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<dt>Revision</dt> <dd><a href="http://trac.webkit.org/projects/webkit/changeset/168606">168606</a></dd>
<dt>Author</dt> <dd>commit-queue@webkit.org</dd>
<dt>Date</dt> <dd>2014-05-11 21:23:12 -0700 (Sun, 11 May 2014)</dd>
</dl>
<h3>Log Message</h3>
<pre>CSS JIT: reduce cost of computing backtracking height
https://bugs.webkit.org/show_bug.cgi?id=132546
Patch by Yusuke Suzuki <utatane.tea@gmail.com> on 2014-05-11
Reviewed by Benjamin Poulain.
Because compiler previously compute backtracking height for
previous child fragment, by leveraging this, we can limit the
`maxPrefixSize` for `computeBacktrackingHeightFromDescendant`.
For example, consider selector "c>a>b>d>a>b e"'s descendant chain,
"c>a>b>d>a>b".
At the <a> position, we have matching pattern [b, a, d, b, a] and
calculate the backtracking height by following method.
pattern: [b, a, d, b, a]
candidate0: [b, a, d, b] => Not matched.
candidate1: [b, a, d] => Not matched.
candidate2: [b, a] => Matched against the pattern.
At this time, first candidate0's pattern size is `pattern.size() - 1`.
And get backtracking height from descendant 3, that is
`pattern.size() - candidate.size()`, `5 - 2`.
And next, at the <c> position, we calcucate the backtracking height
for this pattern.
pattern: [b, a, d, b, a, c]
candidate0: [b, a, d, b, a] => Not matched.
candidate1: [b, a, d, b] => Not matched.
candidate2: [b, a, d] => Not matched.
candidate3: [b, a] => Not matched.
candidate4: [b] => Not matched.
candidate5: [] => Matched against the pattern.
Then, we get the backtracking height, which is 6 (6 - 0).
However, in the above case, we already know that attempts from candidate0
to candidate1 always fail, since parts of these are already tested at
the <b> position trial and we know they don't match.
So in this case, we should start this computation from candidate2,
such as,
pattern: [b, a, d, b, a, c]
candidate2: [b, a, d] => Not matched.
candidate3: [b, a] => Not matched.
candidate4: [b] => Not matched.
candidate5: [] => Matched against the pattern.
We can start computation with candidate size
`pattern.size() - previousChildFragmentBacktrackingHeight`.
In this example, `pattern.size()` is 6 and
`previousChildFragmentBacktrackingHeight` is 3, so candidate size is
3, that is candidate2.
* cssjit/SelectorCompiler.cpp:
(WebCore::SelectorCompiler::computeBacktrackingStartHeightFromDescendant):
(WebCore::SelectorCompiler::computeBacktrackingHeightFromDescendant):</pre>
<h3>Modified Paths</h3>
<ul>
<li><a href="#trunkSourceWebCoreChangeLog">trunk/Source/WebCore/ChangeLog</a></li>
<li><a href="#trunkSourceWebCorecssjitSelectorCompilercpp">trunk/Source/WebCore/cssjit/SelectorCompiler.cpp</a></li>
</ul>
</div>
<div id="patch">
<h3>Diff</h3>
<a id="trunkSourceWebCoreChangeLog"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/ChangeLog (168605 => 168606)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/ChangeLog 2014-05-12 00:08:02 UTC (rev 168605)
+++ trunk/Source/WebCore/ChangeLog 2014-05-12 04:23:12 UTC (rev 168606)
</span><span class="lines">@@ -1,3 +1,64 @@
</span><ins>+2014-05-11 Yusuke Suzuki <utatane.tea@gmail.com>
+
+ CSS JIT: reduce cost of computing backtracking height
+ https://bugs.webkit.org/show_bug.cgi?id=132546
+
+ Reviewed by Benjamin Poulain.
+
+ Because compiler previously compute backtracking height for
+ previous child fragment, by leveraging this, we can limit the
+ `maxPrefixSize` for `computeBacktrackingHeightFromDescendant`.
+
+ For example, consider selector "c>a>b>d>a>b e"'s descendant chain,
+ "c>a>b>d>a>b".
+
+ At the <a> position, we have matching pattern [b, a, d, b, a] and
+ calculate the backtracking height by following method.
+
+ pattern: [b, a, d, b, a]
+ candidate0: [b, a, d, b] => Not matched.
+ candidate1: [b, a, d] => Not matched.
+ candidate2: [b, a] => Matched against the pattern.
+
+ At this time, first candidate0's pattern size is `pattern.size() - 1`.
+ And get backtracking height from descendant 3, that is
+ `pattern.size() - candidate.size()`, `5 - 2`.
+
+ And next, at the <c> position, we calcucate the backtracking height
+ for this pattern.
+
+ pattern: [b, a, d, b, a, c]
+ candidate0: [b, a, d, b, a] => Not matched.
+ candidate1: [b, a, d, b] => Not matched.
+ candidate2: [b, a, d] => Not matched.
+ candidate3: [b, a] => Not matched.
+ candidate4: [b] => Not matched.
+ candidate5: [] => Matched against the pattern.
+
+ Then, we get the backtracking height, which is 6 (6 - 0).
