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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - ARM MC assembler allows backward branching on CBNZ instruction."
href="http://llvm.org/bugs/show_bug.cgi?id=20359">20359</a>
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<th>Summary</th>
<td>ARM MC assembler allows backward branching on CBNZ instruction.
</td>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>sgundapa@codeaurora.org
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
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</tr></table>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=12791" name="attach_12791" title="Test case to demonstrate the issue">attachment 12791</a> <a href="attachment.cgi?id=12791&action=edit" title="Test case to demonstrate the issue">[details]</a></span>
Test case to demonstrate the issue
I have attached an arm assembly file which can demonstrate the issue.
(The test case is an excerpt from compiler-rt ARM builtins library)
$ clang -mcpu=cortex-a9 -c /tmp/temp.s -no-integrated-as -o /tmp/temp.o
/tmp/temp.s: Assembler messages:
/tmp/temp.s:13: Error: branch out of range
clang-3.5: error: assembler command failed with exit code 1 (use -v to see
invocation)
$ clang -mcpu=cortex-a9 -c /tmp/temp.s -integrated-as -o /tmp/temp.o
The integrated-as compiles the file with out an issue and generates bad
assembly
$ arm-none-linux-gnueabi-objdump -S /tmp/temp.o
/tmp/temp.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <__sync_fetch_and_add_8>:
0: b570 push {r4, r5, r6, lr}
2: f3bf 8f5f dmb sy
6: 4684 mov ip, r0
8: e8dc 017f ldrexd r0, r1, [ip]
c: 1884 adds r4, r0, r2
e: eb41 0503 adc.w r5, r1, r3
12: e8cc 4576 strexd r6, r4, r5, [ip]
16: bbbe cbnz r6, 88 <__sync_fetch_and_add_8+0x88>
18: f3bf 8f5f dmb sy
1c: bd70 pop {r4, r5, r6, pc}
There is no address corresponding to 88 <__sync_fetch_and_add_8+0x88>.
The ARM manual on CBNZ says backward branching is not possible.
Refer to
<a href="http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0489c/Cjaghefc.html">http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0489c/Cjaghefc.html</a></pre>
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