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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Use "bt" for switch select from bitmask?"
href="https://bugs.llvm.org/show_bug.cgi?id=39174">39174</a>
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<th>Summary</th>
<td>Use "bt" for switch select from bitmask?
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>hans@chromium.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>For example:
bool f(int x) {
switch (x) {
case 0: return true;
case 1: return true;
case 2: return false;
case 3: return false;
case 4: return true;
}
}
SimplifyCFG will generate a "table lookup" from a bitmask, and on X86_64 we end
up with:
f(int): # @f(int)
# %bb.0:
mov al, 19
mov ecx, edi
shr al, cl
and al, 1
ret
I wonder if we could get tighter code with the BT instruction.</pre>
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