/*
- * Copyright 2013 Facebook, Inc.
+ * Copyright 2017 Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* "Quasi-succinct indices" (arxiv:1206.4300).
*/
-#ifndef FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
-#define FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
+#pragma once
-#ifndef __GNUC__
-#error EliasFanoCoding.h requires GCC
-#endif
-
-#if !defined(__x86_64__)
-#error EliasFanoCoding.h requires x86_64
-#endif
-
-#include <cstdlib>
#include <algorithm>
+#include <cstdlib>
#include <limits>
#include <type_traits>
-#include <boost/noncopyable.hpp>
+
+#include <folly/Assume.h>
+#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 <glog/logging.h>
-#include "folly/Bits.h"
-#include "folly/CpuId.h"
-#include "folly/Likely.h"
-#include "folly/Range.h"
-#if __BYTE_ORDER__ != __ORDER_LITTLE_ENDIAN__
-#error EliasFanoCoding.h requires little endianness
+#if !FOLLY_X64
+#error EliasFanoCoding.h requires x86_64
#endif
namespace folly { namespace compression {
-template <class Value,
- class SkipValue = size_t,
- size_t kSkipQuantum = 0, // 0 = disabled
- size_t kForwardQuantum = 0> // 0 = disabled
-struct EliasFanoCompressedList {
+static_assert(kIsLittleEndian, "EliasFanoCoding.h requires little endianness");
+
+constexpr size_t kCacheLineSize = 64;
+
+template <class Pointer>
+struct EliasFanoCompressedListBase {
+ EliasFanoCompressedListBase() = default;
+
+ template <class OtherPointer>
+ EliasFanoCompressedListBase(
+ const EliasFanoCompressedListBase<OtherPointer>& other)
+ : size(other.size),
+ numLowerBits(other.numLowerBits),
+ data(other.data),
+ skipPointers(reinterpret_cast<Pointer>(other.skipPointers)),
+ forwardPointers(reinterpret_cast<Pointer>(other.forwardPointers)),
+ lower(reinterpret_cast<Pointer>(other.lower)),
+ upper(reinterpret_cast<Pointer>(other.upper)) { }
+
+ template <class T = Pointer>
+ auto free() -> decltype(::free(T(nullptr))) {
+ return ::free(data.data());
+ }
+
+ size_t upperSize() const {
+ return size_t(data.end() - upper);
+ }
+
+ size_t size = 0;
+ uint8_t numLowerBits = 0;
+
+ // WARNING: EliasFanoCompressedList has no ownership of data. The 7
+ // bytes following the last byte should be readable.
+ folly::Range<Pointer> data;
+
+ Pointer skipPointers = nullptr;
+ Pointer forwardPointers = nullptr;
+ Pointer lower = nullptr;
+ Pointer upper = nullptr;
+};
+
+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
+struct EliasFanoEncoderV2 {
static_assert(std::is_integral<Value>::value &&
- std::is_unsigned<Value>::value,
+ std::is_unsigned<Value>::value,
"Value should be unsigned integral");
+ typedef EliasFanoCompressedList CompressedList;
+ typedef MutableEliasFanoCompressedList MutableCompressedList;
+
typedef Value ValueType;
typedef SkipValue SkipValueType;
-
- EliasFanoCompressedList()
- : size(0), numLowerBits(0) { }
+ struct Layout;
static constexpr size_t skipQuantum = kSkipQuantum;
static constexpr size_t forwardQuantum = kForwardQuantum;
- size_t size;
- uint8_t numLowerBits;
-
- // WARNING: EliasFanoCompressedList has no ownership of
- // lower, upper, skipPointers and forwardPointers.
- // The 7 bytes following the last byte of lower and upper
- // sequences should be readable.
