Skip to content

Commit

Permalink
Reapply "[clang][CodeGen] Zero init unspecified fields in initializer…
Browse files Browse the repository at this point in the history 
…s in C" (#109898) (#110051)

This reverts commit d50eaac. Also fixes
a bug calculating offsets for bit fields in the original patch.
  • Loading branch information
@yabinc
yabinc authored Oct 14, 2024
1 parent 9eddc8b commit 6277465
Show file tree
Hide file tree
Showing 23 changed files with 754 additions and 96 deletions.
23 changes: 23 additions & 0 deletions clang/docs/LanguageExtensions.rst
View file
Original file line number Diff line number Diff line change
Expand Up @@ -5883,3 +5883,26 @@ specify the starting offset to begin embedding from. The resources is treated
as being empty if the specified offset is larger than the number of bytes in
the resource. The offset will be applied *before* any ``limit`` parameters are
applied.
Union and aggregate initialization in C
=======================================
In C23 (N2900), when an object is initialized from initializer ``= {}``, all
elements of arrays, all members of structs, and the first members of unions are
empty-initialized recursively. In addition, all padding bits are initialized to
zero.
Clang guarantees the following behaviors:
* ``1:`` Clang supports initializer ``= {}`` mentioned above in all C
standards.
* ``2:`` When unions are initialized from initializer ``= {}``, bytes outside
of the first members of unions are also initialized to zero.
* ``3:`` When unions, structures and arrays are initialized from initializer
``= { initializer-list }``, all members not explicitly initialized in
the initializer list are empty-initialized recursively. In addition, all
padding bits are initialized to zero.
Currently, the above extension only applies to C source code, not C++.
73 changes: 71 additions & 2 deletions clang/lib/CodeGen/CGExprAgg.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@

#include "CGCXXABI.h"
#include "CGObjCRuntime.h"
#include "CGRecordLayout.h"
#include "CodeGenFunction.h"
#include "CodeGenModule.h"
#include "ConstantEmitter.h"
Expand Down Expand Up @@ -64,6 +65,9 @@ class AggExprEmitter : public StmtVisitor<AggExprEmitter> {
void withReturnValueSlot(const Expr *E,
llvm::function_ref<RValue(ReturnValueSlot)> Fn);

void DoZeroInitPadding(uint64_t &PaddingStart, uint64_t PaddingEnd,
const FieldDecl *NextField);

public:
AggExprEmitter(CodeGenFunction &cgf, AggValueSlot Dest, bool IsResultUnused)
: CGF(cgf), Builder(CGF.Builder), Dest(Dest),
Expand Down Expand Up @@ -1698,6 +1702,9 @@ void AggExprEmitter::VisitCXXParenListOrInitListExpr(
// Prepare a 'this' for CXXDefaultInitExprs.
CodeGenFunction::FieldConstructionScope FCS(CGF, Dest.getAddress());

const bool ZeroInitPadding =
CGF.CGM.shouldZeroInitPadding() && !Dest.isZeroed();

if (record->isUnion()) {
// Only initialize one field of a union. The field itself is
// specified by the initializer list.
Expand All @@ -1722,16 +1729,27 @@ void AggExprEmitter::VisitCXXParenListOrInitListExpr(
if (NumInitElements) {
// Store the initializer into the field
EmitInitializationToLValue(InitExprs[0], FieldLoc);
if (ZeroInitPadding) {
uint64_t TotalSize = CGF.getContext().toBits(
Dest.getPreferredSize(CGF.getContext(), DestLV.getType()));
uint64_t FieldSize = CGF.getContext().getTypeSize(FieldLoc.getType());
DoZeroInitPadding(FieldSize, TotalSize, nullptr);
}
} else {
// Default-initialize to null.
EmitNullInitializationToLValue(FieldLoc);
if (ZeroInitPadding)
EmitNullInitializationToLValue(DestLV);
else
EmitNullInitializationToLValue(FieldLoc);
}

