Commit aa19ca17 authored by Nicholas Nethercote's avatar Nicholas Nethercote
Browse files

Bug 1442433 - Remove the refcount from static atoms. r=froydnj 

The refcount is only used for dynamic atoms.

On 64-bit, this reduces sizeof(nsStaticAtom) from 24 bytes to 16 bytes, and the
on-heap size from 32 bytes to 16 bytes. This saves 42 KiB per process.

On 32-bit, this reduces sizeof(nsStaticAtom) from 16 bytes to 12 bytes, but the
on-heap size stays at 16 bytes, so memory usage is unchanged.

MozReview-Commit-ID: 7d9H7MRHN9a

--HG--
extra : rebase_source : d3eafb3aaf44a592afd6c89cd0519d043056e65a
parent 1306e251

xpcom/ds/nsAtom.h

+4 −4
Original line number Diff line number Diff line
@@ -88,15 +88,15 @@ private: 
  friend class nsAtomSubTable;

  friend class nsHtml5AtomEntry;



  // Dynamic atom construction is done by |friend|s.

protected:

  // Used by nsDynamicAtom and directly (by nsHtml5AtomEntry) for HTML5 atoms.

  nsAtom(AtomKind aKind, const nsAString& aString, uint32_t aHash);



protected:

  // Used by nsStaticAtom.

  nsAtom(const char16_t* aString, uint32_t aLength, uint32_t aHash);



  ~nsAtom();



  mozilla::ThreadSafeAutoRefCnt mRefCnt;

  uint32_t mLength: 30;

  uint32_t mKind: 2; // nsAtom::AtomKind

  uint32_t mHash;
@@ -118,7 +118,7 @@ class nsStaticAtom : public nsAtom 
{

public:

  // These are deleted so it's impossible to RefPtr<nsStaticAtom>. Raw

  // nsStaticAtom atoms should be used instead.

  // nsStaticAtom pointers should be used instead.

  MozExternalRefCountType AddRef() = delete;

  MozExternalRefCountType Release() = delete;

xpcom/ds/nsAtomTable.cpp

+55 −26
Original line number Diff line number Diff line
@@ -64,10 +64,37 @@ enum class GCKind { 
// See nsAtom::AddRef() and nsAtom::Release().

static Atomic<int32_t, ReleaseAcquire> gUnusedAtomCount(0);



// Dynamic atoms need a ref count; this class adds that to nsAtom.

class nsDynamicAtom : public nsAtom

{

public:

  // We can't use NS_INLINE_DECL_THREADSAFE_REFCOUNTING because the refcounting

  // of this type is special.

  MozExternalRefCountType AddRef();

  MozExternalRefCountType Release();



  static nsDynamicAtom* As(nsAtom* aAtom)

  {

    MOZ_ASSERT(aAtom->IsDynamicAtom());

    return static_cast<nsDynamicAtom*>(aAtom);

  }



private:

  friend class nsAtomTable;

  friend class nsAtomSubTable;



  // Construction is done by |friend|s.

  nsDynamicAtom(const nsAString& aString, uint32_t aHash)

    : nsAtom(AtomKind::DynamicAtom, aString, aHash)

    , mRefCnt(1)

  {}



  mozilla::ThreadSafeAutoRefCnt mRefCnt;

};



// This constructor is for dynamic atoms and HTML5 atoms.

nsAtom::nsAtom(AtomKind aKind, const nsAString& aString, uint32_t aHash)

  : mRefCnt(1)

  , mLength(aString.Length())

  : mLength(aString.Length())

  , mKind(static_cast<uint32_t>(aKind))

  , mHash(aHash)

{
@@ -88,7 +115,7 @@ nsAtom::nsAtom(AtomKind aKind, const nsAString& aString, uint32_t aHash) 
    mString[mLength] = char16_t(0);

  }



  MOZ_ASSERT_IF(IsDynamicAtom(), mHash == HashString(mString, mLength));

  MOZ_ASSERT_IF(!IsHTML5Atom(), mHash == HashString(mString, mLength));