- folly::ByteRange lower;
- folly::ByteRange upper;
-
- folly::ByteRange skipPointers;
- folly::ByteRange forwardPointers;
-
- void free() {
- ::free(const_cast<unsigned char*>(lower.data()));
- ::free(const_cast<unsigned char*>(upper.data()));
- ::free(const_cast<unsigned char*>(skipPointers.data()));
- ::free(const_cast<unsigned char*>(forwardPointers.data()));
- }
-
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;
- }
-
- // WARNING: encode() mallocates lower, upper, skipPointers
- // and forwardPointers. As EliasFanoCompressedList has
- // no ownership of them, you need to call free() explicitly.
- static void encode(const ValueType* list, size_t size,
- EliasFanoCompressedList& result) {
- encode(list, list + size, result);
+ // 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
+ // crashes if it's not).
+ // WARNING: encode() mallocates EliasFanoCompressedList::data. As
+ // EliasFanoCompressedList has no ownership of it, you need to call
+ // free() explicitly.
template <class RandomAccessIterator>
- static void encode(RandomAccessIterator begin,
- RandomAccessIterator end,
- EliasFanoCompressedList& result) {
- auto list = begin;
- const size_t size = end - begin;
-
- if (size == 0) {
- result = EliasFanoCompressedList();
- return;
+ static MutableCompressedList encode(RandomAccessIterator begin,
+ RandomAccessIterator end) {
+ if (begin == end) {
+ return MutableCompressedList();
}
+ EliasFanoEncoderV2 encoder(size_t(end - begin), *(end - 1));
+ for (; begin != end; ++begin) {
+ encoder.add(*begin);
+ }
+ return encoder.finish();
+ }
- DCHECK(std::is_sorted(list, list + size));
-
- const ValueType upperBound = list[size - 1];
- uint8_t numLowerBits = defaultNumLowerBits(upperBound, size);
+ explicit EliasFanoEncoderV2(const MutableCompressedList& result)
+ : lower_(result.lower),
+ upper_(result.upper),
+ skipPointers_(reinterpret_cast<SkipValueType*>(
+ result.skipPointers)),
+ forwardPointers_(reinterpret_cast<SkipValueType*>(
+ result.forwardPointers)),
+ result_(result) {
+ std::fill(result.data.begin(), result.data.end(), '\0');
+ }
- // This is detail::writeBits56 limitation.
- numLowerBits = std::min<uint8_t>(numLowerBits, 56);
- CHECK_LT(numLowerBits, 8 * sizeof(Value)); // As we shift by numLowerBits.
+ EliasFanoEncoderV2(size_t size, ValueType upperBound)
+ : EliasFanoEncoderV2(
+ Layout::fromUpperBoundAndSize(upperBound, size).allocList()) { }
- // WARNING: Current read/write logic assumes that the 7 bytes
- // following the last byte of lower and upper sequences are
- // readable (stored value doesn't matter and won't be changed),
- // so we allocate additional 7B, but do not include them in size
- // of returned value.
+ void add(ValueType value) {
+ CHECK_LT(value, std::numeric_limits<ValueType>::max());
+ CHECK_GE(value, lastValue_);
- // *** Lower bits.
- const size_t lowerSize = (numLowerBits * size + 7) / 8;
- unsigned char* lower = nullptr;
- if (lowerSize > 0) { // numLowerBits != 0
- lower = static_cast<unsigned char*>(calloc(lowerSize + 7, 1));
- const ValueType lowerMask = (ValueType(1) << numLowerBits) - 1;
- for (size_t i = 0; i < size; ++i) {
- const ValueType lowerBits = list[i] & lowerMask;
- writeBits56(lower, i * numLowerBits, numLowerBits, lowerBits);
- }
- }
+ const auto numLowerBits = result_.numLowerBits;
+ const ValueType upperBits = value >> numLowerBits;
- // *** Upper bits.
- // Upper bits are stored using unary delta encoding.
- // For example, (3 5 5 9) will be encoded as 1000011001000_2.
- const size_t upperSizeBits =
- (upperBound >> numLowerBits) + // Number of 0-bits to be stored.
- size; // 1-bits.