return;
}

// Here we iterate over the fields; this makes it simpler to both
// default-initialize fields and skip over unnamed fields.
const ASTRecordLayout &Layout = CGF.getContext().getASTRecordLayout(record);
uint64_t PaddingStart = 0;

for (const auto *field : record->fields()) {
// We're done once we hit the flexible array member.
if (field->getType()->isIncompleteArrayType())
Expand All @@ -1748,6 +1766,9 @@ void AggExprEmitter::VisitCXXParenListOrInitListExpr(
CGF.getTypes().isZeroInitializable(ExprToVisit->getType()))
break;

if (ZeroInitPadding)
DoZeroInitPadding(PaddingStart,
Layout.getFieldOffset(field->getFieldIndex()), field);

LValue LV = CGF.EmitLValueForFieldInitialization(DestLV, field);
// We never generate write-barries for initialized fields.
Expand All @@ -1774,6 +1795,54 @@ void AggExprEmitter::VisitCXXParenListOrInitListExpr(
}
}
}
if (ZeroInitPadding) {
uint64_t TotalSize = CGF.getContext().toBits(
Dest.getPreferredSize(CGF.getContext(), DestLV.getType()));
DoZeroInitPadding(PaddingStart, TotalSize, nullptr);
}
}

void AggExprEmitter::DoZeroInitPadding(uint64_t &PaddingStart,
uint64_t PaddingEnd,
const FieldDecl *NextField) {

auto InitBytes = [&](uint64_t StartBit, uint64_t EndBit) {
CharUnits Start = CGF.getContext().toCharUnitsFromBits(StartBit);
CharUnits End = CGF.getContext().toCharUnitsFromBits(EndBit);
Address Addr = Dest.getAddress().withElementType(CGF.CharTy);
if (!Start.isZero())
Addr = Builder.CreateConstGEP(Addr, Start.getQuantity());
llvm::Constant *SizeVal = Builder.getInt64((End - Start).getQuantity());
CGF.Builder.CreateMemSet(Addr, Builder.getInt8(0), SizeVal, false);
};

if (NextField != nullptr && NextField->isBitField()) {
// For bitfield, zero init StorageSize before storing the bits. So we don't
// need to handle big/little endian.
const CGRecordLayout &RL =
CGF.getTypes().getCGRecordLayout(NextField->getParent());
const CGBitFieldInfo &Info = RL.getBitFieldInfo(NextField);
uint64_t StorageStart = CGF.getContext().toBits(Info.StorageOffset);
if (StorageStart + Info.StorageSize > PaddingStart) {
if (StorageStart > PaddingStart)
InitBytes(PaddingStart, StorageStart);
Address Addr = Dest.getAddress();
if (!Info.StorageOffset.isZero())
Addr = Builder.CreateConstGEP(Addr.withElementType(CGF.CharTy),
Info.StorageOffset.getQuantity());
Addr = Addr.withElementType(
llvm::Type::getIntNTy(CGF.getLLVMContext(), Info.StorageSize));
Builder.CreateStore(Builder.getIntN(Info.StorageSize, 0), Addr);
PaddingStart = StorageStart + Info.StorageSize;
}
return;
}

if (PaddingStart < PaddingEnd)
InitBytes(PaddingStart, PaddingEnd);
if (NextField != nullptr)
PaddingStart =
PaddingEnd + CGF.getContext().getTypeSize(NextField->getType());
}

void AggExprEmitter::VisitArrayInitLoopExpr(const ArrayInitLoopExpr *E,
Expand Down
107 changes: 96 additions & 11 deletions clang/lib/CodeGen/CGExprConstant.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,16 @@ using namespace CodeGen;
namespace {
class ConstExprEmitter;

llvm::Constant *getPadding(const CodeGenModule &CGM, CharUnits PadSize) {
llvm::Type *Ty = CGM.CharTy;
if (PadSize > CharUnits::One())
Ty = llvm::ArrayType::get(Ty, PadSize.getQuantity());
if (CGM.shouldZeroInitPadding()) {
return llvm::Constant::getNullValue(Ty);
}
return llvm::UndefValue::get(Ty);
}

struct ConstantAggregateBuilderUtils {
CodeGenModule &CGM;

