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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Canonicalize (-X srem Y) to -(X srem Y)"
href="https://bugs.llvm.org/show_bug.cgi?id=41443">41443</a>
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<th>Summary</th>
<td>Canonicalize (-X srem Y) to -(X srem Y)
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
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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>
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<th>Severity</th>
<td>enhancement
</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>chenzheng1030@hotmail.com
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</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>This is from code review comment of <a href="https://reviews.llvm.org/D60395">https://reviews.llvm.org/D60395</a>
In that patch, canonicalization is done for '-X s/ Y', so it can save one div
for -(X/Y) and (-X/Y).
There should be same opportunity for srem.
----------------------------------------
Optimization: -X srem Y -> -(X srem Y)
Precondition: true
%o0 = sub nsw i8 0, %x
%r = srem i8 %o0, %y
=>
%n0 = srem i8 %x, %y
%r = sub nsw i8 0, %n0
Done: 1
Optimization is correct
(X srem -Y) is not equal to -(X srem Y).
----------------------------------------
Optimization: X srem -Y -> -(X srem Y)
Precondition: true
%t0 = sub nsw i8 0, %y
%r = sdiv i8 %x, %t0
=>
%n0 = sdiv i8 %x, %y
%r = sub i8 0, %n0
ERROR: Target introduces undefined behavior for i8 %r
Example:
i8 %x = poison
i8 %y = 0xff (255, -1)
i8 %t0 = 0x01 (1)
source: poison
target: undefined</pre>
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