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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incorrect instcombine transform urem -> icmp+zext with vectors"
href="https://bugs.llvm.org/show_bug.cgi?id=44186">44186</a>
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<th>Summary</th>
<td>Incorrect instcombine transform urem -> icmp+zext with vectors
</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
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<th>Status</th>
<td>NEW
</td>
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<th>Keywords</th>
<td>miscompilation
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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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>lebedev.ri@gmail.com, llvm-bugs@lists.llvm.org, regehr@cs.utah.edu, spatel+llvm@rotateright.com
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<p>
<div>
<pre>Testcase from Transforms/InstCombine/vector-urem.ll:
define <4 x i32> @test_v4i32_one_undef(<4 x i32> %a0) {
%1 = urem <4 x i32> { 1, 1, 1, undef }, %a0
ret <4 x i32> %1
}
=>
define <4 x i32> @test_v4i32_one_undef(<4 x i32> %a0) {
%1 = icmp ne <4 x i32> %a0, { 1, 1, 1, undef }
%2 = zext <4 x i1> %1 to <4 x i32>
ret <4 x i32> %2
}
Transformation doesn't verify!
ERROR: Value mismatch
Example:
<4 x i32> %a0 = < #x00000010 (16), #x00000001 (1), #x00000400 (1024),
#x00000001 (1) >
Source:
<4 x i32> %1 = < #x00000001 (1), #x00000000 (0), #x00000001 (1), #x00000000 (0)
>
Target:
<4 x i1> %1 = < #x1 (1), #x0 (0), #x1 (1), #x1 (1) >
<4 x i32> %2 = < #x00000001 (1), #x00000000 (0), #x00000001 (1), #x00000001 (1)
>
Source value: < #x00000001 (1), #x00000000 (0), #x00000001 (1), #x00000000 (0)
>
Target value: < #x00000001 (1), #x00000000 (0), #x00000001 (1), #x00000001 (1)
>
Essentially, "x urem 1" is 0 for any 'x'. However, the optimized code can
produce 0 or 1.
I guess the easiest fix is to change undef to 1 in the icmp.</pre>
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