CGClass.cpp

//===--- CGClass.cpp - Emit LLVM Code for C++ classes ---------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This contains code dealing with C++ code generation of classes
//
//===----------------------------------------------------------------------===//

#include "CGBlocks.h"
#include "CGDebugInfo.h"
#include "CodeGenFunction.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/EvaluatedExprVisitor.h"
#include "clang/AST/RecordLayout.h"
#include "clang/AST/StmtCXX.h"
#include "clang/Frontend/CodeGenOptions.h"

using namespace clang;
using namespace CodeGen;

static CharUnits 
ComputeNonVirtualBaseClassOffset(ASTContext &Context, 
                                 const CXXRecordDecl *DerivedClass,
                                 CastExpr::path_const_iterator Start,
                                 CastExpr::path_const_iterator End) {
  CharUnits Offset = CharUnits::Zero();
  
  const CXXRecordDecl *RD = DerivedClass;
  
  for (CastExpr::path_const_iterator I = Start; I != End; ++I) {
    const CXXBaseSpecifier *Base = *I;
    assert(!Base->isVirtual() && "Should not see virtual bases here!");

    // Get the layout.
    const ASTRecordLayout &Layout = Context.getASTRecordLayout(RD);
    
    const CXXRecordDecl *BaseDecl = 
      cast<CXXRecordDecl>(Base->getType()->getAs<RecordType>()->getDecl());
    
    // Add the offset.
    Offset += Layout.getBaseClassOffset(BaseDecl);
    
    RD = BaseDecl;
  }
  
  return Offset;
}

llvm::Constant *
CodeGenModule::GetNonVirtualBaseClassOffset(const CXXRecordDecl *ClassDecl,
                                   CastExpr::path_const_iterator PathBegin,
                                   CastExpr::path_const_iterator PathEnd) {
  assert(PathBegin != PathEnd && "Base path should not be empty!");

  CharUnits Offset = 
    ComputeNonVirtualBaseClassOffset(getContext(), ClassDecl,
                                     PathBegin, PathEnd);
  if (Offset.isZero())
    return 0;
  
  llvm::Type *PtrDiffTy = 
  Types.ConvertType(getContext().getPointerDiffType());
  
  return llvm::ConstantInt::get(PtrDiffTy, Offset.getQuantity());
}

/// Gets the address of a direct base class within a complete object.
/// This should only be used for (1) non-virtual bases or (2) virtual bases
/// when the type is known to be complete (e.g. in complete destructors).
///
/// The object pointed to by 'This' is assumed to be non-null.
llvm::Value *
CodeGenFunction::GetAddressOfDirectBaseInCompleteClass(llvm::Value *This,
                                                   const CXXRecordDecl *Derived,
                                                   const CXXRecordDecl *Base,
                                                   bool BaseIsVirtual) {
  // 'this' must be a pointer (in some address space) to Derived.
  assert(This->getType()->isPointerTy() &&
         cast<llvm::PointerType>(This->getType())->getElementType()
           == ConvertType(Derived));

  // Compute the offset of the virtual base.
  CharUnits Offset;
  const ASTRecordLayout &Layout = getContext().getASTRecordLayout(Derived);
  if (BaseIsVirtual)
    Offset = Layout.getVBaseClassOffset(Base);
  else
    Offset = Layout.getBaseClassOffset(Base);

  // Shift and cast down to the base type.
  // TODO: for complete types, this should be possible with a GEP.
  llvm::Value *V = This;
  if (Offset.isPositive()) {
    V = Builder.CreateBitCast(V, Int8PtrTy);
    V = Builder.CreateConstInBoundsGEP1_64(V, Offset.getQuantity());
  }
  V = Builder.CreateBitCast(V, ConvertType(Base)->getPointerTo());

  return V;
}

static llvm::Value *
ApplyNonVirtualAndVirtualOffset(CodeGenFunction &CGF, llvm::Value *ptr,
                                CharUnits nonVirtualOffset,
                                llvm::Value *virtualOffset) {
  // Assert that we have something to do.
  assert(!nonVirtualOffset.isZero() || virtualOffset != 0);

