xpcom/base/CodeAddressService.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef CodeAddressService_h__
   8 #define CodeAddressService_h__
   9
  10 #include <cstddef>
  11 #include <cstdint>
  12 #include <cstring>
  13 #include "mozilla/AllocPolicy.h"
  14 #include "mozilla/Assertions.h"
  15 #include "mozilla/HashFunctions.h"
  16 #include "mozilla/HashTable.h"
  17 #include "mozilla/MemoryReporting.h"
  18 #include "mozilla/StackWalk.h"
  19
  20 namespace mozilla {
  21
  22 namespace detail {
  23
  24 template <class AllocPolicy>
  25 class CodeAddressServiceAllocPolicy : public AllocPolicy {
  26  public:
  27   char* strdup_(const char* aStr) {
  28     char* s = AllocPolicy::template pod_malloc<char>(strlen(aStr) + 1);
  29     if (!s) {
  30       MOZ_CRASH("CodeAddressService OOM");
  31     }
  32     strcpy(s, aStr);
  33     return s;
  34   }
  35 };
  36
  37 // Default implementation of DescribeCodeAddressLock.
  38 struct DefaultDescribeCodeAddressLock {
  39   static void Unlock() {}
  40   static void Lock() {}
  41   // Because CodeAddressService asserts that IsLocked() is true, returning true
  42   // here is a sensible default when there is no relevant lock.
  43   static bool IsLocked() { return true; }
  44 };
  45
  46 }  // namespace detail
  47
  48 // This class is used to print details about code locations.
  49 //
  50 // |AllocPolicy_| must adhere to the description in mfbt/AllocPolicy.h.
  51 //
  52 // |DescribeCodeAddressLock| is needed when the callers may be holding a lock
  53 // used by MozDescribeCodeAddress.  |DescribeCodeAddressLock| must implement
  54 // static methods IsLocked(), Unlock() and Lock().
  55 template <class AllocPolicy_ = MallocAllocPolicy,
  56           class DescribeCodeAddressLock =
  57               detail::DefaultDescribeCodeAddressLock>
  58 class CodeAddressService
  59     : private detail::CodeAddressServiceAllocPolicy<AllocPolicy_> {
  60  protected:
  61   // GetLocation() is the key function in this class.  It's basically a wrapper
  62   // around MozDescribeCodeAddress.
  63   //
  64   // However, MozDescribeCodeAddress is very slow on some platforms, and we
  65   // have lots of repeated (i.e. same PC) calls to it.  So we do some caching
  66   // of results.  Each cached result includes two strings (|mFunction| and
  67   // |mLibrary|), so we also optimize them for space in the following ways.
  68   //
  69   // - The number of distinct library names is small, e.g. a few dozen.  There
  70   //   is lots of repetition, especially of libxul.  So we intern them in their
  71   //   own table, which saves space over duplicating them for each cache entry.
  72   //
  73   // - The number of distinct function names is much higher, so we duplicate
  74   //   them in each cache entry.  That's more space-efficient than interning
  75   //   because entries containing single-occurrence function names are quickly
  76   //   overwritten, and their copies released.  In addition, empty function
  77   //   names are common, so we use nullptr to represent them compactly.
  78
  79   using AllocPolicy = detail::CodeAddressServiceAllocPolicy<AllocPolicy_>;
  80   using StringHashSet = HashSet<const char*, CStringHasher, AllocPolicy>;
  81
  82   StringHashSet mLibraryStrings;
  83
  84   struct Entry : private AllocPolicy {
  85     const void* mPc;
  86     char* mFunction;       // owned by the Entry;  may be null
  87     const char* mLibrary;  // owned by mLibraryStrings;  never null
  88                            //   in a non-empty entry is in use
  89     ptrdiff_t mLOffset;
  90     char* mFileName;  // owned by the Entry; may be null
  91     uint32_t mLineNo : 31;
  92     uint32_t mInUse : 1;  // is the entry used?
  93
  94     Entry()
  95         : mPc(0),
  96           mFunction(nullptr),
  97           mLibrary(nullptr),
  98           mLOffset(0),
  99           mFileName(nullptr),
 100           mLineNo(0),
 101           mInUse(0) {}
 102
 103     ~Entry() {
 104       // We don't free mLibrary because it's externally owned.
 105       AllocPolicy::free_(mFunction);
 106       AllocPolicy::free_(mFileName);
 107     }
 108
 109     void Replace(const void* aPc, const char* aFunction, const char* aLibrary,
 110                  ptrdiff_t aLOffset, const char* aFileName,
 111                  unsigned long aLineNo) {
 112       mPc = aPc;
 113
 114       // Convert "" to nullptr.  Otherwise, make a copy of the name.
 115       AllocPolicy::free_(mFunction);
 116       mFunction = !aFunction[0] ? nullptr : AllocPolicy::strdup_(aFunction);
 117       AllocPolicy::free_(mFileName);
 118       mFileName = !aFileName[0] ? nullptr : AllocPolicy::strdup_(aFileName);
 119
 120       mLibrary = aLibrary;
 121       mLOffset = aLOffset;
 122       mLineNo = aLineNo;
 123
 124       mInUse = 1;
 125     }
 126
 127     size_t SizeOfExcludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
 128       // Don't measure mLibrary because it's externally owned.
