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//===- llvm/ADT/SparseSet.h - Sparse set ------------------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file defines the SparseSet class derived from the version described in
// Briggs, Torczon, "An efficient representation for sparse sets", ACM Letters
// on Programming Languages and Systems, Volume 2 Issue 1-4, March-Dec. 1993.
// A sparse set holds a small number of objects identified by integer keys from
// a moderately sized universe. The sparse set uses more memory than other
// containers in order to provide faster operations.
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/Allocator.h"
#include <cassert>
#include <cstdint>
#include <cstdlib>
#include <limits>
#include <utility>
namespace llvm {
/// SparseSetValTraits - Objects in a SparseSet are identified by keys that can
/// be uniquely converted to a small integer less than the set's universe. This
/// class allows the set to hold values that differ from the set's key type as
/// long as an index can still be derived from the value. SparseSet never
/// directly compares ValueT, only their indices, so it can map keys to
/// arbitrary values. SparseSetValTraits computes the index from the value
/// object. To compute the index from a key, SparseSet uses a separate
/// KeyFunctorT template argument.
/// A simple type declaration, SparseSet<Type>, handles these cases:
/// - unsigned key, identity index, identity value
/// - unsigned key, identity index, fat value providing getSparseSetIndex()
/// The type declaration SparseSet<Type, UnaryFunction> handles:
/// - unsigned key, remapped index, identity value (virtual registers)
/// - pointer key, pointer-derived index, identity value (node+ID)
/// - pointer key, pointer-derived index, fat value with getSparseSetIndex()
/// Only other, unexpected cases require specializing SparseSetValTraits.
/// For best results, ValueT should not require a destructor.
template<typename ValueT>
struct SparseSetValTraits {
static unsigned getValIndex(const ValueT &Val) {
return Val.getSparseSetIndex();
/// SparseSetValFunctor - Helper class for selecting SparseSetValTraits. The
/// generic implementation handles ValueT classes which either provide
/// getSparseSetIndex() or specialize SparseSetValTraits<>.
template<typename KeyT, typename ValueT, typename KeyFunctorT>
struct SparseSetValFunctor {
unsigned operator()(const ValueT &Val) const {
return SparseSetValTraits<ValueT>::getValIndex(Val);
/// SparseSetValFunctor<KeyT, KeyT> - Helper class for the common case of
/// identity key/value sets.
template<typename KeyT, typename KeyFunctorT>
struct SparseSetValFunctor<KeyT, KeyT, KeyFunctorT> {
unsigned operator()(const KeyT &Key) const {
return KeyFunctorT()(Key);
/// SparseSet - Fast set implmentation for objects that can be identified by
/// small unsigned keys.
/// SparseSet allocates memory proportional to the size of the key universe, so
/// it is not recommended for building composite data structures. It is useful
/// for algorithms that require a single set with fast operations.
/// Compared to DenseSet and DenseMap, SparseSet provides constant-time fast
/// clear() and iteration as fast as a vector. The find(), insert(), and
/// erase() operations are all constant time, and typically faster than a hash
/// table. The iteration order doesn't depend on numerical key values, it only
/// depends on the order of insert() and erase() operations. When no elements
/// have been erased, the iteration order is the insertion order.
/// Compared to BitVector, SparseSet<unsigned> uses 8x-40x more memory, but
/// offers constant-time clear() and size() operations as well as fast
/// iteration independent on the size of the universe.
/// SparseSet contains a dense vector holding all the objects and a sparse
/// array holding indexes into the dense vector. Most of the memory is used by
/// the sparse array which is the size of the key universe. The SparseT
/// template parameter provides a space/speed tradeoff for sets holding many
/// elements.
/// When SparseT is uint32_t, find() only touches 2 cache lines, but the sparse
/// array uses 4 x Universe bytes.
/// When SparseT is uint8_t (the default), find() touches up to 2+[N/256] cache
/// lines, but the sparse array is 4x smaller. N is the number of elements in
/// the set.
/// For sets that may grow to thousands of elements, SparseT should be set to
/// uint16_t or uint32_t.
/// @tparam ValueT The type of objects in the set.
/// @tparam KeyFunctorT A functor that computes an unsigned index from KeyT.
/// @tparam SparseT An unsigned integer type. See above.
template<typename ValueT,
typename KeyFunctorT = identity<unsigned>,
typename SparseT = uint8_t>
class SparseSet {
static_assert(std::numeric_limits<SparseT>::is_integer &&
"SparseT must be an unsigned integer type");
using KeyT = typename KeyFunctorT::argument_type;
using DenseT = SmallVector<ValueT, 8>;
using size_type = unsigned;
DenseT Dense;
SparseT *Sparse = nullptr;
unsigned Universe = 0;
KeyFunctorT KeyIndexOf;
SparseSetValFunctor<KeyT, ValueT, KeyFunctorT> ValIndexOf;
using value_type = ValueT;
using reference = ValueT &;
using const_reference = const ValueT &;
using pointer = ValueT *;
using const_pointer = const ValueT *;
SparseSet() = default;
SparseSet(const SparseSet &) = delete;
SparseSet &operator=(const SparseSet &) = delete;
~SparseSet() { free(Sparse); }
/// setUniverse - Set the universe size which determines the largest key the
/// set can hold. The universe must be sized before any elements can be
/// added.
