#include <limits>
#include <type_traits>
-#include <folly/Bits.h>
#include <folly/Likely.h>
#include <folly/Portability.h>
#include <folly/Range.h>
+#include <folly/experimental/CodingDetail.h>
#include <folly/experimental/Instructions.h>
#include <folly/experimental/Select64.h>
+#include <folly/lang/Assume.h>
+#include <folly/lang/Bits.h>
#include <glog/logging.h>
#if !FOLLY_X64
static_assert(kIsLittleEndian, "EliasFanoCoding.h requires little endianness");
+constexpr size_t kCacheLineSize = 64;
+
template <class Pointer>
struct EliasFanoCompressedListBase {
EliasFanoCompressedListBase() = default;
return ::free(data.data());
}
- size_t upperSize() const { return data.end() - upper; }
+ size_t upperSize() const {
+ return size_t(data.end() - upper);
+ }
size_t size = 0;
uint8_t numLowerBits = 0;
typedef EliasFanoCompressedListBase<const uint8_t*> EliasFanoCompressedList;
typedef EliasFanoCompressedListBase<uint8_t*> MutableEliasFanoCompressedList;
-template <class Value,
- class SkipValue = size_t,
- size_t kSkipQuantum = 0, // 0 = disabled
- size_t kForwardQuantum = 0> // 0 = disabled
+template <
+ class Value,
+ class SkipValue = size_t,
+ size_t kSkipQuantum = 0, // 0 = disabled
+ size_t kForwardQuantum = 0> // 0 = disabled
struct EliasFanoEncoderV2 {
static_assert(std::is_integral<Value>::value &&
std::is_unsigned<Value>::value,
static constexpr size_t forwardQuantum = kForwardQuantum;
static uint8_t defaultNumLowerBits(size_t upperBound, size_t size) {
- if (size == 0 || upperBound < size) {
+ if (UNLIKELY(size == 0 || upperBound < size)) {
return 0;
}
- // floor(log(upperBound / size));
- return folly::findLastSet(upperBound / size) - 1;
+ // Result that should be returned is "floor(log(upperBound / size))".
+ // In order to avoid expensive division, we rely on
+ // "floor(a) - floor(b) - 1 <= floor(a - b) <= floor(a) - floor(b)".
+ // Assuming "candidate = floor(log(upperBound)) - floor(log(upperBound))",
+ // then result is either "candidate - 1" or "candidate".
+ auto candidate = folly::findLastSet(upperBound) - folly::findLastSet(size);
+ // NOTE: As size != 0, "candidate" is always < 64.
+ return (size > (upperBound >> candidate)) ? candidate - 1 : candidate;
}
// Requires: input range (begin, end) is sorted (encoding
if (begin == end) {
return MutableCompressedList();
}
- EliasFanoEncoderV2 encoder(end - begin, *(end - 1));
+ EliasFanoEncoderV2 encoder(size_t(end - begin), *(end - 1));
for (; begin != end; ++begin) {
encoder.add(*begin);
}
forwardPointers_(reinterpret_cast<SkipValueType*>(
result.forwardPointers)),
result_(result) {
- std::fill(result.data.begin(), result.data.end(), 0);
+ std::fill(result.data.begin(), result.data.end(), '\0');
}
EliasFanoEncoderV2(size_t size, ValueType upperBound)
/* static */ if (skipQuantum != 0) {
while ((skipPointersSize_ + 1) * skipQuantum <= upperBits) {
// Store the number of preceding 1-bits.