- const size_t upperSize = (upperSizeBits + 7) / 8;
- unsigned char* const upper =
- static_cast<unsigned char*>(calloc(upperSize + 7, 1));
- for (size_t i = 0; i < size; ++i) {
- const ValueType upperBits = list[i] >> numLowerBits;
- const size_t pos = upperBits + i; // upperBits 0-bits and (i + 1) 1-bits.
- upper[pos / 8] |= 1U << (pos % 8);
+ // Upper sequence consists of upperBits 0-bits and (size_ + 1) 1-bits.
+ const size_t pos = upperBits + size_;
+ upper_[pos / 8] |= 1U << (pos % 8);
+ // Append numLowerBits bits to lower sequence.
+ if (numLowerBits != 0) {
+ const ValueType lowerBits = value & ((ValueType(1) << numLowerBits) - 1);
+ writeBits56(lower_, size_ * numLowerBits, numLowerBits, lowerBits);
}
- // *** Skip pointers.
- // Store (1-indexed) position of every skipQuantum-th
- // 0-bit in upper bits sequence.
- SkipValueType* skipPointers = nullptr;
- size_t numSkipPointers = 0;
/* static */ if (skipQuantum != 0) {
- // Workaround to avoid 'division by zero' compile-time error.
- constexpr size_t q = skipQuantum ?: 1;
- CHECK_LT(upperSizeBits, std::numeric_limits<SkipValueType>::max());
- // 8 * upperSize is used here instead of upperSizeBits, as that is
- // more serialization-friendly way.
- numSkipPointers = (8 * upperSize - size) / q;
- skipPointers = static_cast<SkipValueType*>(
- numSkipPointers == 0
- ? nullptr
- : calloc(numSkipPointers, sizeof(SkipValueType)));
-
- for (size_t i = 0, pos = 0; i < size; ++i) {
- const ValueType upperBits = list[i] >> numLowerBits;
- for (; (pos + 1) * q <= upperBits; ++pos) {
- skipPointers[pos] = i + (pos + 1) * q;
- }
+ while ((skipPointersSize_ + 1) * skipQuantum <= upperBits) {
+ // Store the number of preceding 1-bits.
+ skipPointers_[skipPointersSize_++] = SkipValue(size_);
}
}
- // *** Forward pointers.
- // Store (1-indexed) position of every forwardQuantum-th
- // 1-bit in upper bits sequence.
- SkipValueType* forwardPointers = nullptr;
- size_t numForwardPointers = 0;
/* static */ if (forwardQuantum != 0) {
- // Workaround to avoid 'division by zero' compile-time error.
- constexpr size_t q = forwardQuantum ?: 1;
- CHECK_LT(upperSizeBits, std::numeric_limits<SkipValueType>::max());
-
- numForwardPointers = size / q;
- forwardPointers = static_cast<SkipValueType*>(
- numForwardPointers == 0
- ? nullptr
- : malloc(numForwardPointers * sizeof(SkipValueType)));
-
- for (size_t i = q - 1, pos = 0; i < size; i += q, ++pos) {
- const ValueType upperBits = list[i] >> numLowerBits;
- forwardPointers[pos] = upperBits + i + 1;
+ if ((size_ + 1) % forwardQuantum == 0) {
+ const auto k = size_ / forwardQuantum;
+ // Store the number of preceding 0-bits.
+ forwardPointers_[k] = upperBits;
}
}
- // *** Result.
