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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Issue with passing expression to inline assembly in Linux kernel"
href="https://bugs.llvm.org/show_bug.cgi?id=51634">51634</a>
</td>
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<tr>
<th>Summary</th>
<td>Issue with passing expression to inline assembly in Linux kernel
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: PowerPC
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>natechancellor@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, nemanja.i.ibm@gmail.com
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=25195" name="attach_25195" title="Reduced program">attachment 25195</a> <a href="attachment.cgi?id=25195&action=edit" title="Reduced program">[details]</a></span>
Reduced program
Initial explanation of the issue by Michael Ellerman:
<a href="https://lore.kernel.org/r/87h7fcc2m4.fsf@mpe.ellerman.id.au/">https://lore.kernel.org/r/87h7fcc2m4.fsf@mpe.ellerman.id.au/</a>
clang does not emit a clrlwi instruction for the attached program, resulting in
incorrect behavior at runtime, as noted in the above post.
$ clang -O2 --target=powerpc64-linux-gnu -c -o warn_on.o warn_on.c
$ llvm-objdump -dr warn_on.o
warn_on.o: file format elf64-powerpc
Disassembly of section .text:
0000000000000000 <.text>:
0: f8 83 00 00 std 4, 0(3)
4: 0b 04 00 00 tdnei 4, 0
8: 4e 80 00 20 blr
c: 4e 80 00 20 blr
...
Compared to GCC 11.2.0:
$ powerpc64-linux-gcc -O2 -c -o warn_on.o warn_on.c
$ llvm-objdump -dr warn_on.o
warn_on.o: file format elf64-powerpc
Disassembly of section .text:
0000000000000000 <.text>:
0: 54 89 07 fe clrlwi 9, 4, 31
4: f8 83 00 00 std 4, 0(3)
8: 0b 09 00 00 tdnei 9, 0
c: 4e 80 00 20 blr
...
Changing
"r"(knode_dead(knode))
to
"r"(!!knode_dead(knode)
does make clang emit the clrlwi instruction:
$ clang -O2 --target=powerpc64-linux-gnu -c -o warn_on.o warn_on.c
$ llvm-objdump -dr warn_on.o
warn_on.o: file format elf64-powerpc
Disassembly of section .text:
0000000000000000 <.text>:
0: 54 85 07 fe clrlwi 5, 4, 31
4: f8 83 00 00 std 4, 0(3)
8: 0b 05 00 00 tdnei 5, 0
c: 4e 80 00 20 blr
10: 4e 80 00 20 blr
...
but that seems to be more of a workaround than addressing the root cause?</pre>
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