/// @param U Universe size. All object keys must be less than U.
void setUniverse(unsigned U) {
// It's not hard to resize the universe on a non-empty set, but it doesn't
// seem like a likely use case, so we can add that code when we need it.
assert(empty() && "Can only resize universe on an empty map");
// Hysteresis prevents needless reallocations.
if (U >= Universe/4 && U <= Universe)
// The Sparse array doesn't actually need to be initialized, so malloc
// would be enough here, but that will cause tools like valgrind to
// complain about branching on uninitialized data.
Sparse = static_cast<SparseT*>(safe_calloc(U, sizeof(SparseT)));
Universe = U;
// Import trivial vector stuff from DenseT.
using iterator = typename DenseT::iterator;
using const_iterator = typename DenseT::const_iterator;
const_iterator begin() const { return Dense.begin(); }
const_iterator end() const { return Dense.end(); }
iterator begin() { return Dense.begin(); }
iterator end() { return Dense.end(); }
/// empty - Returns true if the set is empty.
/// This is not the same as BitVector::empty().
bool empty() const { return Dense.empty(); }
/// size - Returns the number of elements in the set.
/// This is not the same as BitVector::size() which returns the size of the
/// universe.
size_type size() const { return Dense.size(); }
/// clear - Clears the set. This is a very fast constant time operation.
void clear() {
// Sparse does not need to be cleared, see find().
/// findIndex - Find an element by its index.
/// @param Idx A valid index to find.
/// @returns An iterator to the element identified by key, or end().
iterator findIndex(unsigned Idx) {
assert(Idx < Universe && "Key out of range");
const unsigned Stride = std::numeric_limits<SparseT>::max() + 1u;
for (unsigned i = Sparse[Idx], e = size(); i < e; i += Stride) {
const unsigned FoundIdx = ValIndexOf(Dense[i]);
assert(FoundIdx < Universe && "Invalid key in set. Did object mutate?");
if (Idx == FoundIdx)
return begin() + i;
// Stride is 0 when SparseT >= unsigned. We don't need to loop.
if (!Stride)
return end();
/// find - Find an element by its key.
/// @param Key A valid key to find.
/// @returns An iterator to the element identified by key, or end().
iterator find(const KeyT &Key) {
return findIndex(KeyIndexOf(Key));
const_iterator find(const KeyT &Key) const {
return const_cast<SparseSet*>(this)->findIndex(KeyIndexOf(Key));
/// count - Returns 1 if this set contains an element identified by Key,
/// 0 otherwise.
size_type count(const KeyT &Key) const {
return find(Key) == end() ? 0 : 1;
/// insert - Attempts to insert a new element.
/// If Val is successfully inserted, return (I, true), where I is an iterator
/// pointing to the newly inserted element.
/// If the set already contains an element with the same key as Val, return
/// (I, false), where I is an iterator pointing to the existing element.
/// Insertion invalidates all iterators.
std::pair<iterator, bool> insert(const ValueT &Val) {
unsigned Idx = ValIndexOf(Val);
iterator I = findIndex(Idx);
if (I != end())
return std::make_pair(I, false);
Sparse[Idx] = size();
return std::make_pair(end() - 1, true);
/// array subscript - If an element already exists with this key, return it.
/// Otherwise, automatically construct a new value from Key, insert it,
/// and return the newly inserted element.
ValueT &operator[](const KeyT &Key) {
return *insert(ValueT(Key)).first;
ValueT pop_back_val() {
// Sparse does not need to be cleared, see find().
return Dense.pop_back_val();
/// erase - Erases an existing element identified by a valid iterator.
/// This invalidates all iterators, but erase() returns an iterator pointing
/// to the next element. This makes it possible to erase selected elements
/// while iterating over the set:
/// for (SparseSet::iterator I = Set.begin(); I != Set.end();)
/// if (test(*I))
/// I = Set.erase(I);
/// else
/// ++I;
/// Note that end() changes when elements are erased, unlike std::list.
iterator erase(iterator I) {
assert(unsigned(I - begin()) < size() && "Invalid iterator");
if (I != end() - 1) {
*I = Dense.back();
unsigned BackIdx = ValIndexOf(Dense.back());
assert(BackIdx < Universe && "Invalid key in set. Did object mutate?");
Sparse[BackIdx] = I - begin();
// This depends on SmallVector::pop_back() not invalidating iterators.
// std::vector::pop_back() doesn't give that guarantee.
return I;
/// erase - Erases an element identified by Key, if it exists.
/// @param Key The key identifying the element to erase.
/// @returns True when an element was erased, false if no element was found.
bool erase(const KeyT &Key) {
iterator I = find(Key);
if (I == end())
return false;
return true;
} // end namespace llvm