- result.size = size;
- result.numLowerBits = numLowerBits;
- result.lower.reset(lower, lowerSize);
- result.upper.reset(upper, upperSize);
- result.skipPointers.reset(
- reinterpret_cast<unsigned char*>(skipPointers),
- numSkipPointers * sizeof(SkipValueType));
- result.forwardPointers.reset(
- reinterpret_cast<unsigned char*>(forwardPointers),
- numForwardPointers * sizeof(SkipValueType));
+ lastValue_ = value;
+ ++size_;
+ }
+
+ const MutableCompressedList& finish() const {
+ CHECK_EQ(size_, result_.size);
+ return result_;
}
private:
DCHECK_EQ(0, value & ~((uint64_t(1) << len) - 1));
unsigned char* const ptr = data + (pos / 8);
uint64_t ptrv = folly::loadUnaligned<uint64_t>(ptr);
- ptrv |= value << (pos % 8);
+ ptrv |= value << (pos % 8);
folly::storeUnaligned<uint64_t>(ptr, ptrv);
}
+
+ unsigned char* lower_ = nullptr;
+ unsigned char* upper_ = nullptr;
+ SkipValueType* skipPointers_ = nullptr;
+ SkipValueType* forwardPointers_ = nullptr;
+
+ ValueType lastValue_ = 0;
+ size_t size_ = 0;
+ size_t skipPointersSize_ = 0;
+
+ MutableCompressedList result_;
};
-// NOTE: It's recommended to compile EF coding with -msse4.2, starting
-// with Nehalem, Intel CPUs support POPCNT instruction and gcc will emit
-// it for __builtin_popcountll intrinsic.
-// But we provide an alternative way for the client code: it can switch to
-// the appropriate version of EliasFanoReader<> in realtime (client should
-// implement this switching logic itself) by specifying instruction set to
-// use explicitly.
-namespace instructions {
-
-struct Default {
- static bool supported() {
- return true;
- }
- static inline uint64_t popcount(uint64_t value) {
- return __builtin_popcountll(value);
+template <
+ class Value,
+ class SkipValue,
+ size_t kSkipQuantum,
+ size_t kForwardQuantum>
+struct EliasFanoEncoderV2<Value,
+ SkipValue,
+ kSkipQuantum,
+ kForwardQuantum>::Layout {
+ static Layout fromUpperBoundAndSize(size_t upperBound, size_t size) {
+ // numLowerBits can be at most 56 because of detail::writeBits56.
+ const uint8_t numLowerBits = std::min(defaultNumLowerBits(upperBound,
+ size),
+ uint8_t(56));
+ // *** Upper bits.
+ // Upper bits are stored using unary delta encoding.
+ // For example, (3 5 5 9) will be encoded as 1000011001000_2.
+ const size_t upperSizeBits =
+ (upperBound >> numLowerBits) + // Number of 0-bits to be stored.
+ size; // 1-bits.
+ const size_t upper = (upperSizeBits + 7) / 8;
+
+ // *** Validity checks.
+ // Shift by numLowerBits must be valid.
+ CHECK_LT(numLowerBits, 8 * sizeof(Value));
+ CHECK_LT(size, std::numeric_limits<SkipValueType>::max());
+ CHECK_LT(upperBound >> numLowerBits,
+ std::numeric_limits<SkipValueType>::max());
+
+ return fromInternalSizes(numLowerBits, upper, size);
}
- static inline int ctz(uint64_t value) {
- DCHECK_GT(value, 0);
- return __builtin_ctzll(value);
+
+ static Layout fromInternalSizes(uint8_t numLowerBits,
+ size_t upper,
+ size_t size) {
+ Layout layout;
+ layout.size = size;
+ layout.numLowerBits = numLowerBits;
+
+ layout.lower = (numLowerBits * size + 7) / 8;
+ layout.upper = upper;
+
+ // *** Skip pointers.
+ // Store (1-indexed) position of every skipQuantum-th
+ // 0-bit in upper bits sequence.
+ /* static */ if (skipQuantum != 0) {
+ // 8 * upper is used here instead of upperSizeBits, as that is
+ // more serialization-friendly way (upperSizeBits doesn't need
+ // to be known by this function, unlike upper).
+
+ size_t numSkipPointers = (8 * upper - size) / skipQuantum;
+ layout.skipPointers = numSkipPointers * sizeof(SkipValueType);
+ }
+
+ // *** Forward pointers.
