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diff --git a/js/src/vm/MatchPairs.h b/js/src/vm/MatchPairs.h
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+++ b/js/src/vm/MatchPairs.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 vm_MatchPairs_h
+#define vm_MatchPairs_h
+
+#include "ds/LifoAlloc.h"
+#include "js/AllocPolicy.h"
+#include "js/Vector.h"
+
+/*
+ * RegExp match results are succinctly represented by pairs of integer
+ * indices delimiting (start, limit] segments of the input string.
+ *
+ * The pair count for a given RegExp match is the capturing parentheses
+ * count plus one for the "0 capturing paren" whole text match.
+ */
+
+namespace js {
+
+struct MatchPair final {
+  int32_t start;
+  int32_t limit;
+
+  static constexpr int32_t NoMatch = -1;
+
+  MatchPair() : start(NoMatch), limit(NoMatch) {}
+
+  MatchPair(int32_t start, int32_t limit) : start(start), limit(limit) {}
+
+  size_t length() const {
+    MOZ_ASSERT(!isUndefined());
+    return limit - start;
+  }
+  bool isUndefined() const { return start < 0; }
+
+  inline bool check() const {
+    MOZ_ASSERT(limit >= start);
+    MOZ_ASSERT_IF(start < 0, start == NoMatch);
+    MOZ_ASSERT_IF(limit < 0, limit == NoMatch);
+    return true;
+  }
+};
+
+// MachPairs is used as base class for VectorMatchPairs but can also be
+// stack-allocated (without a Vector) in JIT code.
+class MatchPairs {
+ protected:
+  /* Length of pairs_. */
+  uint32_t pairCount_;
+
+  /* Raw pointer into an allocated MatchPair buffer. */
+  MatchPair* pairs_;
+
+ protected:
+  /* Not used directly: use VectorMatchPairs. */
+  MatchPairs() : pairCount_(0), pairs_(nullptr) {}
+
+ protected:
+  /* Functions used by friend classes. */
+  friend class RegExpShared;
+  friend class RegExpStatics;
+
+  void forgetArray() { pairs_ = nullptr; }
+
+ public:
+  void checkAgainst(size_t inputLength) {
+#ifdef DEBUG
+    for (size_t i = 0; i < pairCount_; i++) {
+      const MatchPair& p = (*this)[i];
+      MOZ_ASSERT(p.check());
+      if (p.isUndefined()) {
+        continue;
+      }
+      MOZ_ASSERT(size_t(p.limit) <= inputLength);
+    }
+#endif
+  }
+
+  /* Querying functions in the style of RegExpStatics. */
+  bool empty() const { return pairCount_ == 0; }
+  size_t pairCount() const {
+    MOZ_ASSERT(pairCount_ > 0);
+    return pairCount_;
+  }
+
+  static size_t offsetOfPairs() { return offsetof(MatchPairs, pairs_); }
+  static size_t offsetOfPairCount() { return offsetof(MatchPairs, pairCount_); }
+
+  int32_t* pairsRaw() { return reinterpret_cast<int32_t*>(pairs_); }
+
+ public:
+  size_t length() const { return pairCount_; }
+
+  const MatchPair& operator[](size_t i) const {
+    MOZ_ASSERT(i < pairCount_);
+    return pairs_[i];
+  }
+  MatchPair& operator[](size_t i) {
+    MOZ_ASSERT(i < pairCount_);
+    return pairs_[i];
+  }
+};
+
+class VectorMatchPairs : public MatchPairs {
+  Vector<MatchPair, 10, SystemAllocPolicy> vec_;
+
+ protected:
+  friend class RegExpShared;
+  friend class RegExpStatics;
+
+  /* MatchPair buffer allocator: set pairs_ and pairCount_. */
+  bool allocOrExpandArray(size_t pairCount);
+
+  bool initArrayFrom(VectorMatchPairs& copyFrom);
+};
+
+} /* namespace js */
+
+#endif /* vm_MatchPairs_h */