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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Miscompilation in arm64 backend"
href="https://bugs.llvm.org/show_bug.cgi?id=33100">33100</a>
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<tr>
<th>Summary</th>
<td>Miscompilation in arm64 backend
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>Other
</td>
</tr>
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<th>OS</th>
<td>MacOS X
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>llc
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>human@apple.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=18471" name="attach_18471" title="Returns 0 when compiled correctly">attachment 18471</a> <a href="attachment.cgi?id=18471&action=edit" title="Returns 0 when compiled correctly">[details]</a></span>
Returns 0 when compiled correctly
I came across a miscompilation bug on arm64. I reduced it to a small test case
that is successful when compiled with -O0 but miscompiles with -O1.
Repro steps:
$ llc -relocation-model=pic -mcpu=cyclone -disable-fp-elim -O0 -o
reduced_good.s reduced_test_case.ll
$ clang -arch arm64 -x assembler -o reduced_good reduced_good.s
Execute on any arm64 device:
$ ./reduced_good
$ echo $?
0
$ llc -relocation-model=pic -mcpu=cyclone -disable-fp-elim -O1 -o reduced_bad.s
reduced_test_case.ll
$ clang -arch arm64 -x assembler -o reduced_bad reduced_bad.s
Execute on any arm64 device:
$ ./reduced_good
$ echo $?
1
After disassembling both, the bad one shows several 32-bit operations which are
somewhat unexpected in a function with only 64-bit operations. The unoptimized
assembly also uses only 64-bit operations. Maybe the > 32-bit left shift
triggers a mistake in the optimization to overaggressively truncate some
values.</pre>
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