1 files changed, 208 insertions, 0 deletions
diff --git a/js/src/vm/SharedMem.h b/js/src/vm/SharedMem.h
new file mode 100644
index 0000000000..04a03bafd0
--- /dev/null
+++ b/js/src/vm/SharedMem.h
@@ -0,0 +1,208 @@
+/* -*- 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_SharedMem_h
+#define vm_SharedMem_h
+
+#include "mozilla/Assertions.h"
+
+#include <type_traits>
+
+template <typename T>
+class SharedMem {
+  static_assert(std::is_pointer_v<T>, "SharedMem encapsulates pointer types");
+
+  enum Sharedness { IsUnshared, IsShared };
+
+  T ptr_;
+#ifdef DEBUG
+  Sharedness sharedness_;
+#endif
+
+  SharedMem(T ptr, Sharedness sharedness)
+      : ptr_(ptr)
+#ifdef DEBUG
+        ,
+        sharedness_(sharedness)
+#endif
+  {
+  }
+
+ public:
+  // Create a SharedMem<T> that is an unshared nullptr.
+  SharedMem()
+      : ptr_(nullptr)
+#ifdef DEBUG
+        ,
+        sharedness_(IsUnshared)
+#endif
+  {
+  }
+
+  // Create a SharedMem<T> that's shared/unshared in the same way as
+  // "forSharedness".
+  SharedMem(T ptr, const SharedMem& forSharedness)
+      : ptr_(ptr)
+#ifdef DEBUG
+        ,
+        sharedness_(forSharedness.sharedness_)
+#endif
+  {
+  }
+
+  // Create a SharedMem<T> that's marked as shared.
+  static SharedMem shared(void* p) {
+    return SharedMem(static_cast<T>(p), IsShared);
+  }
+
+  // Create a SharedMem<T> that's marked as unshared.
+  static SharedMem unshared(void* p) {
+    return SharedMem(static_cast<T>(p), IsUnshared);
+  }
+
+  SharedMem& operator=(const SharedMem& that) {
+    ptr_ = that.ptr_;
+#ifdef DEBUG
+    sharedness_ = that.sharedness_;
+#endif
+    return *this;
+  }
+
+  // Reinterpret-cast the pointer to type U, preserving sharedness.
+  // Eg, "obj->dataPointerEither().cast<uint8_t*>()" yields a
+  // SharedMem<uint8_t*>.
+  template <typename U>
+  inline SharedMem<U> cast() const {
+#ifdef DEBUG
+    MOZ_ASSERT(
+        asValue() %
+            sizeof(std::conditional_t<std::is_void_v<std::remove_pointer_t<U>>,
+                                      char, std::remove_pointer_t<U>>) ==
+        0);
+    if (sharedness_ == IsUnshared) {
+      return SharedMem<U>::unshared(unwrap());
+    }
+#endif
+    return SharedMem<U>::shared(unwrap());
+  }
+
+  explicit operator bool() const { return ptr_ != nullptr; }
+
+  SharedMem operator+(size_t offset) const {
+    return SharedMem(ptr_ + offset, *this);
+  }
+
+  SharedMem operator-(size_t offset) const {
+    return SharedMem(ptr_ - offset, *this);
+  }
+
+  SharedMem operator++() {
+    ptr_++;
+    return *this;
+  }
+
+  SharedMem operator++(int) {
+    SharedMem<T> result(*this);
+    ptr_++;
+    return result;
+  }
+
+  SharedMem operator--() {
+    ptr_--;
+    return *this;
+  }
+
+  SharedMem operator--(int) {
+    SharedMem<T> result(*this);
+    ptr_--;
+    return result;
+  }
+
+  uintptr_t asValue() const { return reinterpret_cast<uintptr_t>(ptr_); }
+
+  // Cast to char*, add nbytes, and cast back to T.  Simplifies code in a few
+  // places.
+  SharedMem addBytes(size_t nbytes) {
+    MOZ_ASSERT(
+        nbytes %
+            sizeof(std::conditional_t<std::is_void_v<std::remove_pointer_t<T>>,
+                                      char, std::remove_pointer_t<T>>) ==
+        0);
+    return SharedMem(
+        reinterpret_cast<T>(reinterpret_cast<char*>(ptr_) + nbytes), *this);
+  }
+
+  T unwrap() const { return ptr_; }
+
+  T unwrapUnshared() const {
+    MOZ_ASSERT(sharedness_ == IsUnshared);
+    return ptr_;
+  }
+
+  uintptr_t unwrapValue() const { return reinterpret_cast<uintptr_t>(ptr_); }
+};
+
+template <typename T>
+inline bool operator>=(const SharedMem<T>& a, const SharedMem<T>& b) {
+  return a.unwrap() >= b.unwrap();
+}
+
+template <typename T>
+inline bool operator>=(const void* a, const SharedMem<T>& b) {
+  return a >= b.unwrap();
+}
+
+template <typename T>
+inline bool operator==(const void* a, const SharedMem<T>& b) {
+  return a == b.unwrap();
+}
+
+template <typename T>
+inline bool operator==(const SharedMem<T>& a, decltype(nullptr) b) {
+  return a.unwrap() == b;
+}
+
+template <typename T>
+inline bool operator==(const SharedMem<T>& a, const SharedMem<T>& b) {
+  return a.unwrap() == b.unwrap();
+}
+
+template <typename T>
+inline bool operator!=(const SharedMem<T>& a, decltype(nullptr) b) {
+  return a.unwrap() != b;
+}
+
+template <typename T>
+inline bool operator!=(const SharedMem<T>& a, const SharedMem<T>& b) {
+  return a.unwrap() != b.unwrap();
+}
+
+template <typename T>
+inline bool operator>(const SharedMem<T>& a, const SharedMem<T>& b) {
+  return a.unwrap() > b.unwrap();
+}
+
+template <typename T>
+inline bool operator>(const void* a, const SharedMem<T>& b) {
+  return a > b.unwrap();
+}
+
+template <typename T>
+inline bool operator<=(const SharedMem<T>& a, const SharedMem<T>& b) {
+  return a.unwrap() <= b.unwrap();
+}
+
+template <typename T>
+inline bool operator<=(const void* a, const SharedMem<T>& b) {
+  return a <= b.unwrap();
+}
+
+template <typename T>
+inline bool operator<(const void* a, const SharedMem<T>& b) {
+  return a < b.unwrap();
+}
+
+#endif  // vm_SharedMem_h