Expand All @@ -61,10 +71,7 @@ struct ConstantAggregateBuilderUtils {
}

llvm::Constant *getPadding(CharUnits PadSize) const {
llvm::Type *Ty = CGM.CharTy;
if (PadSize > CharUnits::One())
Ty = llvm::ArrayType::get(Ty, PadSize.getQuantity());
return llvm::UndefValue::get(Ty);
return ::getPadding(CGM, PadSize);
}

llvm::Constant *getZeroes(CharUnits ZeroSize) const {
Expand Down Expand Up @@ -591,6 +598,11 @@ class ConstStructBuilder {
bool Build(const InitListExpr *ILE, bool AllowOverwrite);
bool Build(const APValue &Val, const RecordDecl *RD, bool IsPrimaryBase,
const CXXRecordDecl *VTableClass, CharUnits BaseOffset);
bool DoZeroInitPadding(const ASTRecordLayout &Layout, unsigned FieldNo,
const FieldDecl &Field, bool AllowOverwrite,
CharUnits &SizeSoFar, bool &ZeroFieldSize);
bool DoZeroInitPadding(const ASTRecordLayout &Layout, bool AllowOverwrite,
CharUnits SizeSoFar);
llvm::Constant *Finalize(QualType Ty);
};

Expand Down Expand Up @@ -715,6 +727,10 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
if (CXXRD->getNumBases())
return false;

const bool ZeroInitPadding = CGM.shouldZeroInitPadding();
bool ZeroFieldSize = false;
CharUnits SizeSoFar = CharUnits::Zero();

for (FieldDecl *Field : RD->fields()) {
++FieldNo;

Expand All @@ -732,8 +748,13 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
const Expr *Init = nullptr;
if (ElementNo < ILE->getNumInits())
Init = ILE->getInit(ElementNo++);
if (isa_and_nonnull<NoInitExpr>(Init))
if (isa_and_nonnull<NoInitExpr>(Init)) {
if (ZeroInitPadding &&
!DoZeroInitPadding(Layout, FieldNo, *Field, AllowOverwrite, SizeSoFar,
ZeroFieldSize))
return false;
continue;
}

// Zero-sized fields are not emitted, but their initializers may still
// prevent emission of this struct as a constant.
Expand All @@ -743,6 +764,11 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
continue;
}

if (ZeroInitPadding &&
!DoZeroInitPadding(Layout, FieldNo, *Field, AllowOverwrite, SizeSoFar,
ZeroFieldSize))
return false;

// When emitting a DesignatedInitUpdateExpr, a nested InitListExpr
// represents additional overwriting of our current constant value, and not
// a new constant to emit independently.
Expand All @@ -768,6 +794,10 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
if (!EltInit)
return false;

if (ZeroInitPadding && ZeroFieldSize)
SizeSoFar += CharUnits::fromQuantity(
CGM.getDataLayout().getTypeAllocSize(EltInit->getType()));

if (!Field->isBitField()) {
// Handle non-bitfield members.
if (!AppendField(Field, Layout.getFieldOffset(FieldNo), EltInit,
Expand All @@ -785,6 +815,9 @@ bool ConstStructBuilder::Build(const InitListExpr *ILE, bool AllowOverwrite) {
}
}

if (ZeroInitPadding && !DoZeroInitPadding(Layout, AllowOverwrite, SizeSoFar))
return false;

return true;
}

Expand Down Expand Up @@ -849,6 +882,9 @@ bool ConstStructBuilder::Build(const APValue &Val, const RecordDecl *RD,

