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diff --git a/js/src/vm/StencilCache.h b/js/src/vm/StencilCache.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_StencilCache_h
+#define vm_StencilCache_h
+
+#include "mozilla/Atomics.h"        // mozilla::Atomic
+#include "mozilla/HashFunctions.h"  // mozilla::HashGeneric
+#include "mozilla/RefPtr.h"         // mozilla::RefPtr
+
+#include "js/HashTable.h"  // js::HashTable
+
+#include "threading/ExclusiveData.h"  // js::ExclusiveData
+
+#include "vm/JSScript.h"       // js::ScriptSource
+#include "vm/SharedStencil.h"  // js::SourceExtent
+
+struct JS_PUBLIC_API JSContext;  // vm/JSContext.h
+
+namespace js {
+
+namespace frontend {
+struct CompilationStencil;            // frontend/CompilationStencil.h
+struct ExtensibleCompilationStencil;  // frontend/CompilationStencil.h
+} /* namespace frontend */
+
+// Note, the key is a RefPtr<ScriptSource>, but neither the PointerHasher nor
+// DefaultHasher seems to handle these correctly.
+struct SourceCachePolicy {
+  using Lookup = const ScriptSource*;
+
+  static HashNumber hash(const Lookup& l) { return mozilla::HashGeneric(l); }
+  static bool match(const Lookup& entry, const Lookup& l) { return entry == l; }
+};
+
+// Immutable information to identify a unique function, as well as the
+// compilation context which will result in a unique Stencil once compiled.
+struct StencilContext {
+  // This pointer is used to isolate a single source. Note the uniqueness of the
+  // ReadOnlyCompileOptions is implied by the fact that we allocate a new
+  // ScriptSource for each CompilationInput, which are initialized with a set of
+  // CompileOptions.
+  RefPtr<ScriptSource> source;
+
+  SourceExtent::FunctionKey funKey;
+
+  StencilContext(RefPtr<ScriptSource>& source, SourceExtent extent)
+      : source(source), funKey(extent.toFunctionKey()) {}
+};
+
+struct StencilCachePolicy {
+  using Lookup = StencilContext;
+
+  static HashNumber hash(const Lookup& l) {
+    const ScriptSource* raw = l.source;
+    return mozilla::HashGeneric(raw, l.funKey);
+  }
+  static bool match(const Lookup& entry, const Lookup& l) {
+    return entry.source == l.source && entry.funKey == l.funKey;
+  }
+};
+
+// Cache stencils which are parsed from the same source, and with identical
+// compilation options.
+//
+// This cache does not check the principals, as the source should not be
+// aliased across different principals.
+//
+// The content provided by this cache is computed by delazification tasks. The
+// delazification task needs contextual information to generate Stencils, which
+// are then registered in this cache.
+//
+// To reclaim memory from this cache, the producers should be shutdown and the
+// cache should be cleared. As the delazification process needs contextual
+// information to generate the Stencils, it is impossible to resume without
+// either keeping memory or recomputing from the beginning.
+//
+// Therefore, this cache cannot be cleared without disabling it at the same
+// time. Disabling this cache ends all threads which attempts to add new
+// stencils.
+//
+// The cache can be in multiple states:
+//
+// - Cache disabled:
+//
+//     The cache can be disabled when running in a steady state for some time,
+//     or after reaching a shrinking GC, which reclaims the memory from this
+//     cache.
+//
+//     This state is expected for the steady state of web pages, once the
+//     pages are loaded for a while and that we do not expect a huge load of
+//     delazification.
+//
+// - Cache enabled, Source is not cached:
+//
+//     This is a rare case which can happen either when testing the
+//     StencilCache, or after some dynamic load of JavaScript code in a page
+//     which already reached as steady state. In which case, we want to prevent
+//     locking for any functions which are not part of the newly loaded source.
+//
+//     This might also happen for eval-ed or inline JavaScript, which is
+//     parsed on the main thread, and also delazified on the main thread.
+//
+// - Cache enabled, Source is cached:
+//
+//     This case is expected to be frequent for any new document. A new
+//     document will register many sources for parsing off-thread, and
+//     triggering off-thread delazification.
+//
+//     All newly parse sources will be registered here until a steady state is
+//     reached, or a shrinking GC is called.
+class StencilCache {
+  using SourceSet =
+      js::HashSet<RefPtr<ScriptSource>, SourceCachePolicy, SystemAllocPolicy>;
+  using StencilMap =
+      js::HashMap<StencilContext, RefPtr<frontend::CompilationStencil>,
+                  StencilCachePolicy, SystemAllocPolicy>;
+
+  struct CacheData {
+    // Sources which are recorded in this cache.
+    SourceSet watched;
+    // Stencils of functions which are recorded in this cache.
+    StencilMap functions;
+  };
+
+  // Map a function to its CompilationStencil.
+  ExclusiveData<CacheData> cache;
+
+  // This flag is mostly read, and changes rarely. We use this Atomic to avoid
+  // locking a Mutex when the cache is disabled.
+  //
+  // The cache can be disabled when running in a steady state for some time, or
+  // after reaching a shrinking GC, which reclaims the memory from this cache
+  // and indirectly ends the threads which are producing stencils for this
+  // cache.
+  mozilla::Atomic<bool, mozilla::ReleaseAcquire> enabled;
+
+ public:
+  StencilCache();
+
+  // An access key is returned when checking whether the cache is enabled. It
+  // should be used in an if statement and provided to all follow-up functions
+  // if it is true-ish.
+  using AccessKey = ExclusiveData<CacheData>::NullableGuard;
+
+  // Not all stencils should be cached, we use the ScriptSource pointer to
+  // identify whether we should look further or not in the cache.
+  AccessKey isSourceCached(ScriptSource* src);
+
+  // Register a source in the cache in order to cache any stencil associated
+  // with this source in the future. To stop caching, the function
+  // clearAndDisable can be used.
+  //
+  // Note: This function should be called once per source. Which is usualy after
+  // creating it.
+  [[nodiscard]] bool startCaching(RefPtr<ScriptSource>&& src);
+
+  // Checks if the cache contains a specific stencil and returns a pointer to
+  // it if it does. Otherwise, returns nullptr.
+  frontend::CompilationStencil* lookup(AccessKey& guard,
+                                       const StencilContext& key);
+
+  // Adds a newly compiled stencil to the cache. The cache should not contain
+  // any entry for this function before calling this function.
+  [[nodiscard]] bool putNew(AccessKey& guard, const StencilContext& key,
+                            frontend::CompilationStencil* value);
+
+  // Prevent any further stencil from being cached, and clear and reclaim the
+  // memory of all stencil held by the cache.
+  //
+  // WARNING: This function should not be called within a scope checking for
+  // isSourceCached, as this would cause a dead-lock.
+  void clearAndDisable();
+};
+
+} /* namespace js */
+
+#endif /* vm_StencilCache_h */