  // Compute the offset from the static and dynamic components.
  llvm::Value *baseOffset;
  if (!nonVirtualOffset.isZero()) {
    baseOffset = llvm::ConstantInt::get(CGF.PtrDiffTy,
                                        nonVirtualOffset.getQuantity());
    if (virtualOffset) {
      baseOffset = CGF.Builder.CreateAdd(virtualOffset, baseOffset);
    }
  } else {
    baseOffset = virtualOffset;
  }
  
  // Apply the base offset.
  ptr = CGF.Builder.CreateBitCast(ptr, CGF.Int8PtrTy);
  ptr = CGF.Builder.CreateInBoundsGEP(ptr, baseOffset, "add.ptr");
  return ptr;
}

llvm::Value *
CodeGenFunction::GetAddressOfBaseClass(llvm::Value *Value, 
                                       const CXXRecordDecl *Derived,
                                       CastExpr::path_const_iterator PathBegin,
                                       CastExpr::path_const_iterator PathEnd,
                                       bool NullCheckValue) {
  assert(PathBegin != PathEnd && "Base path should not be empty!");

  CastExpr::path_const_iterator Start = PathBegin;
  const CXXRecordDecl *VBase = 0;
  
  // Sema has done some convenient canonicalization here: if the
  // access path involved any virtual steps, the conversion path will
  // *start* with a step down to the correct virtual base subobject,
  // and hence will not require any further steps.
  if ((*Start)->isVirtual()) {
    VBase = 
      cast<CXXRecordDecl>((*Start)->getType()->getAs<RecordType>()->getDecl());
    ++Start;
  }

  // Compute the static offset of the ultimate destination within its
  // allocating subobject (the virtual base, if there is one, or else
  // the "complete" object that we see).
  CharUnits NonVirtualOffset = 
    ComputeNonVirtualBaseClassOffset(getContext(), VBase ? VBase : Derived,
                                     Start, PathEnd);

  // If there's a virtual step, we can sometimes "devirtualize" it.
  // For now, that's limited to when the derived type is final.
  // TODO: "devirtualize" this for accesses to known-complete objects.
  if (VBase && Derived->hasAttr<FinalAttr>()) {
    const ASTRecordLayout &layout = getContext().getASTRecordLayout(Derived);
    CharUnits vBaseOffset = layout.getVBaseClassOffset(VBase);
    NonVirtualOffset += vBaseOffset;
    VBase = 0; // we no longer have a virtual step
  }

  // Get the base pointer type.
  llvm::Type *BasePtrTy = 
    ConvertType((PathEnd[-1])->getType())->getPointerTo();

  // If the static offset is zero and we don't have a virtual step,
  // just do a bitcast; null checks are unnecessary.
  if (NonVirtualOffset.isZero() && !VBase) {
    return Builder.CreateBitCast(Value, BasePtrTy);
  }    

  llvm::BasicBlock *origBB = 0;
  llvm::BasicBlock *endBB = 0;
  
  // Skip over the offset (and the vtable load) if we're supposed to
  // null-check the pointer.
  if (NullCheckValue) {
    origBB = Builder.GetInsertBlock();
    llvm::BasicBlock *notNullBB = createBasicBlock("cast.notnull");
    endBB = createBasicBlock("cast.end");
    
    llvm::Value *isNull = Builder.CreateIsNull(Value);
    Builder.CreateCondBr(isNull, endBB, notNullBB);
    EmitBlock(notNullBB);
  }

  // Compute the virtual offset.
  llvm::Value *VirtualOffset = 0;
  if (VBase) {
    VirtualOffset = GetVirtualBaseClassOffset(Value, Derived, VBase);
  }