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Extensively comment bitfield layout, rearrange some 

code for legibility, and fix a bug with bitfields in packed
ms_structs.

rdar://15926990

llvm-svn: 200379
This commit is contained in:
John McCall 2014-01-29 07:53:44 +00:00
parent 6e15ec1442
commit 30268ca2e0
2 changed files with 167 additions and 84 deletions
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230
clang/lib/AST/RecordLayoutBuilder.cpp
View File

@ -1450,127 +1450,191 @@ void RecordLayoutBuilder::LayoutBitField(const FieldDecl *D) {
uint64_t TypeSize = FieldInfo.first;
unsigned FieldAlign = FieldInfo.second;
// UnfilledBitsInLastUnit is the difference between the end of the
// last allocated bitfield (i.e. the first bit offset available for
// bitfields) and the end of the current data size in bits (i.e. the
// first bit offset available for non-bitfields). The current data
// size in bits is always a multiple of the char size; additionally,
// for ms_struct records it's also a multiple of the
// LastBitfieldTypeSize (if set).
// The basic bitfield layout rule for ms_struct is to allocate an
// entire unit of the bitfield's declared type (e.g. 'unsigned
// long'), then parcel it up among successive bitfields whose
// declared types have the same size, making a new unit as soon as
// the last can no longer store the whole value.
// The standard bitfield layout rule for non-ms_struct is to place
// bitfields at the next available bit offset where the entire
// bitfield would fit in an aligned storage unit of the declared
// type (even if there are also non-bitfields within that same
// unit). However, some targets (those that !useBitFieldTypeAlignment())
// don't require this storage unit to be aligned, and therefore
// always put the bit-field at the next available bit offset.
// Such targets generally do interpret zero-width bitfields as
// forcing the use of a new storage unit.
// First, some simple bookkeeping to perform for ms_struct structs.
if (IsMsStruct) {
// The field alignment for integer types in ms_struct structs is
// always the size.
// The field alignment for integer types is always the size.
FieldAlign = TypeSize;
// Ignore zero-length bitfields after non-bitfields in ms_struct structs.
if (!FieldSize && !LastBitfieldTypeSize)
FieldAlign = 1;
// If a bitfield is followed by a bitfield of a different size, don't
// pack the bits together in ms_struct structs.
// If the previous field was not a bitfield, or was a bitfield
// with a different storage unit size, we're done with that
// storage unit.
if (LastBitfieldTypeSize != TypeSize) {
// Also, ignore zero-length bitfields after non-bitfields.
if (!LastBitfieldTypeSize && !FieldSize)
FieldAlign = 1;
UnfilledBitsInLastUnit = 0;
LastBitfieldTypeSize = 0;
}
}
uint64_t UnpaddedFieldOffset = getDataSizeInBits() - UnfilledBitsInLastUnit;
uint64_t FieldOffset = IsUnion ? 0 : UnpaddedFieldOffset;
bool ZeroLengthBitfield = false;
if (!Context.getTargetInfo().useBitFieldTypeAlignment() &&
Context.getTargetInfo().useZeroLengthBitfieldAlignment() &&
FieldSize == 0) {
// The alignment of a zero-length bitfield affects the alignment
// of the next member. The alignment is the max of the zero
// length bitfield's alignment and a target specific fixed value.
ZeroLengthBitfield = true;
unsigned ZeroLengthBitfieldBoundary =
Context.getTargetInfo().getZeroLengthBitfieldBoundary();
if (ZeroLengthBitfieldBoundary > FieldAlign)
FieldAlign = ZeroLengthBitfieldBoundary;
}
// If the field is wider than its declared type, it follows
// different rules in all cases.
if (FieldSize > TypeSize) {
LayoutWideBitField(FieldSize, TypeSize, FieldPacked, D);
return;
}
// The align if the field is not packed. This is to check if the attribute
// was unnecessary (-Wpacked).
// Compute the next available bit offset.
uint64_t FieldOffset =
IsUnion ? 0 : (getDataSizeInBits() - UnfilledBitsInLastUnit);
// Handle targets that don't honor bitfield type alignment.
if (!Context.getTargetInfo().useBitFieldTypeAlignment()) {
// Some such targets do honor it on zero-width bitfields.
