Implement StringImpl sharing for StringView::toString().

third_party/WebKit/Source/wtf/text/StringBuilder.h

Index: third_party/WebKit/Source/wtf/text/StringBuilder.h
diff --git a/third_party/WebKit/Source/wtf/text/StringBuilder.h b/third_party/WebKit/Source/wtf/text/StringBuilder.h
index 4166a553d70c4fd78051c4a7891504488061606e..c967fa6082696ec2c0f232172ddd0837ee13a327 100644
--- a/third_party/WebKit/Source/wtf/text/StringBuilder.h
+++ b/third_party/WebKit/Source/wtf/text/StringBuilder.h
@@ -70,28 +70,42 @@ public:
             append(other.characters16(), other.m_length);
     }
 
-    // TODO(esprehn): This method is just duplicating what StringView itself
-    // does. Remove it and replace callers with append(StringView(string, offset, length)).
+    // NOTE: The semantics of this are different than StringView(..., offset, length)
+    // in that an invalid offset or invalid length is a no-op instead of an
+    // error.
+    // TODO(esprehn): We should probably unify the semantics instead.
     void append(const StringView& string, unsigned offset, unsigned length)
     {
-        if (!string.length())
-            return;
-
         unsigned extent = offset + length;
         if (extent < offset || extent > string.length())
             return;
 
-        if (string.is8Bit())
-            append(string.characters8() + offset, length);
-        else
-            append(string.characters16() + offset, length);
+        // We can't do this before the above check since StringView's constructor
+        // doesn't accept invalid offsets or lengths.
+        append(StringView(string, offset, length));
     }
 
     void append(const StringView& string)
     {
-        if (!string.length())
+        if (string.isEmpty())
             return;
 
+        // If we're appending to an empty builder, and there is not a buffer
+        // (reserveCapacity has not been called), then share the impl if
+        // possible.
+        //
+        // This is important to avoid string copies inside dom operations like
+        // Node::textContent when there's only a single Text node child, or
+        // inside the parser in the common case when flushing buffered text to
+        // a Text node.
+        StringImpl* impl = string.sharedImpl();
+        if (!m_length && !m_buffer && impl) {
+            m_string = impl;

[haraken, 2016/06/06 02:28:52]

Don't we need to set m_buffer to impl?

[esprehn, 2016/06/06 04:42:21]

Nope, m_buffer is for when we have an overallocated string created with the
allocate uninitialized method. Since the string we're adding here is coming from
outside the builder its immutable and we can't set m_buffer, and it's also
correctly sized already, so there's no extra room.

+            m_length = impl->length();
+            m_is8Bit = impl->is8Bit();
+            return;
+        }
+
         if (string.is8Bit())
             append(string.characters8(), string.length());
         else