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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/194510">194510</a></dd>
<dt>Author</dt> <dd>akling@apple.com</dd>
<dt>Date</dt> <dd>2016-01-03 08:12:22 -0800 (Sun, 03 Jan 2016)</dd>
</dl>
<h3>Log Message</h3>
<pre>StringBuilder often creates two StringImpls.
<https://webkit.org/b/152662>
Reviewed by Anders Carlsson.
If StringBuilder didn't manage to guess the exact final length of the
string being built, it would still keep the buffer around, and the final
build process would then return a substring into that buffer.
This effectively yielded two StringImpls instead of one, with the substring
retaining the longer buffer StringImpl.
This patch improves the situation by having StringImpl crop the buffer's
m_length field to the final built string length. That way we never have to
return a substring.
* wtf/text/StringBuilder.cpp:
(WTF::StringBuilder::reifyString):
* wtf/text/StringImpl.h:</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceWTFChangeLog">trunk/Source/WTF/ChangeLog</a></li>
<li><a href="#trunkSourceWTFwtftextStringBuildercpp">trunk/Source/WTF/wtf/text/StringBuilder.cpp</a></li>
<li><a href="#trunkSourceWTFwtftextStringImplh">trunk/Source/WTF/wtf/text/StringImpl.h</a></li>
</ul>
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<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceWTFChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/WTF/ChangeLog (194509 => 194510)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WTF/ChangeLog 2016-01-03 09:48:58 UTC (rev 194509)
+++ trunk/Source/WTF/ChangeLog 2016-01-03 16:12:22 UTC (rev 194510)
</span><span class="lines">@@ -1,5 +1,27 @@
</span><span class="cx"> 2016-01-03 Andreas Kling <akling@apple.com>
</span><span class="cx">
</span><ins>+ StringBuilder often creates two StringImpls.
+ <https://webkit.org/b/152662>
+
+ Reviewed by Anders Carlsson.
+
+ If StringBuilder didn't manage to guess the exact final length of the
+ string being built, it would still keep the buffer around, and the final
+ build process would then return a substring into that buffer.
+
+ This effectively yielded two StringImpls instead of one, with the substring
+ retaining the longer buffer StringImpl.
+
+ This patch improves the situation by having StringImpl crop the buffer's
+ m_length field to the final built string length. That way we never have to
+ return a substring.
+
+ * wtf/text/StringBuilder.cpp:
+ (WTF::StringBuilder::reifyString):
+ * wtf/text/StringImpl.h:
+
+2016-01-03 Andreas Kling <akling@apple.com>
+
</ins><span class="cx"> Remove redundant StringImpl substring creation function.
</span><span class="cx"> <https://webkit.org/b/152652>
</span><span class="cx">
</span></span></pre></div>
<a id="trunkSourceWTFwtftextStringBuildercpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/WTF/wtf/text/StringBuilder.cpp (194509 => 194510)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WTF/wtf/text/StringBuilder.cpp 2016-01-03 09:48:58 UTC (rev 194509)
+++ trunk/Source/WTF/wtf/text/StringBuilder.cpp 2016-01-03 16:12:22 UTC (rev 194510)
</span><span class="lines">@@ -54,12 +54,14 @@
</span><span class="cx"> return;
</span><span class="cx"> }
</span><span class="cx">
</span><del>- // Must be valid in the buffer, take a substring (unless string fills the buffer).
- ASSERT(m_buffer && m_length <= m_buffer->length());
- if (m_length == m_buffer->length())
- m_string = m_buffer.get();
- else
- m_string = StringImpl::createSubstringSharingImpl(m_buffer, 0, m_length);
</del><ins>+ ASSERT(m_buffer);
+ ASSERT(m_length <= m_buffer->length());
+
+ // "Crop" the StringImpl by updating its m_length field. Note that this will
+ // result in some StringImpl objects being backed by a slightly larger allocation
+ // than their m_length field indicates.
+ m_buffer->m_length = m_length;
+ m_string = m_buffer.get();
</ins><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> void StringBuilder::resize(unsigned newSize)
</span></span></pre></div>
<a id="trunkSourceWTFwtftextStringImplh"></a>
<div class="modfile"><h4>Modified: trunk/Source/WTF/wtf/text/StringImpl.h (194509 => 194510)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WTF/wtf/text/StringImpl.h 2016-01-03 09:48:58 UTC (rev 194509)
+++ trunk/Source/WTF/wtf/text/StringImpl.h 2016-01-03 16:12:22 UTC (rev 194510)
</span><span class="lines">@@ -135,6 +135,7 @@
</span><span class="cx"> friend struct WTF::UCharBufferTranslator;
</span><span class="cx"> friend class JSC::LLInt::Data;
</span><span class="cx"> friend class JSC::LLIntOffsetsExtractor;
</span><ins>+ friend class WTF::StringBuilder;
</ins><span class="cx">
</span><span class="cx"> private:
</span><span class="cx"> enum BufferOwnership {
</span></span></pre>
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