unsigned FieldNo = 0;
uint64_t OffsetBits = CGM.getContext().toBits(Offset);
const bool ZeroInitPadding = CGM.shouldZeroInitPadding();
bool ZeroFieldSize = false;
CharUnits SizeSoFar = CharUnits::Zero();

bool AllowOverwrite = false;
for (RecordDecl::field_iterator Field = RD->field_begin(),
Expand All @@ -870,6 +906,15 @@ bool ConstStructBuilder::Build(const APValue &Val, const RecordDecl *RD,
if (!EltInit)
return false;

if (ZeroInitPadding) {
if (!DoZeroInitPadding(Layout, FieldNo, **Field, AllowOverwrite,
SizeSoFar, ZeroFieldSize))
return false;
if (ZeroFieldSize)
SizeSoFar += CharUnits::fromQuantity(
CGM.getDataLayout().getTypeAllocSize(EltInit->getType()));
}

if (!Field->isBitField()) {
// Handle non-bitfield members.
if (!AppendField(*Field, Layout.getFieldOffset(FieldNo) + OffsetBits,
Expand All @@ -886,7 +931,49 @@ bool ConstStructBuilder::Build(const APValue &Val, const RecordDecl *RD,
return false;
}
}
if (ZeroInitPadding && !DoZeroInitPadding(Layout, AllowOverwrite, SizeSoFar))
return false;

return true;
}

bool ConstStructBuilder::DoZeroInitPadding(
const ASTRecordLayout &Layout, unsigned FieldNo, const FieldDecl &Field,
bool AllowOverwrite, CharUnits &SizeSoFar, bool &ZeroFieldSize) {
uint64_t StartBitOffset = Layout.getFieldOffset(FieldNo);
CharUnits StartOffset = CGM.getContext().toCharUnitsFromBits(StartBitOffset);
if (SizeSoFar < StartOffset)
if (!AppendBytes(SizeSoFar, getPadding(CGM, StartOffset - SizeSoFar),
AllowOverwrite))
return false;

if (!Field.isBitField()) {
CharUnits FieldSize = CGM.getContext().getTypeSizeInChars(Field.getType());
SizeSoFar = StartOffset + FieldSize;
ZeroFieldSize = FieldSize.isZero();
} else {
const CGRecordLayout &RL =
CGM.getTypes().getCGRecordLayout(Field.getParent());
const CGBitFieldInfo &Info = RL.getBitFieldInfo(&Field);
uint64_t EndBitOffset = StartBitOffset + Info.Size;
SizeSoFar = CGM.getContext().toCharUnitsFromBits(EndBitOffset);
if (EndBitOffset % CGM.getContext().getCharWidth() != 0) {
SizeSoFar++;
}
ZeroFieldSize = Info.Size == 0;
}
return true;
}

bool ConstStructBuilder::DoZeroInitPadding(const ASTRecordLayout &Layout,
bool AllowOverwrite,
CharUnits SizeSoFar) {
CharUnits TotalSize = Layout.getSize();
if (SizeSoFar < TotalSize)
if (!AppendBytes(SizeSoFar, getPadding(CGM, TotalSize - SizeSoFar),
AllowOverwrite))
return false;
SizeSoFar = TotalSize;
return true;
}

Expand Down Expand Up @@ -1127,12 +1214,10 @@ class ConstExprEmitter

assert(CurSize <= TotalSize && "Union size mismatch!");
if (unsigned NumPadBytes = TotalSize - CurSize) {
llvm::Type *Ty = CGM.CharTy;
if (NumPadBytes > 1)
Ty = llvm::ArrayType::get(Ty, NumPadBytes);

Elts.push_back(llvm::UndefValue::get(Ty));
Types.push_back(Ty);
llvm::Constant *Padding =
getPadding(CGM, CharUnits::fromQuantity(NumPadBytes));
Elts.push_back(Padding);
Types.push_back(Padding->getType());
}

llvm::StructType *STy = llvm::StructType::get(VMContext, Types, false);
Expand Down
Loading

0 comments on commit 6277465

Please sign in to comment.