+ // Store (1-indexed) position of every forwardQuantum-th
+ // 1-bit in upper bits sequence.
+ /* static */ if (forwardQuantum != 0) {
+ size_t numForwardPointers = size / forwardQuantum;
+ layout.forwardPointers = numForwardPointers * sizeof(SkipValueType);
+ }
+
+ return layout;
}
-};
-struct Fast : public Default {
- static bool supported() {
- folly::CpuId cpuId;
- return cpuId.popcnt();
+ size_t bytes() const {
+ return lower + upper + skipPointers + forwardPointers;
}
- static inline uint64_t popcount(uint64_t value) {
- uint64_t result;
- asm ("popcntq %1, %0" : "=r" (result) : "r" (value));
+
+ template <class Range>
+ EliasFanoCompressedListBase<typename Range::iterator>
+ openList(Range& buf) const {
+ EliasFanoCompressedListBase<typename Range::iterator> result;
+ result.size = size;
+ result.numLowerBits = numLowerBits;
+ result.data = buf.subpiece(0, bytes());
+
+ auto advance = [&] (size_t n) {
+ auto begin = buf.data();
+ buf.advance(n);
+ return begin;
+ };
+
+ result.skipPointers = advance(skipPointers);
+ result.forwardPointers = advance(forwardPointers);
+ result.lower = advance(lower);
+ result.upper = advance(upper);
+
return result;
}
-};
-} // namespace instructions
+ MutableCompressedList allocList() const {
+ uint8_t* buf = nullptr;
+ // WARNING: Current read/write logic assumes that the 7 bytes
+ // following the last byte of lower and upper sequences are
+ // readable (stored value doesn't matter and won't be changed), so
+ // we allocate additional 7 bytes, but do not include them in size
+ // of returned value.
+ if (size > 0) {
+ buf = static_cast<uint8_t*>(malloc(bytes() + 7));
+ }
+ folly::MutableByteRange bufRange(buf, bytes());
+ return openList(bufRange);
+ }
+
+ size_t size = 0;
+ uint8_t numLowerBits = 0;
+
+ // Sizes in bytes.
+ size_t lower = 0;
+ size_t upper = 0;
+ size_t skipPointers = 0;
+ size_t forwardPointers = 0;
+};
namespace detail {
-template <class CompressedList, class Instructions>
-class UpperBitsReader {
- typedef typename CompressedList::SkipValueType SkipValueType;
+template <class Encoder, class Instructions, class SizeType>
+class UpperBitsReader : ForwardPointers<Encoder::forwardQuantum>,
+ SkipPointers<Encoder::skipQuantum> {
+ typedef typename Encoder::SkipValueType SkipValueType;
public:
- typedef typename CompressedList::ValueType ValueType;
+ typedef typename Encoder::ValueType ValueType;
- explicit UpperBitsReader(const CompressedList& list)
- : forwardPointers_(list.forwardPointers.data()),
- skipPointers_(list.skipPointers.data()),
- start_(list.upper.data()),
- block_(start_ != nullptr ? folly::loadUnaligned<block_t>(start_) : 0),
- outer_(0), // outer offset: number of consumed bytes in upper.
- inner_(-1), // inner offset: (bit) position in current block.
- position_(-1), // index of current value (= #reads - 1).
- value_(0) { }
+ explicit UpperBitsReader(const typename Encoder::CompressedList& list)
+ : ForwardPointers<Encoder::forwardQuantum>(list.forwardPointers),
+ SkipPointers<Encoder::skipQuantum>(list.skipPointers),
+ start_(list.upper) {
+ reset();
+ }
- size_t position() const { return position_; }
- ValueType value() const { return value_; }
+ void reset() {
+ block_ = start_ != nullptr ? folly::loadUnaligned<block_t>(start_) : 0;
+ position_ = std::numeric_limits<SizeType>::max();
+ outer_ = 0;
+ value_ = 0;
+ }
+
+ SizeType position() const {
+ return position_;
+ }
+ ValueType value() const {
+ return value_;
+ }
ValueType next() {
// Skip to the first non-zero block.