if (FieldSize == 0 &&
Context.getTargetInfo().useZeroLengthBitfieldAlignment()) {
// The alignment to round up to is the max of the field's natural
// alignment and a target-specific fixed value (sometimes zero).
unsigned ZeroLengthBitfieldBoundary =
Context.getTargetInfo().getZeroLengthBitfieldBoundary();
FieldAlign = std::max(FieldAlign, ZeroLengthBitfieldBoundary);
// If that doesn't apply, just ignore the field alignment.
} else {
FieldAlign = 1;
}
}
// Remember the alignment we would have used if the field were not packed.
unsigned UnpackedFieldAlign = FieldAlign;
uint64_t UnpackedFieldOffset = FieldOffset;
if (!Context.getTargetInfo().useBitFieldTypeAlignment() && !ZeroLengthBitfield)
UnpackedFieldAlign = 1;
if (FieldPacked ||
(!Context.getTargetInfo().useBitFieldTypeAlignment() && !ZeroLengthBitfield))
// Ignore the field alignment if the field is packed.
if (FieldPacked)
FieldAlign = 1;
FieldAlign = std::max(FieldAlign, D->getMaxAlignment());
UnpackedFieldAlign = std::max(UnpackedFieldAlign, D->getMaxAlignment());
// The maximum field alignment overrides the aligned attribute.
if (!MaxFieldAlignment.isZero() && FieldSize != 0) {
// But, if there's an 'aligned' attribute on the field, honor that.
if (unsigned ExplicitFieldAlign = D->getMaxAlignment()) {
FieldAlign = std::max(FieldAlign, ExplicitFieldAlign);
UnpackedFieldAlign = std::max(UnpackedFieldAlign, ExplicitFieldAlign);
}
// But, if there's a #pragma pack in play, that takes precedent over
// even the 'aligned' attribute, for non-zero-width bitfields.
if (!MaxFieldAlignment.isZero() && FieldSize) {
unsigned MaxFieldAlignmentInBits = Context.toBits(MaxFieldAlignment);
FieldAlign = std::min(FieldAlign, MaxFieldAlignmentInBits);
UnpackedFieldAlign = std::min(UnpackedFieldAlign, MaxFieldAlignmentInBits);
}
// ms_struct bitfields always have to start at a round alignment.
if (IsMsStruct && !LastBitfieldTypeSize) {
FieldOffset = llvm::RoundUpToAlignment(FieldOffset, FieldAlign);
UnpackedFieldOffset = llvm::RoundUpToAlignment(UnpackedFieldOffset,
UnpackedFieldAlign);
// For purposes of diagnostics, we're going to simultaneously
// compute the field offsets that we would have used if we weren't
// adding any alignment padding or if the field weren't packed.
uint64_t UnpaddedFieldOffset = FieldOffset;
uint64_t UnpackedFieldOffset = FieldOffset;
// Check if we need to add padding to fit the bitfield within an
// allocation unit with the right size and alignment. The rules are
// somewhat different here for ms_struct structs.
if (IsMsStruct) {
// If it's not a zero-width bitfield, and we can fit the bitfield
// into the active storage unit (and we haven't already decided to
// start a new storage unit), just do so, regardless of any other
// other consideration. Otherwise, round up to the right alignment.
if (FieldSize == 0 || FieldSize > UnfilledBitsInLastUnit) {
FieldOffset = llvm::RoundUpToAlignment(FieldOffset, FieldAlign);
UnpackedFieldOffset = llvm::RoundUpToAlignment(UnpackedFieldOffset,
UnpackedFieldAlign);
UnfilledBitsInLastUnit = 0;
}
} else {
// #pragma pack, with any value, suppresses the insertion of padding.
bool AllowPadding = MaxFieldAlignment.isZero();
// Compute the real offset.
if (FieldSize == 0 ||
(AllowPadding &&
(FieldOffset & (FieldAlign-1)) + FieldSize > TypeSize)) {
FieldOffset = llvm::RoundUpToAlignment(FieldOffset, FieldAlign);
}
// Repeat the computation for diagnostic purposes.
if (FieldSize == 0 ||
(AllowPadding &&
(UnpackedFieldOffset & (UnpackedFieldAlign-1)) + FieldSize > TypeSize))
UnpackedFieldOffset = llvm::RoundUpToAlignment(UnpackedFieldOffset,
UnpackedFieldAlign);
}
// Check if we need to add padding to give the field the correct alignment.
if (FieldSize == 0 ||
(MaxFieldAlignment.isZero() &&
(FieldOffset & (FieldAlign-1)) + FieldSize > TypeSize))
FieldOffset = llvm::RoundUpToAlignment(FieldOffset, FieldAlign);
if (FieldSize == 0 ||
(MaxFieldAlignment.isZero() &&
(UnpackedFieldOffset & (UnpackedFieldAlign-1)) + FieldSize > TypeSize))
UnpackedFieldOffset = llvm::RoundUpToAlignment(UnpackedFieldOffset,
