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diff --git a/js/src/jit/FixedList.h b/js/src/jit/FixedList.h
new file mode 100644
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+++ b/js/src/jit/FixedList.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 jit_FixedList_h
+#define jit_FixedList_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Likely.h"
+
+#include <stddef.h>
+
+#include "jit/JitAllocPolicy.h"
+#include "js/Utility.h"
+
+namespace js {
+namespace jit {
+
+// List of a fixed length, but the length is unknown until runtime.
+template <typename T>
+class FixedList {
+  T* list_;
+  size_t length_;
+
+ private:
+  FixedList(const FixedList&);       // no copy definition.
+  void operator=(const FixedList*);  // no assignment definition.
+
+ public:
+  FixedList() : list_(nullptr), length_(0) {}
+
+  // Dynamic memory allocation requires the ability to report failure.
+  [[nodiscard]] bool init(TempAllocator& alloc, size_t length) {
+    if (length == 0) {
+      return true;
+    }
+
+    list_ = alloc.allocateArray<T>(length);
+    if (!list_) {
+      return false;
+    }
+
+    length_ = length;
+    return true;
+  }
+
+  size_t empty() const { return length_ == 0; }
+
+  size_t length() const { return length_; }
+
+  void shrink(size_t num) {
+    MOZ_ASSERT(num < length_);
+    length_ -= num;
+  }
+
+  [[nodiscard]] bool growBy(TempAllocator& alloc, size_t num) {
+    size_t newlength = length_ + num;
+    if (newlength < length_) {
+      return false;
+    }
+    size_t bytes;
+    if (MOZ_UNLIKELY(!CalculateAllocSize<T>(newlength, &bytes))) {
+      return false;
+    }
+    T* list = (T*)alloc.allocate(bytes);
+    if (MOZ_UNLIKELY(!list)) {
+      return false;
+    }
+
+    for (size_t i = 0; i < length_; i++) {
+      list[i] = list_[i];
+    }
+
+    length_ += num;
+    list_ = list;
+    return true;
+  }
+
+  T& operator[](size_t index) {
+    MOZ_ASSERT(index < length_);
+    return list_[index];
+  }
+  const T& operator[](size_t index) const {
+    MOZ_ASSERT(index < length_);
+    return list_[index];
+  }
+
+  T* data() { return list_; }
+
+  T* begin() { return list_; }
+  T* end() { return list_ + length_; }
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif /* jit_FixedList_h */