}
++position_;
- inner_ = Instructions::ctz(block_);
- block_ &= block_ - 1;
+ 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 (CompressedList::forwardQuantum > 0 &&
- n > CompressedList::forwardQuantum) {
- // Workaround to avoid 'division by zero' compile-time error.
- constexpr size_t q = CompressedList::forwardQuantum ?: 1;
-
- const size_t steps = position_ / q;
- const size_t dest =
- folly::loadUnaligned<SkipValueType>(
- forwardPointers_ + (steps - 1) * sizeof(SkipValueType));
-
- reposition(dest);
- n = position_ + 1 - steps * q; // n is > 0.
- // correct inner_ will be set at the end.
+ if (Encoder::forwardQuantum > 0 && n > Encoder::forwardQuantum) {
+ const size_t steps = position_ / Encoder::forwardQuantum;
+ 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.
}
size_t cnt;
block_ = folly::loadUnaligned<block_t>(start_ + outer_);
}
- // NOTE: Trying to skip half-block here didn't show any
- // performance improvements.
-
+ // Skip to the n-th one in the block.
DCHECK_GT(n, 0);
+ size_t inner = select64<Instructions>(block_, n - 1);
+ block_ &= (block_t(-1) << inner) << 1;
- // Kill n - 1 least significant 1-bits.
- for (size_t i = 0; i < n - 1; ++i) {
- block_ &= block_ - 1;
- }
-
- inner_ = Instructions::ctz(block_);
- block_ &= block_ - 1;
-
- return setValue();
+ return setValue(inner);
}
// Skip to the first element that is >= v and located *after* the current
DCHECK_GE(v, value_);
// Use skip pointer.
- if (CompressedList::skipQuantum > 0 &&
- v >= value_ + CompressedList::skipQuantum) {
- // Workaround to avoid 'division by zero' compile-time error.
- constexpr size_t q = CompressedList::skipQuantum ?: 1;
+ if (Encoder::skipQuantum > 0 && v >= value_ + Encoder::skipQuantum) {
+ const size_t steps = v / Encoder::skipQuantum;
+ const size_t dest = folly::loadUnaligned<SkipValueType>(
+ this->skipPointers_ + (steps - 1) * sizeof(SkipValueType));
- const size_t steps = v / q;
- const size_t dest =
- folly::loadUnaligned<SkipValueType>(
- skipPointers_ + (steps - 1) * sizeof(SkipValueType));
+ reposition(dest + Encoder::skipQuantum * steps);
+ position_ = dest - 1;
- reposition(dest);
- position_ = dest - q * steps - 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
block_ = folly::loadUnaligned<block_t>(start_ + outer_);
}
- // Try to skip half-block.
- constexpr size_t kBitsPerHalfBlock = 4 * sizeof(block_t);
- constexpr block_t halfBlockMask = (block_t(1) << kBitsPerHalfBlock) - 1;
- if ((cnt = Instructions::popcount(~block_ & halfBlockMask)) < skip) {
- position_ += kBitsPerHalfBlock - cnt;
- block_ &= ~halfBlockMask;
+ if (LIKELY(skip)) {
+ auto inner = select64<Instructions>(~block_, skip - 1);
+ position_ += inner - skip + 1;
+ block_ &= block_t(-1) << inner;
}
- // Just skip until we see expected value.
- while (next() < v) { }
+ next();
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 {
+ // Avoid reading the head, skip() will reposition.
+ position_ = std::numeric_limits<SizeType>::max();
+ }
+ return skip(n);
+ }
+
+ ValueType jumpToNext(ValueType v) {
+ if (Encoder::skipQuantum == 0 || v < Encoder::skipQuantum) {
+ reset();
+ } else {
+ value_ = 0; // Avoid reading the head, skipToNext() will reposition.
