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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef ds_Bitmap_h
#define ds_Bitmap_h
#include "mozilla/Array.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/MemoryChecking.h"
#include <algorithm>
#include <stddef.h>
#include <stdint.h>
#include "js/AllocPolicy.h"
#include "js/HashTable.h"
#include "js/Vector.h"
// This file provides two classes for representing bitmaps.
// DenseBitmap is an array of words of bits, with size linear in the maximum
// bit which has been set on it.
// SparseBitmap provides a reasonably simple, reasonably efficient (in time and
// space) implementation of a sparse bitmap. The basic representation is a hash
// table whose entries are fixed length malloc'ed blocks of bits.
namespace js {
class DenseBitmap {
using Data = Vector<uintptr_t, 0, SystemAllocPolicy>;
Data data;
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
return data.sizeOfExcludingThis(mallocSizeOf);
bool ensureSpace(size_t numWords) {
return data.appendN(0, numWords);
size_t numWords() const { return data.length(); }
uintptr_t word(size_t i) const { return data[i]; }
uintptr_t& word(size_t i) { return data[i]; }
void copyBitsFrom(size_t wordStart, size_t numWords, uintptr_t* source) {
MOZ_ASSERT(wordStart + numWords <= data.length());
// Use std::copy and not std::copy_n because the former requires no
// overlap and so provides extra opportunity to optimize.
std::copy(source, source + numWords, &data[wordStart]);
void bitwiseOrRangeInto(size_t wordStart, size_t numWords,
uintptr_t* target) const {
for (size_t i = 0; i < numWords; i++) {
target[i] |= data[wordStart + i];
class SparseBitmap {
// The number of words of bits to use for each block mainly affects the
// memory usage of the bitmap. To minimize overhead, bitmaps which are
// expected to be fairly dense should have a large block size, and bitmaps
// which are expected to be very sparse should have a small block size.
static const size_t WordsInBlock = 4096 / sizeof(uintptr_t);
using BitBlock = mozilla::Array<uintptr_t, WordsInBlock>;
using Data =
HashMap<size_t, BitBlock*, DefaultHasher<size_t>, SystemAllocPolicy>;
Data data;
static size_t blockStartWord(size_t word) {
return word & ~(WordsInBlock - 1);
// Return the number of words in a BitBlock starting at |blockWord| which
// are in |other|.
static size_t wordIntersectCount(size_t blockWord, const DenseBitmap& other) {
long count = other.numWords() - blockWord;
return std::min<size_t>((size_t)WordsInBlock, std::max<long>(count, 0));
BitBlock& createBlock(Data::AddPtr p, size_t blockId,
AutoEnterOOMUnsafeRegion& oomUnsafe);
BitBlock* createBlock(Data::AddPtr p, size_t blockId);
MOZ_ALWAYS_INLINE BitBlock* getBlock(size_t blockId) const {
Data::Ptr p = data.lookup(blockId);
return p ? p->value() : nullptr;
MOZ_ALWAYS_INLINE BitBlock& getOrCreateBlock(size_t blockId) {
// The lookupForAdd() needs protection against injected OOMs, as does
// the add() within createBlock().
AutoEnterOOMUnsafeRegion oomUnsafe;
Data::AddPtr p = data.lookupForAdd(blockId);
if (p) {
return *p->value();
return createBlock(p, blockId, oomUnsafe);
MOZ_ALWAYS_INLINE BitBlock* getOrCreateBlockFallible(size_t blockId) {
Data::AddPtr p = data.lookupForAdd(blockId);
if (p) {
return p->value();
return createBlock(p, blockId);
size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf);
MOZ_ALWAYS_INLINE void setBit(size_t bit) {
size_t word = bit / JS_BITS_PER_WORD;
size_t blockWord = blockStartWord(word);
BitBlock& block = getOrCreateBlock(blockWord / WordsInBlock);
block[word - blockWord] |= uintptr_t(1) << (bit % JS_BITS_PER_WORD);
MOZ_ALWAYS_INLINE bool setBitFallible(size_t bit) {
size_t word = bit / JS_BITS_PER_WORD;
size_t blockWord = blockStartWord(word);
BitBlock* block = getOrCreateBlockFallible(blockWord / WordsInBlock);
if (!block) {
return false;
(*block)[word - blockWord] |= uintptr_t(1) << (bit % JS_BITS_PER_WORD);
return true;
bool getBit(size_t bit) const;
void bitwiseAndWith(const DenseBitmap& other);
void bitwiseOrWith(const SparseBitmap& other);
void bitwiseOrInto(DenseBitmap& other) const;
// Currently, this API only supports a range of words that is in a single bit
// block.
void bitwiseOrRangeInto(size_t wordStart, size_t numWords,
uintptr_t* target) const;
} // namespace js
#endif // ds_Bitmap_h