the virtual bases of ReusingBase. This may
/// or may not be the same class as VBPtrSubobject.Base. A derived class will
/// reuse the vbptr of the first non-virtual base subobject that has one.
const CXXRecordDecl *ReusingBase;
/// The vbptr is stored inside this subobject.
BaseSubobject VBPtrSubobject;
/// The GlobalVariable for this vbtable.
llvm::GlobalVariable *GV;
/// \brief Emits a definition for GV by setting it's initializer.
void EmitVBTableDefinition(CodeGenModule &CGM, const CXXRecordDecl *RD,
llvm::GlobalVariable::LinkageTypes Linkage) const;
};
// These are embedded in a DenseMap and the elements are large, so we don't want
// SmallVector.
typedef std::vector<VBTableInfo> VBTableVector;
struct VBTablePath;
typedef llvm::SmallVector<VBTablePath *, 6> VBTablePathVector;
/// Produces MSVC-compatible vbtable data. The symbols produced by this builder
/// match those produced by MSVC 2012, which is different from MSVC 2010.
///
/// Unlike Itanium, which uses only one vtable per class, MSVC uses a different
/// symbol for every "address point" installed in base subobjects. As a result,
/// we have to compute unique symbols for every table. Since there can be
/// multiple non-virtual base subobjects of the same class, combining the most
/// derived class with the base containing the vtable is insufficient. The most
/// trivial algorithm would be to mangle in the entire path from base to most
/// derived, but that would be too easy and would create unnecessarily large
/// symbols. ;)
///
/// MSVC 2012 appears to minimize the vbtable names using the following
/// algorithm. First, walk the class hierarchy in the usual order, depth first,
/// left to right, to find all of the subobjects which contain a vbptr field.
/// Visiting each class node yields a list of inheritance paths to vbptrs. Each
/// record with a vbptr creates an initially empty path.
///
/// To combine paths from child nodes, the paths are compared to check for
/// ambiguity. Paths are "ambiguous" if multiple paths have the same set of
/// components in the same order. Each group of ambiguous paths is extended by
/// appending the class of the base from which it came. If the current class
/// node produced an ambiguous path, its path is extended with the current class.
/// After extending paths, MSVC again checks for ambiguity, and extends any
/// ambiguous path which wasn't already extended. Because each node yields an
/// unambiguous set of paths, MSVC doesn't need to extend any path more than once
/// to produce an unambiguous set of paths.
///
/// The VBTableBuilder class attempts to implement this algorithm by repeatedly
/// bucketing paths together by sorting them.
///
/// TODO: Presumably vftables use the same algorithm.
///
/// TODO: Implement the MSVC 2010 name mangling scheme to avoid emitting
/// duplicate vbtables with different symbols.
class VBTableBuilder {
public:
VBTableBuilder(CodeGenModule &CGM, const CXXRecordDecl *MostDerived);
void enumerateVBTables(VBTableVector &VBTables);
private:
bool hasVBPtr(const CXXRecordDecl *RD);
llvm::GlobalVariable *getAddrOfVBTable(const CXXRecordDecl *ReusingBase,
ArrayRef<const CXXRecordDecl *> BasePath);
/// Enumerates paths to bases with vbptrs. The paths elements are compressed
/// to contain only the classes necessary to form an unambiguous path.
void findUnambiguousPaths(const CXXRecordDecl *ReusingBase,
BaseSubobject CurSubobject,
VBTablePathVector &Paths);
void extendPath(VBTablePath *Info, bool SecondPass);
bool rebucketPaths(VBTablePathVector &Paths, size_t PathsStart,
bool SecondPass = false);
CodeGenModule &CGM;
const CXXRecordDecl *MostDerived;
/// Caches the layout of the most derived class.
const ASTRecordLayout &DerivedLayout;
/// Set of vbases to avoid re-visiting the same vbases.
llvm::SmallPtrSet<const CXXRecordDecl*, 4> VBasesSeen;
};
} // namespace CodeGen
} // namespace clang
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