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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 - [DebugInfo@O2][Dexter] Loop strength reduction preserves little debug info"
href="https://bugs.llvm.org/show_bug.cgi?id=38815">38815</a>
</td>
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<th>Summary</th>
<td>[DebugInfo@O2][Dexter] Loop strength reduction preserves little debug info
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Keywords</th>
<td>wrong-debug
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jeremy.morse.llvm@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>chackz0x12@gmail.com, greg.bedwell@sony.com, international.phantom@gmail.com, llvm-bugs@lists.llvm.org, paul_robinson@playstation.sony.com
</td>
</tr>
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<th>Blocks</th>
<td>38768
</td>
</tr></table>
<p>
<div>
<pre>It seems the loop-strength-reduction pass doesn't preserve debug information,
for any transformation it makes. Compiling the example below with llvm/clang @
r340912 and options "-O2 -g -fno-inline" for x86_64, using both gdb and lldb,
both the value of "i" and "blah" are reported as being optimised out, after
being rewritten by LSR:
-------->8--------
int
foo(int blah)
{
int i;
for (i = blah; i < blah + 10; i++) {
if (i < 3)
return 0;
}
return 1;
}
int
main()
{
return foo(3);
}
--------8<--------
Which would be fairly frustrating if one landed in the loop and were unable to
determine either the loop bound or which iteration it was on. (The example is
contrived, but LSR still knackers more complicated loops).
This isn't a bug per se, and the DebugInfo doesn't mislead anyone, it's just a
poor debug experience that will annoy developers. Particularly galling is that
the values of "i" and "blah" remain in registers for the whole loop, but are
still not available to the debugger.</pre>
</div>
</p>
<div id="referenced">
<hr style="border: 1px dashed #969696">
<b>Referenced Bugs:</b>
<ul>
<li>
[<a class="bz_bug_link
bz_status_NEW "
title="NEW - [meta][DebugInfo] Umbrella bug for poor debug experiences"
href="https://bugs.llvm.org/show_bug.cgi?id=38768">Bug 38768</a>] [meta][DebugInfo] Umbrella bug for poor debug experiences
</li>
</ul>
</div>
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