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diff --git a/xpcom/base/CodeAddressService.h b/xpcom/base/CodeAddressService.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 CodeAddressService_h__
+#define CodeAddressService_h__
+
+#include "mozilla/Assertions.h"
+#include "mozilla/HashFunctions.h"
+#include "mozilla/IntegerPrintfMacros.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Types.h"
+
+#include "mozilla/StackWalk.h"
+
+namespace mozilla {
+
+// This class is used to print details about code locations.
+//
+// |StringTable| must implement an Intern() method that returns an interned
+// copy of the string that was passed in, as well as a standard
+// SizeOfExcludingThis() method.
+//
+// |StringAlloc| must implement |copy| and |free|. |copy| copies a string,
+// while |free| is used to free strings created by |copy|.
+//
+// |DescribeCodeAddressLock| is needed when the callers may be holding a lock
+// used by MozDescribeCodeAddress.  |DescribeCodeAddressLock| must implement
+// static methods IsLocked(), Unlock() and Lock().
+template <class StringTable,
+          class StringAlloc,
+          class DescribeCodeAddressLock>
+class CodeAddressService
+{
+  // GetLocation() is the key function in this class.  It's basically a wrapper
+  // around MozDescribeCodeAddress.
+  //
+  // However, MozDescribeCodeAddress is very slow on some platforms, and we
+  // have lots of repeated (i.e. same PC) calls to it.  So we do some caching
+  // of results.  Each cached result includes two strings (|mFunction| and
+  // |mLibrary|), so we also optimize them for space in the following ways.
+  //
+  // - The number of distinct library names is small, e.g. a few dozen.  There
+  //   is lots of repetition, especially of libxul.  So we intern them in their
+  //   own table, which saves space over duplicating them for each cache entry.
+  //
+  // - The number of distinct function names is much higher, so we duplicate
+  //   them in each cache entry.  That's more space-efficient than interning
+  //   because entries containing single-occurrence function names are quickly
+  //   overwritten, and their copies released.  In addition, empty function
+  //   names are common, so we use nullptr to represent them compactly.
+
+  StringTable mLibraryStrings;
+
+  struct Entry
+  {
+    const void* mPc;
+    char*       mFunction;  // owned by the Entry;  may be null
+    const char* mLibrary;   // owned by mLibraryStrings;  never null
+                            //   in a non-empty entry is in use
+    ptrdiff_t   mLOffset;
+    char*       mFileName;  // owned by the Entry; may be null
+    uint32_t    mLineNo:31;
+    uint32_t    mInUse:1;   // is the entry used?
+
+    Entry()
+      : mPc(0), mFunction(nullptr), mLibrary(nullptr), mLOffset(0),
+        mFileName(nullptr), mLineNo(0), mInUse(0)
+    {}
+
+    ~Entry()
+    {
+      // We don't free mLibrary because it's externally owned.
+      StringAlloc::free(mFunction);
+      StringAlloc::free(mFileName);
+    }
+
+    void Replace(const void* aPc, const char* aFunction,
+                 const char* aLibrary, ptrdiff_t aLOffset,
+                 const char* aFileName, unsigned long aLineNo)
+    {
+      mPc = aPc;
+
+      // Convert "" to nullptr.  Otherwise, make a copy of the name.
+      StringAlloc::free(mFunction);
+      mFunction = !aFunction[0] ? nullptr : StringAlloc::copy(aFunction);
+      StringAlloc::free(mFileName);
+      mFileName = !aFileName[0] ? nullptr : StringAlloc::copy(aFileName);
+
+      mLibrary = aLibrary;
+      mLOffset = aLOffset;
+      mLineNo = aLineNo;
+
+      mInUse = 1;
+    }
+
+    size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+    {
+      // Don't measure mLibrary because it's externally owned.
+      size_t n = 0;
+      n += aMallocSizeOf(mFunction);
+      n += aMallocSizeOf(mFileName);
+      return n;
+    }
+  };
+
+  // A direct-mapped cache.  When doing dmd::Analyze() just after starting
+  // desktop Firefox (which is similar to analyzing after a longer-running
+  // session, thanks to the limit on how many records we print), a cache with
+  // 2^24 entries (which approximates an infinite-entry cache) has a ~91% hit
+  // rate.  A cache with 2^12 entries has a ~83% hit rate, and takes up ~85 KiB
+  // (on 32-bit platforms) or ~150 KiB (on 64-bit platforms).
+  static const size_t kNumEntries = 1 << 12;
+  static const size_t kMask = kNumEntries - 1;
+  Entry mEntries[kNumEntries];
+
+  size_t mNumCacheHits;
+  size_t mNumCacheMisses;
+
+public:
+  CodeAddressService()
+    : mEntries(), mNumCacheHits(0), mNumCacheMisses(0)
+  {
+  }
+
+  void GetLocation(uint32_t aFrameNumber, const void* aPc, char* aBuf,
+                   size_t aBufLen)
+  {
+    MOZ_ASSERT(DescribeCodeAddressLock::IsLocked());
+
+    uint32_t index = HashGeneric(aPc) & kMask;
+    MOZ_ASSERT(index < kNumEntries);
+    Entry& entry = mEntries[index];
+
+    if (!entry.mInUse || entry.mPc != aPc) {
+      mNumCacheMisses++;
+
+      // MozDescribeCodeAddress can (on Linux) acquire a lock inside
+      // the shared library loader.  Another thread might call malloc
+      // while holding that lock (when loading a shared library).  So
+      // we have to exit the lock around this call.  For details, see
+      // https://bugzilla.mozilla.org/show_bug.cgi?id=363334#c3
+      MozCodeAddressDetails details;
+      {
+        DescribeCodeAddressLock::Unlock();
+        (void)MozDescribeCodeAddress(const_cast<void*>(aPc), &details);
+        DescribeCodeAddressLock::Lock();
+      }
+
+      const char* library = mLibraryStrings.Intern(details.library);
+      entry.Replace(aPc, details.function, library, details.loffset,
+                    details.filename, details.lineno);
+
+    } else {
+      mNumCacheHits++;
+    }
+
+    MOZ_ASSERT(entry.mPc == aPc);
+
+    MozFormatCodeAddress(aBuf, aBufLen, aFrameNumber, entry.mPc,
+                         entry.mFunction, entry.mLibrary, entry.mLOffset,
+                         entry.mFileName, entry.mLineNo);
+  }
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const
+  {
+    size_t n = aMallocSizeOf(this);
+    for (uint32_t i = 0; i < kNumEntries; i++) {
+      n += mEntries[i].SizeOfExcludingThis(aMallocSizeOf);
+    }
+
+    n += mLibraryStrings.SizeOfExcludingThis(aMallocSizeOf);
+
+    return n;
+  }
+
+  size_t CacheCapacity() const { return kNumEntries; }
+
+  size_t CacheCount() const
+  {
+    size_t n = 0;
+    for (size_t i = 0; i < kNumEntries; i++) {
+      if (mEntries[i].mInUse) {
+        n++;
+      }
+    }
+    return n;
+  }
+
+  size_t NumCacheHits()   const { return mNumCacheHits; }
+  size_t NumCacheMisses() const { return mNumCacheMisses; }
+};
+
+} // namespace mozilla
+
+#endif // CodeAddressService_h__