1 files changed, 88 insertions, 0 deletions
diff --git a/js/src/jit/PcScriptCache.h b/js/src/jit/PcScriptCache.h
new file mode 100644
index 0000000000..c83c479c85
--- /dev/null
+++ b/js/src/jit/PcScriptCache.h
@@ -0,0 +1,88 @@
+/* -*- 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_PcScriptCache_h
+#define jit_PcScriptCache_h
+
+#include "mozilla/Array.h"
+#include "js/TypeDecls.h"
+#include "vm/Runtime.h"
+
+// Defines a fixed-size hash table solely for the purpose of caching
+// jit::GetPcScript(). One cache is attached to each JSRuntime; it functions as
+// if cleared on GC.
+
+namespace js {
+namespace jit {
+
+struct PcScriptCacheEntry {
+  uint8_t* returnAddress;  // Key into the hash table.
+  jsbytecode* pc;          // Cached PC.
+  JSScript* script;        // Cached script.
+};
+
+struct PcScriptCache {
+ private:
+  static const uint32_t Length = 73;
+
+  // GC number at the time the cache was filled or created.
+  // Storing and checking against this number allows us to not bother
+  // clearing this cache on every GC -- only when actually necessary.
+  uint64_t gcNumber;
+
+  // List of cache entries.
+  mozilla::Array<PcScriptCacheEntry, Length> entries;
+
+ public:
+  explicit PcScriptCache(uint64_t gcNumber) { clear(gcNumber); }
+
+  void clear(uint64_t gcNumber) {
+    for (uint32_t i = 0; i < Length; i++) {
+      entries[i].returnAddress = nullptr;
+    }
+    this->gcNumber = gcNumber;
+  }
+
+  // Get a value from the cache. May perform lazy allocation.
+  [[nodiscard]] bool get(JSRuntime* rt, uint32_t hash, uint8_t* addr,
+                         JSScript** scriptRes, jsbytecode** pcRes) {
+    // If a GC occurred, lazily clear the cache now.
+    if (gcNumber != rt->gc.gcNumber()) {
+      clear(rt->gc.gcNumber());
+      return false;
+    }
+
+    if (entries[hash].returnAddress != addr) {
+      return false;
+    }
+
+    *scriptRes = entries[hash].script;
+    if (pcRes) {
+      *pcRes = entries[hash].pc;
+    }
+
+    return true;
+  }
+
+  void add(uint32_t hash, uint8_t* addr, jsbytecode* pc, JSScript* script) {
+    MOZ_ASSERT(addr);
+    MOZ_ASSERT(pc);
+    MOZ_ASSERT(script);
+    entries[hash].returnAddress = addr;
+    entries[hash].pc = pc;
+    entries[hash].script = script;
+  }
+
+  static uint32_t Hash(uint8_t* addr) {
+    uint32_t key = (uint32_t)((uintptr_t)addr);
+    return ((key >> 3) * 2654435761u) % Length;
+  }
+};
+
+}  // namespace jit
+}  // namespace js
+
+#endif /* jit_PcScriptCache_h */