- skipPointers_[skipPointersSize_++] = size_;
+ skipPointers_[skipPointersSize_++] = SkipValue(size_);
}
}
MutableCompressedList result_;
};
-template <class Value,
- class SkipValue,
- size_t kSkipQuantum,
- size_t kForwardQuantum>
+template <
+ class Value,
+ class SkipValue,
+ size_t kSkipQuantum,
+ size_t kForwardQuantum>
struct EliasFanoEncoderV2<Value,
SkipValue,
kSkipQuantum,
namespace detail {
-template <class Encoder, class Instructions>
-class UpperBitsReader {
+template <class Encoder, class Instructions, class SizeType>
+class UpperBitsReader : ForwardPointers<Encoder::forwardQuantum>,
+ SkipPointers<Encoder::skipQuantum> {
typedef typename Encoder::SkipValueType SkipValueType;
public:
typedef typename Encoder::ValueType ValueType;
explicit UpperBitsReader(const typename Encoder::CompressedList& list)
- : forwardPointers_(list.forwardPointers),
- skipPointers_(list.skipPointers),
+ : ForwardPointers<Encoder::forwardQuantum>(list.forwardPointers),
+ SkipPointers<Encoder::skipQuantum>(list.skipPointers),
start_(list.upper) {
reset();
}
void reset() {
block_ = start_ != nullptr ? folly::loadUnaligned<block_t>(start_) : 0;
+ position_ = std::numeric_limits<SizeType>::max();
outer_ = 0;
- inner_ = -1;
- position_ = -1;
value_ = 0;
}
- size_t position() const { return position_; }
- ValueType value() const { return value_; }
+ SizeType position() const {
+ return position_;
+ }
+ ValueType value() const {
+ return value_;
+ }
ValueType next() {
// Skip to the first non-zero block.
}
++position_;
- inner_ = Instructions::ctz(block_);
+ size_t inner = Instructions::ctz(block_);
block_ = Instructions::blsr(block_);
- return setValue();
+ return setValue(inner);
}
- ValueType skip(size_t n) {
+ ValueType skip(SizeType n) {
DCHECK_GT(n, 0);
position_ += n; // n 1-bits will be read.
// Use forward pointer.
if (Encoder::forwardQuantum > 0 && n > Encoder::forwardQuantum) {
const size_t steps = position_ / Encoder::forwardQuantum;
- const size_t dest =
- folly::loadUnaligned<SkipValueType>(
- forwardPointers_ + (steps - 1) * sizeof(SkipValueType));
+ const size_t dest = folly::loadUnaligned<SkipValueType>(
+ this->forwardPointers_ + (steps - 1) * sizeof(SkipValueType));
reposition(dest + steps * Encoder::forwardQuantum);
n = position_ + 1 - steps * Encoder::forwardQuantum; // n is > 0.
- // Correct inner_ will be set at the end.
}
size_t cnt;
// Skip to the n-th one in the block.
DCHECK_GT(n, 0);
- inner_ = select64<Instructions>(block_, n - 1);
- block_ &= (block_t(-1) << inner_) << 1;
+ size_t inner = select64<Instructions>(block_, n - 1);
+ block_ &= (block_t(-1) << inner) << 1;
- return setValue();
+ return setValue(inner);
}
// Skip to the first element that is >= v and located *after* the current
// Use skip pointer.
if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
const size_t steps = v / Encoder::skipQuantum;
- const size_t dest =
- folly::loadUnaligned<SkipValueType>(
- skipPointers_ + (steps - 1) * sizeof(SkipValueType));
+ const size_t dest = folly::loadUnaligned<SkipValueType>(
+ this->skipPointers_ + (steps - 1) * sizeof(SkipValueType));
reposition(dest + Encoder::skipQuantum * steps);
position_ = dest - 1;
- // Correct inner_ and value_ will be set during the next()
- // call at the end.
+ // Correct value_ will be set during the next() call at the end.
// NOTE: Corresponding block of lower bits sequence may be
// prefetched here (via __builtin_prefetch), but experiments
return value_;
}
+ /**
+ * Prepare to skip to `value`. This is a constant-time operation that will
+ * prefetch memory required for a `skipTo(value)` call.
+ *
+ * @return position of reader
+ */
+ SizeType prepareSkipTo(ValueType v) const {
+ auto position = position_;
+
+ if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
+ auto outer = outer_;
+ const size_t steps = v / Encoder::skipQuantum;
+ const size_t dest = folly::loadUnaligned<SkipValueType>(
+ this->skipPointers_ + (steps - 1) * sizeof(SkipValueType));
+
+ position = dest - 1;
+ outer = (dest + Encoder::skipQuantum * steps) / 8;
+
+ // Prefetch up to the beginning of where we linear search. After that,
+ // hardware prefetching will outperform our own. In addition, this
+ // simplifies calculating what to prefetch as we don't have to calculate
+ // the entire destination address. Two cache lines are prefetched because
+ // this results in fewer cycles used (based on practical results) than
+ // one. However, three cache lines does not have any additional effect.