+ }
+ return skipToNext(v);
+ }
+
+ ValueType previousValue() const {
+ DCHECK_NE(position(), std::numeric_limits<SizeType>::max());
+ DCHECK_GT(position(), 0);
+
+ 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;
+
+ while (UNLIKELY(block == 0)) {
+ DCHECK_GT(outer, 0);
+ outer -= std::min<OuterType>(sizeof(block_t), outer);
+ block = folly::loadUnaligned<block_t>(start_ + outer);
+ }
+
+ inner = 8 * sizeof(block_t) - 1 - Instructions::clz(block);
+ return static_cast<ValueType>(8 * outer + inner - (position_ - 1));
+ }
+
+ 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 unsigned long long 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_;
- size_t inner_;
- size_t position_;
+ SizeType position_; // Index of current value (= #reads - 1).
+ OuterType outer_; // Outer offset: number of consumed bytes in upper.
ValueType value_;
};
-} // namespace detail
-
-template <class CompressedList,
- class Instructions = instructions::Default>
-class EliasFanoReader : private boost::noncopyable {
+} // 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,
+ class SizeType = size_t>
+class EliasFanoReader {
public:
- typedef typename CompressedList::ValueType ValueType;
-
- explicit EliasFanoReader(const CompressedList& list)
- : list_(list),
- lowerMask_((ValueType(1) << list_.numLowerBits) - 1),
- upper_(list),
- progress_(0),
- value_(0) {
+ typedef Encoder EncoderType;
+ typedef typename Encoder::ValueType ValueType;
+
+ explicit EliasFanoReader(const typename Encoder::CompressedList& list)
+ : upper_(list),
+ lower_(list.lower),
+ size_(list.size),
+ numLowerBits_(list.numLowerBits) {
DCHECK(Instructions::supported());
// To avoid extra branching during skipTo() while reading
// upper sequence we need to know the last element.
- if (UNLIKELY(list_.size == 0)) {
+ // If kUnchecked == true, we do not check that skipTo() is called
+ // within the bounds, so we can avoid initializing lastValue_.
+ if (kUnchecked || UNLIKELY(list.size == 0)) {
lastValue_ = 0;
return;
}
- ValueType lastUpperValue = 8 * list_.upper.size() - list_.size;
- auto it = list_.upper.end() - 1;
+ ValueType lastUpperValue = ValueType(8 * list.upperSize() - size_);
+ auto it = list.upper + list.upperSize() - 1;
DCHECK_NE(*it, 0);
lastUpperValue -= 8 - folly::findLastSet(*it);
- lastValue_ = readLowerPart(list_.size - 1) |
- (lastUpperValue << list_.numLowerBits);
+ lastValue_ = readLowerPart(size_ - 1) | (lastUpperValue << numLowerBits_);
}
- size_t size() const { return list_.size; }
-
- size_t position() const { return progress_ - 1; }
- ValueType value() const { return value_; }
+ void reset() {
+ upper_.reset();
+ value_ = kInvalidValue;
+ }
bool next() {
- if (UNLIKELY(progress_ == list_.size)) {
- value_ = std::numeric_limits<ValueType>::max();
- return false;
+ if (!kUnchecked && UNLIKELY(position() + 1 >= size_)) {
+ return setDone();
}
- value_ = readLowerPart(progress_) |
- (upper_.next() << list_.numLowerBits);
- ++progress_;
+ upper_.next();
+ value_ = readLowerPart(upper_.position()) |
+ (upper_.value() << numLowerBits_);
return true;
}
- bool skip(size_t n) {
+ bool skip(SizeType n) {
CHECK_GT(n, 0);
- progress_ += n - 1;
- if (LIKELY(progress_ < list_.size)) {
- value_ = readLowerPart(progress_) |
- (upper_.skip(n) << list_.numLowerBits);
- ++progress_;
+ if (kUnchecked || LIKELY(position() + n < size_)) {
+ if (LIKELY(n < kLinearScanThreshold)) {
+ for (SizeType i = 0; i < n; ++i) {
+ upper_.next();
+ }
+ } else {
+ upper_.skip(n);
+ }
+ value_ = readLowerPart(upper_.position()) |
+ (upper_.value() << numLowerBits_);
return true;
}
- progress_ = list_.size;
- value_ = std::numeric_limits<ValueType>::max();
- return false;
+ return setDone();
}
bool skipTo(ValueType value) {
- DCHECK_GE(value, value_);
- if (value <= value_) {
+ // Also works when value_ == kInvalidValue.
