CGRecordLayoutBuilder.cpp | Explore in Territory

//===--- CGRecordLayoutBuilder.cpp - CGRecordLayout builder  ----*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// Builder implementation for CGRecordLayout objects.
//
//===----------------------------------------------------------------------===//

#include "ABIInfoImpl.h"
#include "CGCXXABI.h"
#include "CGRecordLayout.h"
#include "CodeGenTypes.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Attr.h"
#include "clang/AST/CXXInheritance.h"
#include "clang/AST/DeclCXX.h"
#include "clang/AST/Expr.h"
#include "clang/AST/RecordLayout.h"
#include "clang/Basic/CodeGenOptions.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Type.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
usingnamespaceclang;
usingnamespaceCodeGen;

namespace {
/// The CGRecordLowering is responsible for lowering an ASTRecordLayout to an
/// llvm::Type.  Some of the lowering is straightforward, some is not.  Here we
/// detail some of the complexities and weirdnesses here.
/// * LLVM does not have unions - Unions can, in theory be represented by any
///   llvm::Type with correct size.  We choose a field via a specific heuristic
///   and add padding if necessary.
/// * LLVM does not have bitfields - Bitfields are collected into contiguous
///   runs and allocated as a single storage type for the run.  ASTRecordLayout
///   contains enough information to determine where the runs break.  Microsoft
///   and Itanium follow different rules and use different codepaths.
/// * It is desired that, when possible, bitfields use the appropriate iN type
///   when lowered to llvm types. For example unsigned x : 24 gets lowered to
///   i24.  This isn't always possible because i24 has storage size of 32 bit
///   and if it is possible to use that extra byte of padding we must use [i8 x
///   3] instead of i24. This is computed when accumulating bitfields in
///   accumulateBitfields.
///   C++ examples that require clipping:
///   struct { int a : 24; char b; }; // a must be clipped, b goes at offset 3
///   struct A { int a : 24; ~A(); }; // a must be clipped because:
///   struct B : A { char b; }; // b goes at offset 3
/// * The allocation of bitfield access units is described in more detail in
///   CGRecordLowering::accumulateBitFields.
/// * Clang ignores 0 sized bitfields and 0 sized bases but *not* zero sized
///   fields.  The existing asserts suggest that LLVM assumes that *every* field
///   has an underlying storage type.  Therefore empty structures containing
///   zero sized subobjects such as empty records or zero sized arrays still get
///   a zero sized (empty struct) storage type.
/// * Clang reads the complete type rather than the base type when generating
///   code to access fields.  Bitfields in tail position with tail padding may
///   be clipped in the base class but not the complete class (we may discover
///   that the tail padding is not used in the complete class.) However,
///   because LLVM reads from the complete type it can generate incorrect code
///   if we do not clip the tail padding off of the bitfield in the complete
///   layout.
/// * Itanium allows nearly empty primary virtual bases.  These bases don't get
///   get their own storage because they're laid out as part of another base
///   or at the beginning of the structure.  Determining if a VBase actually
///   gets storage awkwardly involves a walk of all bases.
/// * VFPtrs and VBPtrs do *not* make a record NotZeroInitializable.
struct CGRecordLowering { … };
} // namespace {

CGRecordLowering::CGRecordLowering(CodeGenTypes &Types, const RecordDecl *D,
                                   bool Packed)
    : … { … }

void CGRecordLowering::setBitFieldInfo(
    const FieldDecl *FD, CharUnits StartOffset, llvm::Type *StorageType) { … }

void CGRecordLowering::lower(bool NVBaseType) { … }

void CGRecordLowering::lowerUnion(bool isNoUniqueAddress) { … }

void CGRecordLowering::accumulateFields(bool isNonVirtualBaseType) { … }

// Create members for bitfields. Field is a bitfield, and FieldEnd is the end
// iterator of the record. Return the first non-bitfield encountered.  We need
// to know whether this is the base or complete layout, as virtual bases could
// affect the upper bound of bitfield access unit allocation.
RecordDecl::field_iterator
CGRecordLowering::accumulateBitFields(bool isNonVirtualBaseType,
                                      RecordDecl::field_iterator Field,
                                      RecordDecl::field_iterator FieldEnd) { … }

void CGRecordLowering::accumulateBases() { … }

/// The AAPCS that defines that, when possible, bit-fields should
/// be accessed using containers of the declared type width:
/// When a volatile bit-field is read, and its container does not overlap with
/// any non-bit-field member or any zero length bit-field member, its container
/// must be read exactly once using the access width appropriate to the type of
/// the container. When a volatile bit-field is written, and its container does
/// not overlap with any non-bit-field member or any zero-length bit-field
/// member, its container must be read exactly once and written exactly once
/// using the access width appropriate to the type of the container. The two
/// accesses are not atomic.
///
/// Enforcing the width restriction can be disabled using
/// -fno-aapcs-bitfield-width.
void CGRecordLowering::computeVolatileBitfields() { … }

void CGRecordLowering::accumulateVPtrs() { … }

CharUnits
CGRecordLowering::calculateTailClippingOffset(bool isNonVirtualBaseType) const { … }

void CGRecordLowering::accumulateVBases() { … }

bool CGRecordLowering::hasOwnStorage(const CXXRecordDecl *Decl,
                                     const CXXRecordDecl *Query) const { … }

void CGRecordLowering::calculateZeroInit() { … }

// Verify accumulateBitfields computed the correct storage representations.
void CGRecordLowering::checkBitfieldClipping(bool IsNonVirtualBaseType) const { … }

void CGRecordLowering::determinePacked(bool NVBaseType) { … }

void CGRecordLowering::insertPadding() { … }

void CGRecordLowering::fillOutputFields() { … }

CGBitFieldInfo CGBitFieldInfo::MakeInfo(CodeGenTypes &Types,
                                        const FieldDecl *FD,
                                        uint64_t Offset, uint64_t Size,
                                        uint64_t StorageSize,
                                        CharUnits StorageOffset) { … }

std::unique_ptr<CGRecordLayout>
CodeGenTypes::ComputeRecordLayout(const RecordDecl *D, llvm::StructType *Ty) { … }

void CGRecordLayout::print(raw_ostream &OS) const { … }

LLVM_DUMP_METHOD void CGRecordLayout::dump() const { … }

void CGBitFieldInfo::print(raw_ostream &OS) const { … }

LLVM_DUMP_METHOD void CGBitFieldInfo::dump() const { … }
llvm/clang/lib/CodeGen/CGRecordLayoutBuilder.cpp
