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diff --git a/js/src/jit/BitSet.h b/js/src/jit/BitSet.h
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+++ b/js/src/jit/BitSet.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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 jit_BitSet_h
+#define jit_BitSet_h
+
+#include "mozilla/MathAlgorithms.h"
+
+#include "jit/JitAllocPolicy.h"
+
+namespace js {
+namespace jit {
+
+// Provides constant time set insertion and removal, and fast linear
+// set operations such as intersection, difference, and union.
+// N.B. All set operations must be performed on sets with the same number
+// of bits.
+class BitSet
+{
+  public:
+    static const size_t BitsPerWord = 8 * sizeof(uint32_t);
+
+    static size_t RawLengthForBits(size_t bits) {
+        return (bits + BitsPerWord - 1) / BitsPerWord;
+    }
+
+  private:
+    uint32_t* bits_;
+    const unsigned int numBits_;
+
+    static inline uint32_t bitForValue(unsigned int value) {
+        return 1l << uint32_t(value % BitsPerWord);
+    }
+
+    static inline unsigned int wordForValue(unsigned int value) {
+        return value / BitsPerWord;
+    }
+
+    inline unsigned int numWords() const {
+        return RawLengthForBits(numBits_);
+    }
+
+    BitSet(const BitSet&) = delete;
+    void operator=(const BitSet&) = delete;
+
+  public:
+    class Iterator;
+
+    explicit BitSet(unsigned int numBits) :
+        bits_(nullptr),
+        numBits_(numBits) {}
+
+    MOZ_MUST_USE bool init(TempAllocator& alloc);
+
+    unsigned int getNumBits() const {
+        return numBits_;
+    }
+
+    // O(1): Check if this set contains the given value.
+    bool contains(unsigned int value) const {
+        MOZ_ASSERT(bits_);
+        MOZ_ASSERT(value < numBits_);
+
+        return !!(bits_[wordForValue(value)] & bitForValue(value));
+    }
+
+    // O(numBits): Check if this set contains any value.
+    bool empty() const;
+
+    // O(1): Insert the given value into this set.
+    void insert(unsigned int value) {
+        MOZ_ASSERT(bits_);
+        MOZ_ASSERT(value < numBits_);
+
+        bits_[wordForValue(value)] |= bitForValue(value);
+    }
+
+    // O(numBits): Insert every element of the given set into this set.
+    void insertAll(const BitSet& other);
+
+    // O(1): Remove the given value from this set.
+    void remove(unsigned int value) {
+        MOZ_ASSERT(bits_);
+        MOZ_ASSERT(value < numBits_);
+
+        bits_[wordForValue(value)] &= ~bitForValue(value);
+    }
+
+    // O(numBits): Remove the every element of the given set from this set.
+    void removeAll(const BitSet& other);
+
+    // O(numBits): Intersect this set with the given set.
+    void intersect(const BitSet& other);
+
+    // O(numBits): Intersect this set with the given set; return whether the
+    // intersection caused the set to change.
+    bool fixedPointIntersect(const BitSet& other);
+
+    // O(numBits): Does inplace complement of the set.
+    void complement();
+
+    // O(numBits): Clear this set.
+    void clear();
+
+    uint32_t* raw() const {
+        return bits_;
+    }
+    size_t rawLength() const {
+        return numWords();
+    }
+};
+
+class BitSet::Iterator
+{
+  private:
+    BitSet& set_;
+    unsigned index_;
+    unsigned word_;
+    uint32_t value_;
+
+    void skipEmpty() {
+        // Skip words containing only zeros.
+        unsigned numWords = set_.numWords();
+        const uint32_t* bits = set_.bits_;
+        while (value_ == 0) {
+            word_++;
+            if (word_ == numWords)
+                return;
+
+            index_ = word_ * BitSet::BitsPerWord;
+            value_ = bits[word_];
+        }
+
+        // Be careful: the result of CountTrailingZeroes32 is undefined if the
+        // input is 0.
+        int numZeros = mozilla::CountTrailingZeroes32(value_);
+        index_ += numZeros;
+        value_ >>= numZeros;
+
+        MOZ_ASSERT_IF(index_ < set_.numBits_, set_.contains(index_));
+    }
+
+  public:
+    explicit Iterator(BitSet& set) :
+      set_(set),
+      index_(0),
+      word_(0),
+      value_(set.bits_[0])
+    {
+        skipEmpty();
+    }
+
+    inline bool more() const {
+        return word_ < set_.numWords();
+    }
+    explicit operator bool() const {
+        return more();
+    }
+
+    inline void operator++() {
+        MOZ_ASSERT(more());
+        MOZ_ASSERT(index_ < set_.numBits_);
+
+        index_++;
+        value_ >>= 1;
+
+        skipEmpty();
+    }
+
+    unsigned int operator*() {
+        MOZ_ASSERT(index_ < set_.numBits_);
+        return index_;
+    }
+};
+
+} // namespace jit
+} // namespace js
+
+#endif /* jit_BitSet_h */