+ const auto addr = start_ + outer;
+ __builtin_prefetch(addr);
+ __builtin_prefetch(addr + kCacheLineSize);
+ }
+
+ return position;
+ }
+
ValueType jump(size_t n) {
if (Encoder::forwardQuantum == 0 || n <= Encoder::forwardQuantum) {
reset();
} else {
- position_ = -1; // Avoid reading the head, skip() will reposition.
+ // Avoid reading the head, skip() will reposition.
+ position_ = std::numeric_limits<SizeType>::max();
}
return skip(n);
}
}
ValueType previousValue() const {
- DCHECK_NE(position(), -1);
+ DCHECK_NE(position(), std::numeric_limits<SizeType>::max());
DCHECK_GT(position(), 0);
- size_t outer = outer_;
+ auto outer = outer_;
+ auto inner = size_t(value_) - 8 * outer_ + position_;
block_t block = folly::loadUnaligned<block_t>(start_ + outer);
- block &= (block_t(1) << inner_) - 1;
+ block &= (block_t(1) << inner) - 1;
while (UNLIKELY(block == 0)) {
- DCHECK_GE(outer, sizeof(block_t));
- outer -= sizeof(block_t);
+ DCHECK_GT(outer, 0);
+ outer -= std::min<OuterType>(sizeof(block_t), outer);
block = folly::loadUnaligned<block_t>(start_ + outer);
}
- auto inner = 8 * sizeof(block_t) - 1 - Instructions::clz(block);
+ inner = 8 * sizeof(block_t) - 1 - Instructions::clz(block);
return static_cast<ValueType>(8 * outer + inner - (position_ - 1));
}
- void setDone(size_t endPos) {
+ void setDone(SizeType endPos) {
position_ = endPos;
}
private:
- ValueType setValue() {
- value_ = static_cast<ValueType>(8 * outer_ + inner_ - position_);
+ ValueType setValue(size_t inner) {
+ value_ = static_cast<ValueType>(8 * outer_ + inner - position_);
return value_;
}
- void reposition(size_t dest) {
+ void reposition(SizeType dest) {
outer_ = dest / 8;
block_ = folly::loadUnaligned<block_t>(start_ + outer_);
block_ &= ~((block_t(1) << (dest % 8)) - 1);
}
- typedef uint64_t block_t;
- const unsigned char* const forwardPointers_;
- const unsigned char* const skipPointers_;
+ using block_t = uint64_t;
+ // The size in bytes of the upper bits is limited by n + universe / 8,
+ // so a type that can hold either sizes or values is sufficient.
+ using OuterType = typename std::common_type<ValueType, SizeType>::type;
+
const unsigned char* const start_;
block_t block_;
- size_t outer_; // Outer offset: number of consumed bytes in upper.
- size_t inner_; // Inner offset: (bit) position in current block.
- size_t position_; // Index of current value (= #reads - 1).
+ SizeType position_; // Index of current value (= #reads - 1).
+ OuterType outer_; // Outer offset: number of consumed bytes in upper.
ValueType value_;
};
-} // namespace detail
+} // namespace detail
// If kUnchecked = true the caller must guarantee that all the
// operations return valid elements, i.e., they would never return
// false if checked.
-template <class Encoder,
- class Instructions = instructions::Default,
- bool kUnchecked = false>
+template <
+ class Encoder,
+ class Instructions = instructions::Default,
+ bool kUnchecked = false,
+ class SizeType = size_t>
class EliasFanoReader {
public:
typedef Encoder EncoderType;
typedef typename Encoder::ValueType ValueType;
explicit EliasFanoReader(const typename Encoder::CompressedList& list)
- : size_(list.size),
+ : upper_(list),
lower_(list.lower),
- upper_(list),
- lowerMask_((ValueType(1) << list.numLowerBits) - 1),
+ size_(list.size),
numLowerBits_(list.numLowerBits) {
DCHECK(Instructions::supported());
// To avoid extra branching during skipTo() while reading
lastValue_ = 0;
return;
}
- ValueType lastUpperValue = 8 * list.upperSize() - size_;
+ ValueType lastUpperValue = ValueType(8 * list.upperSize() - size_);
auto it = list.upper + list.upperSize() - 1;
DCHECK_NE(*it, 0);
lastUpperValue -= 8 - folly::findLastSet(*it);
return true;
}
- bool skip(size_t n) {
+ bool skip(SizeType n) {
CHECK_GT(n, 0);
if (kUnchecked || LIKELY(position() + n < size_)) {
if (LIKELY(n < kLinearScanThreshold)) {
- for (size_t i = 0; i < n; ++i) upper_.next();
+ for (SizeType i = 0; i < n; ++i) {
+ upper_.next();
+ }
} else {
upper_.skip(n);
}
return true;
}
- size_t upperValue = (value >> numLowerBits_);
- size_t upperSkip = upperValue - upper_.value();
+ ValueType upperValue = (value >> numLowerBits_);
+ ValueType upperSkip = upperValue - upper_.value();
// The average density of ones in upper bits is 1/2.
