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diff --git a/js/src/jit/JitRealm.h b/js/src/jit/JitRealm.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_JitRealm_h
+#define jit_JitRealm_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+#include "mozilla/EnumeratedArray.h"
+#include "mozilla/MemoryReporting.h"
+
+#include <stddef.h>
+#include <stdint.h>
+
+#include "gc/Barrier.h"
+#include "gc/ZoneAllocator.h"
+#include "js/GCHashTable.h"
+#include "js/RootingAPI.h"
+#include "js/TracingAPI.h"
+#include "js/TypeDecls.h"
+
+namespace js {
+
+MOZ_COLD void ReportOutOfMemory(JSContext* cx);
+
+namespace jit {
+
+class JitCode;
+
+class JitRealm {
+  friend class JitActivation;
+
+  // The JitRealm stores stubs to concatenate strings inline and perform RegExp
+  // calls inline. These bake in zone and realm specific pointers and can't be
+  // stored in JitRuntime. They also are dependent on the value of
+  // 'initialStringHeap' and must be flushed when its value changes.
+  //
+  // These are weak pointers, but they can by accessed during off-thread Ion
+  // compilation and therefore can't use the usual read barrier. Instead, we
+  // record which stubs have been read and perform the appropriate barriers in
+  // CodeGenerator::link().
+
+  enum StubIndex : uint32_t {
+    StringConcat = 0,
+    RegExpMatcher,
+    RegExpSearcher,
+    RegExpTester,
+    Count
+  };
+
+  mozilla::EnumeratedArray<StubIndex, StubIndex::Count, WeakHeapPtr<JitCode*>>
+      stubs_;
+
+  gc::InitialHeap initialStringHeap;
+
+  JitCode* generateStringConcatStub(JSContext* cx);
+  JitCode* generateRegExpMatcherStub(JSContext* cx);
+  JitCode* generateRegExpSearcherStub(JSContext* cx);
+  JitCode* generateRegExpTesterStub(JSContext* cx);
+
+  JitCode* getStubNoBarrier(StubIndex stub,
+                            uint32_t* requiredBarriersOut) const {
+    MOZ_ASSERT(CurrentThreadIsIonCompiling());
+    *requiredBarriersOut |= 1 << uint32_t(stub);
+    return stubs_[stub].unbarrieredGet();
+  }
+
+ public:
+  JitRealm();
+
+  void initialize(bool zoneHasNurseryStrings);
+
+  // Initialize code stubs only used by Ion, not Baseline.
+  [[nodiscard]] bool ensureIonStubsExist(JSContext* cx) {
+    if (stubs_[StringConcat]) {
+      return true;
+    }
+    stubs_[StringConcat] = generateStringConcatStub(cx);
+    return stubs_[StringConcat];
+  }
+
+  void traceWeak(JSTracer* trc, JS::Realm* realm);
+
+  void discardStubs() {
+    for (WeakHeapPtr<JitCode*>& stubRef : stubs_) {
+      stubRef = nullptr;
+    }
+  }
+
+  bool hasStubs() const {
+    for (const WeakHeapPtr<JitCode*>& stubRef : stubs_) {
+      if (stubRef) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  void setStringsCanBeInNursery(bool allow) {
+    MOZ_ASSERT(!hasStubs());
+    initialStringHeap = allow ? gc::DefaultHeap : gc::TenuredHeap;
+  }
+
+  JitCode* stringConcatStubNoBarrier(uint32_t* requiredBarriersOut) const {
+    return getStubNoBarrier(StringConcat, requiredBarriersOut);
+  }
+
+  JitCode* regExpMatcherStubNoBarrier(uint32_t* requiredBarriersOut) const {
+    return getStubNoBarrier(RegExpMatcher, requiredBarriersOut);
+  }
+
+  [[nodiscard]] bool ensureRegExpMatcherStubExists(JSContext* cx) {
+    if (stubs_[RegExpMatcher]) {
+      return true;
+    }
+    stubs_[RegExpMatcher] = generateRegExpMatcherStub(cx);
+    return stubs_[RegExpMatcher];
+  }
+
+  JitCode* regExpSearcherStubNoBarrier(uint32_t* requiredBarriersOut) const {
+    return getStubNoBarrier(RegExpSearcher, requiredBarriersOut);
+  }
+
+  [[nodiscard]] bool ensureRegExpSearcherStubExists(JSContext* cx) {
+    if (stubs_[RegExpSearcher]) {
+      return true;
+    }
+    stubs_[RegExpSearcher] = generateRegExpSearcherStub(cx);
+    return stubs_[RegExpSearcher];
+  }
+
+  JitCode* regExpTesterStubNoBarrier(uint32_t* requiredBarriersOut) const {
+    return getStubNoBarrier(RegExpTester, requiredBarriersOut);
+  }
+
+  [[nodiscard]] bool ensureRegExpTesterStubExists(JSContext* cx) {
+    if (stubs_[RegExpTester]) {
+      return true;
+    }
+    stubs_[RegExpTester] = generateRegExpTesterStub(cx);
+    return stubs_[RegExpTester];
+  }
+
+  // Perform the necessary read barriers on stubs described by the bitmasks
+  // passed in. This function can only be called from the main thread.
+  //
+  // The stub pointers must still be valid by the time these methods are
+  // called. This is arranged by cancelling off-thread Ion compilation at the
+  // start of GC and at the start of sweeping.
+  void performStubReadBarriers(uint32_t stubsToBarrier) const;
+
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const;
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif /* jit_JitRealm_h */