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<base href="https://bugs.webkit.org/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - B3->Air compare-branch fusion should fuse even if the result of the comparison is used more than once"
href="https://bugs.webkit.org/show_bug.cgi?id=152198">152198</a>
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<th>Summary</th>
<td>B3->Air compare-branch fusion should fuse even if the result of the comparison is used more than once
</td>
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<th>Classification</th>
<td>Unclassified
</td>
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<th>Product</th>
<td>WebKit
</td>
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<th>Version</th>
<td>WebKit Nightly Build
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>Normal
</td>
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<th>Priority</th>
<td>P2
</td>
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<th>Component</th>
<td>JavaScriptCore
</td>
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<th>Assignee</th>
<td>webkit-unassigned@lists.webkit.org
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<th>Reporter</th>
<td>fpizlo@apple.com
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<p>
<div>
<pre>This code:
cmp
setXX
movzx -> put result in %res
test %res, %res
jnz
...
test %res, %res
jnz
is always worse than this code:
cmp
jXX
...
cmp
jXX
But our instruction selector emits the worse code because it's afraid of emitting the compare instruction more than once. It's also afraid of duplicating loads. The solution is:
- Don't be afraid of duplicating comparisons if you're emitting a branch, since for a shared comparison result, the branch will have to do a second comparison anyway.
- When we decide to match a shared comparison we could simply disable load fusion.</pre>
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