 129       size_t n = 0;
 130       n += aMallocSizeOf(mFunction);
 131       n += aMallocSizeOf(mFileName);
 132       return n;
 133     }
 134   };
 135
 136   const char* InternLibraryString(const char* aString) {
 137     auto p = mLibraryStrings.lookupForAdd(aString);
 138     if (p) {
 139       return *p;
 140     }
 141
 142     const char* newString = AllocPolicy::strdup_(aString);
 143     if (!mLibraryStrings.add(p, newString)) {
 144       MOZ_CRASH("CodeAddressService OOM");
 145     }
 146     return newString;
 147   }
 148
 149   Entry& GetEntry(const void* aPc) {
 150     MOZ_ASSERT(DescribeCodeAddressLock::IsLocked());
 151
 152     uint32_t index = HashGeneric(aPc) & kMask;
 153     MOZ_ASSERT(index < kNumEntries);
 154     Entry& entry = mEntries[index];
 155
 156     if (!entry.mInUse || entry.mPc != aPc) {
 157       mNumCacheMisses++;
 158
 159       // MozDescribeCodeAddress can (on Linux) acquire a lock inside
 160       // the shared library loader.  Another thread might call malloc
 161       // while holding that lock (when loading a shared library).  So
 162       // we have to exit the lock around this call.  For details, see
 163       // https://bugzilla.mozilla.org/show_bug.cgi?id=363334#c3
 164       MozCodeAddressDetails details;
 165       {
 166         DescribeCodeAddressLock::Unlock();
 167         (void)MozDescribeCodeAddress(const_cast<void*>(aPc), &details);
 168         DescribeCodeAddressLock::Lock();
 169       }
 170
 171       const char* library = InternLibraryString(details.library);
 172       entry.Replace(aPc, details.function, library, details.loffset,
 173                     details.filename, details.lineno);
 174
 175     } else {
 176       mNumCacheHits++;
 177     }
 178
 179     MOZ_ASSERT(entry.mPc == aPc);
 180
 181     return entry;
 182   }
 183
 184   // A direct-mapped cache.  When doing dmd::Analyze() just after starting
 185   // desktop Firefox (which is similar to analyzing after a longer-running
 186   // session, thanks to the limit on how many records we print), a cache with
 187   // 2^24 entries (which approximates an infinite-entry cache) has a ~91% hit
 188   // rate.  A cache with 2^12 entries has a ~83% hit rate, and takes up ~85 KiB
 189   // (on 32-bit platforms) or ~150 KiB (on 64-bit platforms).
 190   static const size_t kNumEntries = 1 << 12;
 191   static const size_t kMask = kNumEntries - 1;
 192   Entry mEntries[kNumEntries];
 193
 194   size_t mNumCacheHits;
 195   size_t mNumCacheMisses;
 196
 197  public:
 198   CodeAddressService()
 199       : mLibraryStrings(64), mEntries(), mNumCacheHits(0), mNumCacheMisses(0) {}
 200
 201   ~CodeAddressService() {
 202     for (auto iter = mLibraryStrings.iter(); !iter.done(); iter.next()) {
 203       AllocPolicy::free_(const_cast<char*>(iter.get()));
 204     }
 205   }
 206
 207   void GetLocation(uint32_t aFrameNumber, const void* aPc, char* aBuf,
 208                    size_t aBufLen) {
 209     Entry& entry = GetEntry(aPc);
 210     MozFormatCodeAddress(aBuf, aBufLen, aFrameNumber, entry.mPc,
 211                          entry.mFunction, entry.mLibrary, entry.mLOffset,
 212                          entry.mFileName, entry.mLineNo);
 213   }
 214
 215   size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const {
 216     size_t n = aMallocSizeOf(this);
 217     for (uint32_t i = 0; i < kNumEntries; i++) {
 218       n += mEntries[i].SizeOfExcludingThis(aMallocSizeOf);
 219     }
 220
 221     n += mLibraryStrings.shallowSizeOfExcludingThis(aMallocSizeOf);
 222     for (auto iter = mLibraryStrings.iter(); !iter.done(); iter.next()) {
 223       n += aMallocSizeOf(iter.get());
 224     }
 225
 226     return n;
 227   }
 228
 229   size_t CacheCapacity() const { return kNumEntries; }
 230
 231   size_t CacheCount() const {
 232     size_t n = 0;
 233     for (size_t i = 0; i < kNumEntries; i++) {
 234       if (mEntries[i].mInUse) {
 235         n++;
 236       }
 237     }
 238     return n;
 239   }
 240
 241   size_t NumCacheHits() const { return mNumCacheHits; }
 242   size_t NumCacheMisses() const { return mNumCacheMisses; }
 243 };
 244
 245 }  // namespace mozilla
 246
 247 #endif  // CodeAddressService_h__