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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW " title="NEW --- - Single step atomic sequences" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D23944&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=DDUMf06MYELAe1Nlv7KChiwJLLHbYha4jtK_AOiWqwQ&m=rGPT4hEoDeDJelHlTRdo6wORV15THV-MIBKL6CIl5vs&s=HheDQI6rr6w95Irzn19OPXKv_-OTXLlMdJiCElYTpz0&e=">23944</a>
</td>
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<th>Summary</th>
<td>Single step atomic sequences
</td>
</tr>
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<th>Product</th>
<td>lldb
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
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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>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>All Bugs
</td>
</tr>
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<th>Assignee</th>
<td>lldb-dev@cs.uiuc.edu
</td>
</tr>
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<th>Reporter</th>
<td>tberghammer@google.com
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>On several architectures there are atomic instruction sequences where placing a
breakpoint in the middle of them changes the behavior of the code. The expected
behavior from the debugger is to treat these sequences as a "single instruction
block" and step through it and place breakpoints only outside of it.
Example on ARM (code came from libc.so`pthread_mutex_lock):
libc.so[0x17738] <+190>: ldrex lr, [r4]
libc.so[0x1773c] <+194>: mov.w r2, #0x0
libc.so[0x17740] <+198>: teq.w lr, r5
libc.so[0x17744] <+202>: it eq
libc.so[0x17746] <+204>: strexeq r2, r12, [r4]
libc.so[0x1774a] <+208>: cmp r2, #0x0
libc.so[0x1774c] <+210>: bne 0x17738 ; <+190>
There is an internal link (implemented inside of the processor) between the
ldrex and the strex instruction. If the evaluation of the code brakes between
these 2 instructions then this link broke resulting in a change in the behavior
of the code (in this case strex will always fail).
P.S.: The issue (in theory) handled by GDB properly on ARM
(<a href="https://urldefense.proofpoint.com/v2/url?u=https-3A__sourceware.org_ml_gdb-2Dpatches_2011-2D12_msg00235.html&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=DDUMf06MYELAe1Nlv7KChiwJLLHbYha4jtK_AOiWqwQ&m=rGPT4hEoDeDJelHlTRdo6wORV15THV-MIBKL6CIl5vs&s=DufXso7sZ8QqtDG6NoBDf7u1g2TZkj2qBxMSGdHr-7w&e=">https://sourceware.org/ml/gdb-patches/2011-12/msg00235.html</a>)</pre>
</div>
</p>
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