Improve nsHtml5AtomTable performance (#693)

* cpp change
* h change

2 files changed, 20 insertions, 0 deletions

diff --git a/parser/html/nsHtml5AtomTable.cpp b/parser/html/nsHtml5AtomTable.cpp
index d523f58b01..d9c18485d1 100644
--- a/parser/html/nsHtml5AtomTable.cpp
+++ b/parser/html/nsHtml5AtomTable.cpp
@@ -31,7 +31,11 @@ nsHtml5AtomTable::nsHtml5AtomTable()
 }
 
 nsHtml5AtomTable::~nsHtml5AtomTable()
+  : mRecentlyUsedParserAtoms{}
 {
+#ifdef DEBUG
+  NS_GetMainThread(getter_AddRefs(mPermittedLookupThread));
+#endif
 }
 
 nsIAtom*
@@ -44,13 +48,23 @@ nsHtml5AtomTable::GetAtom(const nsAString& aKey)
     NS_ASSERTION(mPermittedLookupThread == currentThread, "Wrong thread!");
   }
 #endif
+
+  uint32_t index = mozilla::HashString(aKey) % RECENTLY_USED_PARSER_ATOMS_SIZE;
+  nsIAtom* cachedAtom = mRecentlyUsedParserAtoms[index];
+  if (cachedAtom && cachedAtom->Equals(aKey)) {
+    return cachedAtom;
+  }
+
   nsIAtom* atom = NS_GetStaticAtom(aKey);
   if (atom) {
+    mRecentlyUsedParserAtoms[index] = atom;
     return atom;
   }
   nsHtml5AtomEntry* entry = mTable.PutEntry(aKey);
   if (!entry) {
     return nullptr;
   }
+
+  mRecentlyUsedParserAtoms[index] = entry->GetAtom();
   return entry->GetAtom();
 }
diff --git a/parser/html/nsHtml5AtomTable.h b/parser/html/nsHtml5AtomTable.h
index 43f9b5f2f2..b0dc2f678e 100644
--- a/parser/html/nsHtml5AtomTable.h
+++ b/parser/html/nsHtml5AtomTable.h
@@ -11,6 +11,8 @@
 #include "nsIAtom.h"
 #include "nsIThread.h"
 
+#define RECENTLY_USED_PARSER_ATOMS_SIZE 31
+
 class nsHtml5Atom;
 
 class nsHtml5AtomEntry : public nsStringHashKey
@@ -87,6 +89,9 @@ class nsHtml5AtomTable
      */
     void Clear()
     {
+      for (uint32_t i = 0; i < RECENTLY_USED_PARSER_ATOMS_SIZE; ++i) {
+        mRecentlyUsedParserAtoms[i] = nullptr;
+      }
       mTable.Clear();
     }
     
@@ -99,6 +104,7 @@ class nsHtml5AtomTable
   
   private:
     nsTHashtable<nsHtml5AtomEntry> mTable;
+    nsIAtom* mRecentlyUsedParserAtoms[RECENTLY_USED_PARSER_ATOMS_SIZE];
 #ifdef DEBUG
     nsCOMPtr<nsIThread>            mPermittedLookupThread;
 #endif