diff -r 000000000000 -r 6474c204b198 js/src/jit/PcScriptCache.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jit/PcScriptCache.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,78 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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
+
+// 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.
+
+struct JSRuntime;
+
+namespace js {
+namespace jit {
+
+struct PcScriptCacheEntry
+{
+    uint8_t *returnAddress; // Key into the hash table.
+    jsbytecode *pc;         // Cached PC.
+    JSScript *script;       // Cached script.
+};
+
+struct PcScriptCache
+{
+    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;
+
+    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.
+    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->gcNumber) {
+            clear(rt->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) {
+        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 */