Remove SegmentedString::m_pushedChar{1,2} optimization

Source/platform/text/SegmentedString.h

Index: Source/platform/text/SegmentedString.h
diff --git a/Source/platform/text/SegmentedString.h b/Source/platform/text/SegmentedString.h
index c08742a569b058c081d66e6366fbe5813c0d9c31..bdfdf2205d54476a6fc363ade0ecb073876dce35 100644
--- a/Source/platform/text/SegmentedString.h
+++ b/Source/platform/text/SegmentedString.h
@@ -81,6 +81,29 @@ public:
         }
     }
 
+    void push(UChar c)
+    {
+        if (m_is8Bit) {
+            if (m_data.string8Ptr == m_string.characters8()) {
+                m_string.insert(&c, 1, 0);

[Yoav Weiss, 2015/08/27 13:19:03]

AFAICT this goes back to String::append which looks pretty awful in terms of
performance :/
Can we somehow use a StringBuilder for that?

[kouhei (in TOK), 2015/08/28 05:38:47]

I replaced this codepath with prepend() call in SegmentedString::push(). Not
optimal, but better than insert() call.

SegmentedString::push() is only used for undo-ing advance(), so chances are that
--m_data.string{8,16}Ptr path is taken almost all of the time. I have never seen
this codepath being taken except for some unittests/LayoutTests.

+                m_data.string8Ptr = m_string.characters8();
+                m_length = m_string.length();
+            } else {
+                --m_data.string8Ptr;
+                ASSERT(*m_data.string8Ptr == c);
+            }
+        } else {
+            if (m_data.string16Ptr == m_string.characters16()) {
+                m_string.insert(&c, 1, 0);
+                m_data.string16Ptr = m_string.characters16();
+                m_length = m_string.length();
+            } else {
+                --m_data.string16Ptr;
+                ASSERT(*m_data.string16Ptr == static_cast<LChar>(c));
+            }
+        }
+    }
+
     UChar getCurrentChar8()
     {
         return *m_data.string8Ptr;
@@ -148,9 +171,7 @@ private:
 class PLATFORM_EXPORT SegmentedString {
 public:
     SegmentedString()
-        : m_pushedChar1(0)
-        , m_pushedChar2(0)
-        , m_currentChar(0)
+        : m_currentChar(0)
         , m_numberOfCharactersConsumedPriorToCurrentString(0)
         , m_numberOfCharactersConsumedPriorToCurrentLine(0)
         , m_currentLine(0)
@@ -163,9 +184,7 @@ public:
     }
 
     SegmentedString(const String& str)
-        : m_pushedChar1(0)
-        , m_pushedChar2(0)
-        , m_currentString(str)
+        : m_currentString(str)
         , m_currentChar(0)
         , m_numberOfCharactersConsumedPriorToCurrentString(0)
         , m_numberOfCharactersConsumedPriorToCurrentLine(0)
@@ -188,19 +207,7 @@ public:
     bool excludeLineNumbers() const { return m_currentString.excludeLineNumbers(); }
     void setExcludeLineNumbers();
 
-    void push(UChar c)
-    {
-        ASSERT(c);
-
-        if (!m_pushedChar1) {
-            m_currentChar = m_pushedChar1 = c;
-            updateSlowCaseFunctionPointers();
-        } else {
-            ASSERT(!m_pushedChar2);
-            m_pushedChar2 = m_pushedChar1;
-            m_currentChar = m_pushedChar1 = c;
-        }
-    }
+    void push(UChar);
 
     bool isEmpty() const { return m_empty; }
     unsigned length() const;
@@ -219,7 +226,6 @@ public:
     void advance()
     {
         if (m_fastPathFlags & Use8BitAdvance) {
-            ASSERT(!m_pushedChar1);
             m_currentChar = m_currentString.incrementAndGetCurrentChar8();
             decrementAndCheckLength();
             return;
@@ -231,8 +237,6 @@ public:
     inline void advanceAndUpdateLineNumber()
     {
         if (m_fastPathFlags & Use8BitAdvance) {
-            ASSERT(!m_pushedChar1);
-
             bool haveNewLine = (m_currentChar == '\n') & !!(m_fastPathFlags & Use8BitAdvanceAndUpdateLineNumbers);
             m_currentChar = m_currentString.incrementAndGetCurrentChar8();
             decrementAndCheckLength();
@@ -269,7 +273,7 @@ public:
     void advancePastNewlineAndUpdateLineNumber()
     {
         ASSERT(currentChar() == '\n');
-        if (!m_pushedChar1 && m_currentString.length() > 1) {
+        if (m_currentString.length() > 1) {
             int newLineFlag = m_currentString.doNotExcludeLineNumbers();
             m_currentLine += newLineFlag;
             if (newLineFlag)
@@ -285,16 +289,11 @@ public:
     // have space for at least |count| characters.
     void advance(unsigned count, UChar* consumedCharacters);
 
-    bool escaped() const { return m_pushedChar1; }
+    bool escaped() const { return false; }
 
     int numberOfCharactersConsumed() const
     {
         int numberOfPushedCharacters = 0;
-        if (m_pushedChar1) {
-            ++numberOfPushedCharacters;
-            if (m_pushedChar2)
-                ++numberOfPushedCharacters;
-        }
         return m_numberOfCharactersConsumedPriorToCurrentString + m_currentString.numberOfCharactersConsumed() - numberOfPushedCharacters;
     }
 
@@ -340,7 +339,7 @@ private:
 
     void updateAdvanceFunctionPointers()
     {
-        if ((m_currentString.length() > 1) && !m_pushedChar1) {
+        if (m_currentString.length() > 1) {
             if (m_currentString.is8Bit()) {
                 m_advanceFunc = &SegmentedString::advance8;
                 m_fastPathFlags = Use8BitAdvance;
@@ -373,7 +372,7 @@ private:
 
     inline LookAheadResult lookAheadInline(const String& string, TextCaseSensitivity caseSensitivity)
     {
-        if (!m_pushedChar1 && string.length() <= static_cast<unsigned>(m_currentString.length())) {
+        if (string.length() <= static_cast<unsigned>(m_currentString.length())) {
             String currentSubstring = m_currentString.currentSubString(string.length());
             if (currentSubstring.startsWith(string, caseSensitivity))
                 return DidMatch;
@@ -399,8 +398,6 @@ private:
 
     bool isComposite() const { return !m_substrings.isEmpty(); }
 
-    UChar m_pushedChar1;
-    UChar m_pushedChar2;
     SegmentedSubstring m_currentString;
     UChar m_currentChar;
     int m_numberOfCharactersConsumedPriorToCurrentString;