+ However, in the above case, we already know that attempts from candidate0
+ to candidate1 always fail, since parts of these are already tested at
+ the <b> position trial and we know they don't match.
+
+ So in this case, we should start this computation from candidate2,
+ such as,
+
+ pattern: [b, a, d, b, a, c]
+ candidate2: [b, a, d] => Not matched.
+ candidate3: [b, a] => Not matched.
+ candidate4: [b] => Not matched.
+ candidate5: [] => Matched against the pattern.
+
+ We can start computation with candidate size
+ `pattern.size() - previousChildFragmentBacktrackingHeight`.
+ In this example, `pattern.size()` is 6 and
+ `previousChildFragmentBacktrackingHeight` is 3, so candidate size is
+ 3, that is candidate2.
+
+ * cssjit/SelectorCompiler.cpp:
+ (WebCore::SelectorCompiler::computeBacktrackingStartHeightFromDescendant):
+ (WebCore::SelectorCompiler::computeBacktrackingHeightFromDescendant):
+
</ins><span class="cx"> 2014-05-11 Beth Dakin <bdakin@apple.com>
</span><span class="cx">
</span><span class="cx"> Headers and footers are not positioned correctly with topContentInset
</span></span></pre></div>
<a id="trunkSourceWebCorecssjitSelectorCompilercpp"></a>
<div class="modfile"><h4>Modified: trunk/Source/WebCore/cssjit/SelectorCompiler.cpp (168605 => 168606)</h4>
<pre class="diff"><span>
<span class="info">--- trunk/Source/WebCore/cssjit/SelectorCompiler.cpp 2014-05-12 00:08:02 UTC (rev 168605)
+++ trunk/Source/WebCore/cssjit/SelectorCompiler.cpp 2014-05-12 04:23:12 UTC (rev 168606)
</span><span class="lines">@@ -711,12 +711,12 @@
</span><span class="cx">
</span><span class="cx"> // Find the largest matching prefix from already known tagNames.
</span><span class="cx"> // And by using this, compute an appropriate height of backtracking start element from the closest descendant.
</span><del>-static inline unsigned computeBacktrackingStartHeightFromDescendant(const TagNameList& tagNames)
</del><ins>+static inline unsigned computeBacktrackingStartHeightFromDescendant(const TagNameList& tagNames, unsigned maxPrefixSize)
</ins><span class="cx"> {
</span><span class="cx"> RELEASE_ASSERT(!tagNames.isEmpty());
</span><ins>+ RELEASE_ASSERT(maxPrefixSize < tagNames.size());
</ins><span class="cx">
</span><del>- unsigned largestPrefixSize = tagNames.size();
- while (--largestPrefixSize) {
</del><ins>+ for (unsigned largestPrefixSize = maxPrefixSize; largestPrefixSize > 0; --largestPrefixSize) {
</ins><span class="cx"> unsigned offsetToLargestPrefix = tagNames.size() - largestPrefixSize;
</span><span class="cx"> bool matched = true;
</span><span class="cx"> // Since TagNamePatterns are pushed to a tagNames, check tagNames with reverse order.
</span><span class="lines">@@ -752,15 +752,21 @@
</span><span class="cx"> pattern.tagName = fragment.tagName;
</span><span class="cx"> tagNames.append(pattern);
</span><span class="cx">
</span><ins>+ unsigned maxPrefixSize = tagNames.size() - 1;
+ if (previousChildFragmentInDescendantBacktrackingChain) {
+ RELEASE_ASSERT(tagNames.size() >= previousChildFragmentInDescendantBacktrackingChain->tagNameMatchedBacktrackingStartHeightFromDescendant);
+ maxPrefixSize = tagNames.size() - previousChildFragmentInDescendantBacktrackingChain->tagNameMatchedBacktrackingStartHeightFromDescendant;
+ }
+
</ins><span class="cx"> if (pattern.tagName) {
</span><span class="cx"> // Compute height from descendant in the case that tagName is not matched.
</span><span class="cx"> tagNames.last().inverted = true;
</span><del>- fragment.tagNameNotMatchedBacktrackingStartHeightFromDescendant = computeBacktrackingStartHeightFromDescendant(tagNames);
</del><ins>+ fragment.tagNameNotMatchedBacktrackingStartHeightFromDescendant = computeBacktrackingStartHeightFromDescendant(tagNames, maxPrefixSize);
</ins><span class="cx"> }
</span><span class="cx">
</span><span class="cx"> // Compute height from descendant in the case that tagName is matched.
</span><span class="cx"> tagNames.last().inverted = false;
</span><del>- fragment.tagNameMatchedBacktrackingStartHeightFromDescendant = computeBacktrackingStartHeightFromDescendant(tagNames);
</del><ins>+ fragment.tagNameMatchedBacktrackingStartHeightFromDescendant = computeBacktrackingStartHeightFromDescendant(tagNames, maxPrefixSize);
</ins><span class="cx"> fragment.heightFromDescendant = tagNames.size() - 1;
</span><span class="cx"> previousChildFragmentInDescendantBacktrackingChain = &fragment;
</span><span class="cx"> } else {
</span></span></pre>
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