Bug 1460674 - part 0a - store the hash table entry size in iterators; r=njn

This change is satisfying insofar as it removes a load from every
iteration step over the hashtable, but it doesn't seem to affect
our collection benchmarks in any way.  The change does not make
iterators any larger, as there is padding at the end of the iterator
structure on both 32- and 64-bit platforms.

Author: froydnj

xpcom/ds/PLDHashTable.cpp

@@ -728,6 +728,7 @@ PLDHashTable::Iterator::Iterator(Iterator&& aOther)
   , mNexts(aOther.mNexts)
   , mNextsLimit(aOther.mNextsLimit)
   , mHaveRemoved(aOther.mHaveRemoved)
+  , mEntrySize(aOther.mEntrySize)
 {
   // No need to change |mChecker| here.
   aOther.mTable = nullptr;
@@ -736,6 +737,7 @@ PLDHashTable::Iterator::Iterator(Iterator&& aOther)
   aOther.mNexts = 0;
   aOther.mNextsLimit = 0;
   aOther.mHaveRemoved = false;
+  aOther.mEntrySize = 0;
 }
 
 PLDHashTable::Iterator::Iterator(PLDHashTable* aTable)
@@ -745,6 +747,7 @@ PLDHashTable::Iterator::Iterator(PLDHashTable* aTable)
   , mNexts(0)
   , mNextsLimit(mTable->EntryCount())
   , mHaveRemoved(false)
+  , mEntrySize(aTable->mEntrySize)
 {
 #ifdef DEBUG
   mTable->mChecker.StartReadOp();
@@ -788,7 +791,7 @@ PLDHashTable::Iterator::IsOnNonLiveEntry() const
 MOZ_ALWAYS_INLINE void
 PLDHashTable::Iterator::MoveToNextEntry()
 {
-  mCurrent += mTable->mEntrySize;
+  mCurrent += mEntrySize;
   if (mCurrent == mLimit) {
     mCurrent = mTable->mEntryStore.Get();  // Wrap-around. Possible due to Chaos Mode.
   }

xpcom/ds/PLDHashTable.h

@@ -491,6 +491,7 @@ class PLDHashTable
     uint32_t mNextsLimit;             // Next() call limit.
 
     bool mHaveRemoved;                // Have any elements been removed?
+    uint8_t mEntrySize;               // Size of entries.
 
     bool IsOnNonLiveEntry() const;
     void MoveToNextEntry();