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diff --git a/js/src/ds/Bitmap.h b/js/src/ds/Bitmap.h
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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 http://mozilla.org/MPL/2.0/. */
+
+#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/HeapAPI.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;
+
+ public:
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) {
+    return data.sizeOfExcludingThis(mallocSizeOf);
+  }
+
+  bool ensureSpace(size_t numWords) {
+    MOZ_ASSERT(data.empty());
+    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]; }
+
+  template <typename T>
+  typename std::enable_if_t<std::is_convertible_v<T, uintptr_t>, void>
+  copyBitsFrom(size_t wordStart, size_t numWords, T* source) {
+    MOZ_ASSERT(wordStart + numWords <= data.length());
+    for (size_t i = 0; i < numWords; i++) {
+      data[wordStart + i] = source[i];
+    }
+  }
+
+  template <typename T>
+  typename std::enable_if_t<std::is_convertible_v<T, uintptr_t>, void>
+  bitwiseOrRangeInto(size_t wordStart, size_t numWords, 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;
+
+  MOZ_ALWAYS_INLINE static size_t blockStartWord(size_t word) {
+    return word & ~(WordsInBlock - 1);
+  }
+
+  MOZ_ALWAYS_INLINE static uintptr_t bitMask(size_t bit) {
+    return uintptr_t(1) << (bit % JS_BITS_PER_WORD);
+  }
+
+  // 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 const BitBlock* readonlyThreadsafeGetBlock(
+      size_t blockId) const {
+    Data::Ptr p = data.readonlyThreadsafeLookup(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);
+  }
+
+ public:
+  ~SparseBitmap();
+
+  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] |= bitMask(bit);
+  }
+
+  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] |= bitMask(bit);
+    return true;
+  }
+
+  bool getBit(size_t bit) const;
+  bool readonlyThreadsafeGetBit(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.
+  template <typename T>
+  typename std::enable_if_t<std::is_convertible_v<T, uintptr_t>, void>
+  bitwiseOrRangeInto(size_t wordStart, size_t numWords, T* target) const {
+    size_t blockWord = blockStartWord(wordStart);
+
+    // We only support using a single bit block in this API.
+    MOZ_ASSERT(numWords &&
+               (blockWord == blockStartWord(wordStart + numWords - 1)));
+
+    BitBlock* block = getBlock(blockWord / WordsInBlock);
+    if (block) {
+      for (size_t i = 0; i < numWords; i++) {
+        target[i] |= (*block)[wordStart - blockWord + i];
+      }
+    }
+  }
+};
+
+}  // namespace js
+
+#endif  // ds_Bitmap_h