UnpackedFieldAlign);
// Padding members don't affect overall alignment, unless zero length bitfield
// alignment is enabled.
if (!D->getIdentifier() &&
!Context.getTargetInfo().useZeroLengthBitfieldAlignment() &&
!IsMsStruct)
FieldAlign = UnpackedFieldAlign = 1;
// If we're using external layout, give the external layout a chance
// to override this information.
if (ExternalLayout)
FieldOffset = updateExternalFieldOffset(D, FieldOffset);
// Place this field at the current location.
// Okay, place the bitfield at the calculated offset.
FieldOffsets.push_back(FieldOffset);
// Bookkeeping:
// Anonymous members don't affect the overall record alignment,
// except on targets where they do.
if (!IsMsStruct &&
!Context.getTargetInfo().useZeroLengthBitfieldAlignment() &&
!D->getIdentifier())
FieldAlign = UnpackedFieldAlign = 1;
// Diagnose differences in layout due to padding or packing.
if (!ExternalLayout)
CheckFieldPadding(FieldOffset, UnpaddedFieldOffset, UnpackedFieldOffset,
UnpackedFieldAlign, FieldPacked, D);
// Update DataSize to include the last byte containing (part of) the bitfield.
// For unions, this is just a max operation, as usual.
if (IsUnion) {
// FIXME: I think FieldSize should be TypeSize here.
setDataSize(std::max(getDataSizeInBits(), FieldSize));
} else {
if (IsMsStruct && FieldSize) {
// Under ms_struct, a bitfield always takes up space equal to the size
// of the type. We can't just change the alignment computation on the
// other codepath because of the way this interacts with #pragma pack:
// in a packed struct, we need to allocate misaligned space in the
// struct to hold the bitfield.
if (!UnfilledBitsInLastUnit) {
setDataSize(FieldOffset + TypeSize);
UnfilledBitsInLastUnit = TypeSize - FieldSize;
} else if (UnfilledBitsInLastUnit < FieldSize) {
setDataSize(getDataSizeInBits() + TypeSize);
UnfilledBitsInLastUnit = TypeSize - FieldSize;
} else {
UnfilledBitsInLastUnit -= FieldSize;
}
LastBitfieldTypeSize = TypeSize;
} else {
uint64_t NewSizeInBits = FieldOffset + FieldSize;
uint64_t BitfieldAlignment = Context.getTargetInfo().getCharAlign();
setDataSize(llvm::RoundUpToAlignment(NewSizeInBits, BitfieldAlignment));
UnfilledBitsInLastUnit = getDataSizeInBits() - NewSizeInBits;
LastBitfieldTypeSize = 0;
// For non-zero-width bitfields in ms_struct structs, allocate a new
// storage unit if necessary.
} else if (IsMsStruct && FieldSize) {
// We should have cleared UnfilledBitsInLastUnit in every case
// where we changed storage units.
if (!UnfilledBitsInLastUnit) {
setDataSize(FieldOffset + TypeSize);
UnfilledBitsInLastUnit = TypeSize;
}
UnfilledBitsInLastUnit -= FieldSize;
LastBitfieldTypeSize = TypeSize;
// Otherwise, bump the data size up to include the bitfield,
// including padding up to char alignment, and then remember how
// bits we didn't use.
} else {
uint64_t NewSizeInBits = FieldOffset + FieldSize;
uint64_t CharAlignment = Context.getTargetInfo().getCharAlign();
setDataSize(llvm::RoundUpToAlignment(NewSizeInBits, CharAlignment));
UnfilledBitsInLastUnit = getDataSizeInBits() - NewSizeInBits;
// The only time we can get here for an ms_struct is if this is a
// zero-width bitfield, which doesn't count as anything for the
// purposes of unfilled bits.
LastBitfieldTypeSize = 0;
}
// Update the size.

21
clang/test/CodeGen/ms_struct-pack.c
View File

@ -1,4 +1,4 @@
// RUN: %clang_cc1 -emit-llvm-only -triple i386-apple-darwin9 %s
// RUN: %clang_cc1 -emit-llvm-only -triple i386-apple-darwin9 -fdump-record-layouts %s | FileCheck %s
// rdar://8823265
#pragma pack(1)
@ -123,3 +123,22 @@ typedef struct _eight_ms eight_ms;
static int a8[(sizeof(eight_ms) == 48) - 1];
// rdar://15926990
#pragma pack(2)
struct test0 {
unsigned long a : 8;
unsigned long b : 8;
unsigned long c : 8;
unsigned long d : 10;
unsigned long e : 1;
} __attribute__((__ms_struct__));
// CHECK: Type: struct test0
// CHECK-NEXT: Record:
// CHECK-NEXT: Layout:
// CHECK-NEXT: Size:64
// CHECK-NEXT: DataSize:64
// CHECK-NEXT: Alignment:16
// CHECK-NEXT: FieldOffsets: [0, 8, 16, 32, 42]>
static int test0[(sizeof(struct test0) == 8) ? 1 : -1];