// LIKELY here seems to make things worse, even for small skips.
if (upperSkip < 2 * kLinearScanThreshold) {
return true;
}
- bool jump(size_t n) {
+ /**
+ * Prepare to skip to `value` by prefetching appropriate memory in both the
+ * upper and lower bits.
+ */
+ void prepareSkipTo(ValueType value) const {
+ // Also works when value_ == kInvalidValue.
+ if (value != kInvalidValue) {
+ DCHECK_GE(value + 1, value_ + 1);
+ }
+
+ if ((!kUnchecked && value > lastValue_) || (value == value_)) {
+ return;
+ }
+
+ // Do minimal computation required to prefetch address used in
+ // `readLowerPart()`.
+ ValueType upperValue = (value >> numLowerBits_);
+ const auto upperPosition = upper_.prepareSkipTo(upperValue);
+ const auto addr = lower_ + (upperPosition * numLowerBits_ / 8);
+ __builtin_prefetch(addr);
+ __builtin_prefetch(addr + kCacheLineSize);
+ }
+
+ bool jump(SizeType n) {
if (LIKELY(n < size_)) { // Also checks that n != -1.
value_ = readLowerPart(n) | (upper_.jump(n + 1) << numLowerBits_);
return true;
(upper_.previousValue() << numLowerBits_);
}
- size_t size() const { return size_; }
+ SizeType size() const {
+ return size_;
+ }
bool valid() const {
return position() < size(); // Also checks that position() != -1.
}
- size_t position() const { return upper_.position(); }
+ SizeType position() const {
+ return upper_.position();
+ }
ValueType value() const {
DCHECK(valid());
return value_;
}
private:
+ // Must hold kInvalidValue + 1 == 0.
constexpr static ValueType kInvalidValue =
- std::numeric_limits<ValueType>::max(); // Must hold kInvalidValue + 1 == 0.
+ std::numeric_limits<ValueType>::max();
bool setDone() {
value_ = kInvalidValue;
return false;
}
- ValueType readLowerPart(size_t i) const {
+ ValueType readLowerPart(SizeType i) const {
DCHECK_LT(i, size_);
const size_t pos = i * numLowerBits_;
const unsigned char* ptr = lower_ + (pos / 8);
const uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
- return lowerMask_ & (ptrv >> (pos % 8));
+ // This removes the branch in the fallback implementation of
+ // bzhi. The condition is verified at encoding time.
+ assume(numLowerBits_ < sizeof(ValueType) * 8);
+ return Instructions::bzhi(ptrv >> (pos % 8), numLowerBits_);
}
void iterateTo(ValueType value) {
while (true) {
value_ = readLowerPart(upper_.position()) |
(upper_.value() << numLowerBits_);
- if (LIKELY(value_ >= value)) break;
+ if (LIKELY(value_ >= value)) {
+ break;
+ }
upper_.next();
}
}
constexpr static size_t kLinearScanThreshold = 8;
- size_t size_;
+ detail::UpperBitsReader<Encoder, Instructions, SizeType> upper_;
const uint8_t* lower_;
- detail::UpperBitsReader<Encoder, Instructions> upper_;
- const ValueType lowerMask_;
+ SizeType size_;
ValueType value_ = kInvalidValue;
ValueType lastValue_;
uint8_t numLowerBits_;
};
-}} // namespaces
+} // namespace compression
+} // namespace folly
projects / folly.git / blobdiff