+ if (value != kInvalidValue) { DCHECK_GE(value + 1, value_ + 1); }
+
+ if (!kUnchecked && value > lastValue_) {
+ return setDone();
+ } else if (value == value_) {
return true;
}
- if (value > lastValue_) {
- progress_ = list_.size;
- value_ = std::numeric_limits<ValueType>::max();
- return false;
+
+ 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) {
+ do {
+ upper_.next();
+ } while (UNLIKELY(upper_.value() < upperValue));
+ } else {
+ upper_.skipToNext(upperValue);
+ }
+
+ iterateTo(value);
+ return true;
+ }
+
+ /**
+ * 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;
}
+ return setDone();
+ }
- upper_.skipToNext(value >> list_.numLowerBits);
- progress_ = upper_.position();
- value_ = readLowerPart(progress_) |
- (upper_.value() << list_.numLowerBits);
- ++progress_;
- while (value_ < value) {
- value_ = readLowerPart(progress_) |
- (upper_.next() << list_.numLowerBits);
- ++progress_;
+ bool jumpTo(ValueType value) {
+ if (!kUnchecked && value > lastValue_) {
+ return setDone();
}
+ upper_.jumpToNext(value >> numLowerBits_);
+ iterateTo(value);
return true;
}
+ ValueType previousValue() const {
+ DCHECK_GT(position(), 0);
+ DCHECK_LT(position(), size());
+ return readLowerPart(upper_.position() - 1) |
+ (upper_.previousValue() << numLowerBits_);
+ }
+
+ SizeType size() const {
+ return size_;
+ }
+
+ bool valid() const {
+ return position() < size(); // Also checks that position() != -1.
+ }
+
+ SizeType position() const {
+ return upper_.position();
+ }
+ ValueType value() const {
+ DCHECK(valid());
+ return value_;
+ }
+
private:
- ValueType readLowerPart(size_t i) const {
- const size_t pos = i * list_.numLowerBits;
- const unsigned char* ptr = list_.lower.data() + (pos / 8);
+ // Must hold kInvalidValue + 1 == 0.
+ constexpr static ValueType kInvalidValue =
+ std::numeric_limits<ValueType>::max();
+
+ bool setDone() {
+ value_ = kInvalidValue;
+ upper_.setDone(size_);
+ return false;
+ }
+
+ 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_);
}
- const CompressedList list_;
- const ValueType lowerMask_;
- detail::UpperBitsReader<CompressedList, Instructions> upper_;
- size_t progress_;
- ValueType value_;
+ void iterateTo(ValueType value) {
+ while (true) {
+ value_ = readLowerPart(upper_.position()) |
+ (upper_.value() << numLowerBits_);
+ if (LIKELY(value_ >= value)) {
+ break;
+ }
+ upper_.next();
+ }
+ }
+
+ constexpr static size_t kLinearScanThreshold = 8;
+
+ detail::UpperBitsReader<Encoder, Instructions, SizeType> upper_;
+ const uint8_t* lower_;
+ SizeType size_;
+ ValueType value_ = kInvalidValue;
ValueType lastValue_;
+ uint8_t numLowerBits_;
};
-}} // namespaces
-
-#endif // FOLLY_EXPERIMENTAL_ELIAS_FANO_CODING_H
+} // namespace compression
+} // namespace folly
projects / folly.git / blobdiff