static CGBitFieldInfo ComputeBitFieldInfo(CodeGenTypes &Types,
const FieldDecl *FD,
uint64_t FieldOffset,
uint64_t FieldSize) {
const llvm::Type *Ty = Types.ConvertTypeForMemRecursive(FD->getType());
uint64_t TypeSizeInBytes = Types.getTargetData().getTypeAllocSize(Ty);
uint64_t TypeSizeInBits = TypeSizeInBytes * 8;
unsigned StartBit = FieldOffset % TypeSizeInBits;
bool IsSigned = FD->getType()->isSignedIntegerType();
// The current policy is to always access the bit-field using the source type
// of the bit-field. With the C bit-field rules, this implies that we always
// use either one or two accesses, and two accesses can only occur with a
// packed structure when the bit-field straddles an alignment boundary.
CGBitFieldInfo::AccessInfo Components[2];
unsigned LowBits = std::min(FieldSize, TypeSizeInBits - StartBit);
bool NeedsHighAccess = LowBits != FieldSize;
unsigned NumComponents = 1 + NeedsHighAccess;
// FIXME: This access policy is probably wrong on big-endian systems.
CGBitFieldInfo::AccessInfo &LowAccess = Components[0];
LowAccess.FieldIndex = 0;
LowAccess.FieldByteOffset =
TypeSizeInBytes * ((FieldOffset / 8) / TypeSizeInBytes);
LowAccess.FieldBitStart = StartBit;
LowAccess.AccessWidth = TypeSizeInBits;
// FIXME: This might be wrong!
LowAccess.AccessAlignment = 0;
LowAccess.TargetBitOffset = 0;
LowAccess.TargetBitWidth = LowBits;
if (NeedsHighAccess) {
CGBitFieldInfo::AccessInfo &HighAccess = Components[1];
HighAccess.FieldIndex = 0;
HighAccess.FieldByteOffset = LowAccess.FieldByteOffset + TypeSizeInBytes;
HighAccess.FieldBitStart = 0;
HighAccess.AccessWidth = TypeSizeInBits;
// FIXME: This might be wrong!
HighAccess.AccessAlignment = 0;
HighAccess.TargetBitOffset = LowBits;
HighAccess.TargetBitWidth = FieldSize - LowBits;
}
return CGBitFieldInfo(FieldSize, NumComponents, Components, IsSigned);
}
CGBitFieldInfo CGBitFieldInfo::MakeInfo(CodeGenTypes &Types,
const FieldDecl *FD,
uint64_t FieldOffset,
uint64_t FieldSize,
uint64_t ContainingTypeSizeInBits,
unsigned ContainingTypeAlign) {
llvm::Type *Ty = Types.ConvertTypeForMem(FD->getType());
CharUnits TypeSizeInBytes =
CharUnits::fromQuantity(Types.getTargetData().getTypeAllocSize(Ty));
uint64_t TypeSizeInBits = Types.getContext().toBits(TypeSizeInBytes);
bool IsSigned = FD->getType()->isSignedIntegerOrEnumerationType();
if (FieldSize > TypeSizeInBits) {
// We have a wide bit-field. The extra bits are only used for padding, so
// if we have a bitfield of type T, with size N:
//
// T t : N;
//
// We can just assume that it's:
//
// T t : sizeof(T);
//
FieldSize = TypeSizeInBits;
}
// in big-endian machines the first fields are in higher bit positions,
// so revert the offset. The byte offsets are reversed(back) later.
if (Types.getTargetData().isBigEndian()) {
FieldOffset = ((ContainingTypeSizeInBits)-FieldOffset-FieldSize);
}
// Compute the access components. The policy we use is to start by attempting
// to access using the width of the bit-field type itself and to always access
// at aligned indices of that type. If such an access would fail because it
// extends past the bound of the type, then we reduce size to the next smaller
// power of two and retry. The current algorithm assumes pow2 sized types,
// although this is easy to fix.
//
assert(llvm::isPowerOf2_32(TypeSizeInBits) && "Unexpected type size!");
CGBitFieldInfo::AccessInfo Components[3];
unsigned NumComponents = 0;
unsigned AccessedTargetBits = 0; // The number of target bits accessed.
unsigned AccessWidth = TypeSizeInBits; // The current access width to attempt.
// If requested, widen the initial bit-field access to be register sized. The
// theory is that this is most likely to allow multiple accesses into the same
// structure to be coalesced, and that the backend should be smart enough to
// narrow the store if no coalescing is ever done.
//
// The subsequent code will handle align these access to common boundaries and
// guaranteeing that we do not access past the end of the structure.
if (Types.getCodeGenOpts().UseRegisterSizedBitfieldAccess) {
if (AccessWidth < Types.getTarget().getRegisterWidth())
AccessWidth = Types.getTarget().getRegisterWidth();
}
// Round down from the field offset to find the first access position that is
// at an aligned offset of the initial access type.
uint64_t AccessStart = FieldOffset - (FieldOffset % AccessWidth);
// Adjust initial access size to fit within record.
while (AccessWidth > Types.getTarget().getCharWidth() &&
AccessStart + AccessWidth > ContainingTypeSizeInBits) {
AccessWidth >>= 1;
AccessStart = FieldOffset - (FieldOffset % AccessWidth);
}
while (AccessedTargetBits < FieldSize) {
// Check that we can access using a type of this size, without reading off
// the end of the structure. This can occur with packed structures and
// -fno-bitfield-type-align, for example.
if (AccessStart + AccessWidth > ContainingTypeSizeInBits) {
// If so, reduce access size to the next smaller power-of-two and retry.
AccessWidth >>= 1;
assert(AccessWidth >= Types.getTarget().getCharWidth()
&& "Cannot access under byte size!");
continue;
}
// Otherwise, add an access component.
// First, compute the bits inside this access which are part of the
// target. We are reading bits [AccessStart, AccessStart + AccessWidth); the
// intersection with [FieldOffset, FieldOffset + FieldSize) gives the bits
// in the target that we are reading.
assert(FieldOffset < AccessStart + AccessWidth && "Invalid access start!");
assert(AccessStart < FieldOffset + FieldSize && "Invalid access start!");
uint64_t AccessBitsInFieldStart = std::max(AccessStart, FieldOffset);
uint64_t AccessBitsInFieldSize =
std::min(AccessWidth + AccessStart,
FieldOffset + FieldSize) - AccessBitsInFieldStart;
assert(NumComponents < 3 && "Unexpected number of components!");
CGBitFieldInfo::AccessInfo &AI = Components[NumComponents++];
AI.FieldIndex = 0;
// FIXME: We still follow the old access pattern of only using the field
// byte offset. We should switch this once we fix the struct layout to be
// pretty.
// on big-endian machines we reverted the bit offset because first fields are
// in higher bits. But this also reverts the bytes, so fix this here by reverting
// the byte offset on big-endian machines.
if (Types.getTargetData().isBigEndian()) {
AI.FieldByteOffset = Types.getContext().toCharUnitsFromBits(
ContainingTypeSizeInBits - AccessStart - AccessWidth);
} else {
AI.FieldByteOffset = Types.getContext().toCharUnitsFromBits(AccessStart);
}
AI.FieldBitStart = AccessBitsInFieldStart - AccessStart;
AI.AccessWidth = AccessWidth;
AI.AccessAlignment = Types.getContext().toCharUnitsFromBits(
llvm::MinAlign(ContainingTypeAlign, AccessStart));
AI.TargetBitOffset = AccessedTargetBits;
AI.TargetBitWidth = AccessBitsInFieldSize;
AccessStart += AccessWidth;
AccessedTargetBits += AI.TargetBitWidth;
}