Bug 1352888 - Don't set the collision flag when adding to PLDHashTable if we've already found the entry we're going to add. r=njn

PLDHashTable's entry store has two types of unoccupied entries:  free
entries and removed entries.  The search of a chain of entries
(determined by the hash value) in the entry store to search for an entry
can stop at free entries, but it continues across removed entries,
because removed entries are entries that may have been skipped over when
we were adding the value we're searching for to the hash, but have since
been removed.  For live entries, we also maintain this distinction by
using one bit of storage for a collision flag, which notes that if the
hashtable entry is removed, its place in the entry store must become a
removed entry rather than a free entry.

When we add a new entry to the table, Add's semantics require that we
return an existing entry if there is one, and only create a new entry if
no existing entry exists.  (Bug 1352198 suggests the possibility of a
faster alternative Add API where the caller guarantees that the key is
not already in the hashtable.)  When we search for the existing entry,
we must thus continue the search across removed entries, even though we
record the first removed entry found to return if the search for an
existing entry fails.

The existing code adds the collision flag through the entire table
search during an Add.  This patch changes that behavior so that we only
add the collision flag prior to finding the first removed entry.  Adding
it after we find the first removed entry is unnecessary, since we are
not making that entry part of a path to a new entry.  If it is part of a
path to an existing entry, it will already have the collision flag set.

This patch effectively puts an if (!firstRemoved) around the else branch
of the if (MOZ_UNLIKELY(EntryIsRemoved(entry))), and then refactors that
condition outwards since it is now around the contents of both the if
and else branches.

MozReview-Commit-ID: CsXnMYttHVy

--HG--
extra : transplant_source : W4%B8%BA%D5p%102%1B%8D%83%23%E0s%B3%B0f%0D%05%AE

xpcom/ds/PLDHashTable.cpp

@@ -366,11 +366,9 @@ PLDHashTable::SearchTable(const void* aKey, PLDHashNumber aKeyHash)
   PLDHashEntryHdr* firstRemoved = nullptr;
 
   for (;;) {
-    if (Reason == ForAdd) {
+    if (Reason == ForAdd && !firstRemoved) {
       if (MOZ_UNLIKELY(EntryIsRemoved(entry))) {
-        if (!firstRemoved) {
-          firstRemoved = entry;
-        }
+        firstRemoved = entry;
       } else {
         entry->mKeyHash |= kCollisionFlag;
       }