  MOZ_ASSERT(mString[mLength] == char16_t(0), "null terminated");

  MOZ_ASSERT(buf && buf->StorageSize() >= (mLength + 1) * sizeof(char16_t),
@@ -474,7 +501,8 @@ nsAtomSubTable::GCLocked(GCKind aKind) 
    }



    nsAtom* atom = entry->mAtom;

    if (atom->mRefCnt == 0) {

    MOZ_ASSERT(!atom->IsHTML5Atom());

    if (atom->IsDynamicAtom() && nsDynamicAtom::As(atom)->mRefCnt == 0) {

      i.Remove();

      delete atom;

      ++removedCount;
@@ -516,15 +544,9 @@ GCAtomTable() 
  }

}



MozExternalRefCountType

nsAtom::AddRef()

MOZ_ALWAYS_INLINE MozExternalRefCountType

nsDynamicAtom::AddRef()

{

  MOZ_ASSERT(!IsHTML5Atom(), "Attempt to AddRef an HTML5 atom");

  if (!IsDynamicAtom()) {

    MOZ_ASSERT(IsStaticAtom());

    return 2;

  }



  MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");

  nsrefcnt count = ++mRefCnt;

  if (count == 1) {
@@ -533,15 +555,9 @@ nsAtom::AddRef() 
  return count;

}



MozExternalRefCountType

nsAtom::Release()

MOZ_ALWAYS_INLINE MozExternalRefCountType

nsDynamicAtom::Release()

{

  MOZ_ASSERT(!IsHTML5Atom(), "Attempt to Release an HTML5 atom");

  if (!IsDynamicAtom()) {

    MOZ_ASSERT(IsStaticAtom());

    return 1;

  }



  #ifdef DEBUG

  // We set a lower GC threshold for atoms in debug builds so that we exercise

  // the GC machinery more often.
@@ -561,6 +577,22 @@ nsAtom::Release() 
  return count;

}



MozExternalRefCountType

nsAtom::AddRef()

{

  MOZ_ASSERT(!IsHTML5Atom(), "Attempt to AddRef an HTML5 atom");



  return IsStaticAtom() ? 2 : nsDynamicAtom::As(this)->AddRef();

}



MozExternalRefCountType

nsAtom::Release()

{

  MOZ_ASSERT(!IsHTML5Atom(), "Attempt to Release an HTML5 atom");



  return IsStaticAtom() ? 1 : nsDynamicAtom::As(this)->Release();

}



//----------------------------------------------------------------------



// Have the static atoms been inserted into the table?
@@ -706,8 +738,7 @@ nsAtomTable::Atomize(const nsACString& aUTF8String) 
  // Actually, now there is, sort of: ForgetSharedBuffer.

  nsString str;

  CopyUTF8toUTF16(aUTF8String, str);

  RefPtr<nsAtom> atom =

    dont_AddRef(new nsAtom(nsAtom::AtomKind::DynamicAtom, str, hash));

  RefPtr<nsAtom> atom = dont_AddRef(new nsDynamicAtom(str, hash));



  he->mAtom = atom;
@@ -743,8 +774,7 @@ nsAtomTable::Atomize(const nsAString& aUTF16String) 
    return atom.forget();

  }



  RefPtr<nsAtom> atom =

    dont_AddRef(new nsAtom(nsAtom::AtomKind::DynamicAtom, aUTF16String, hash));

  RefPtr<nsAtom> atom = dont_AddRef(new nsDynamicAtom(aUTF16String, hash));

  he->mAtom = atom;



  return atom.forget();
@@ -783,8 +813,7 @@ nsAtomTable::AtomizeMainThread(const nsAString& aUTF16String) 
  if (he->mAtom) {

    retVal = he->mAtom;

  } else {

    RefPtr<nsAtom> newAtom = dont_AddRef(

      new nsAtom(nsAtom::AtomKind::DynamicAtom, aUTF16String, hash));

    RefPtr<nsAtom> newAtom = dont_AddRef(new nsDynamicAtom(aUTF16String, hash));

    he->mAtom = newAtom;

    retVal = newAtom.forget();

  }