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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - InstCombine: (X << C1) / C2 -> X / (C2 >> C1) isn't always possible"
href="http://llvm.org/bugs/show_bug.cgi?id=21245">21245</a>
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<th>Summary</th>
<td>InstCombine: (X << C1) / C2 -> X / (C2 >> C1) isn't always possible
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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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>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</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>nunoplopes@sapo.pt
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</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>In InstCombiner::commonIDivTransforms(), we have the following transformation:
// (X << C1) / C2 -> X / (C2 >> C1) if C2 is a multiple of C1.
However, this is not correct if C1 == width(C1) - 1.
Optimization: 775
Pre: C2 % (1<<C1) == 0
%s = shl nsw i4 %X, C1
%r = sdiv %s, C2
=>
%r = sdiv %X, (C2 / (1 << C1))
ERROR: Mismatch in values of i4 %r
Example:
%X i4 = 15 (0xf)
C1 i4 = 3 (0x3)
C2 i4 = 8 (0x8)
%s i4 = 8 (0x8)
Source value: 1 (0x1)
Target value: 15 (0xf)
This version is correct though:
Name: 775
Pre: C2 % (1<<C1) == 0 && C1 != width(C1)-1
%s = shl nsw %X, C1
%r = sdiv %s, C2
=>
%r = sdiv %X, C2/(1<<C1)</pre>
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