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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [PostRA Machine Sink] Ensure dbg value consistency when moving dbg instructions across other dbg instructions."
href="https://bugs.llvm.org/show_bug.cgi?id=37897">37897</a>
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<th>Summary</th>
<td>[PostRA Machine Sink] Ensure dbg value consistency when moving dbg instructions across other dbg instructions.
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</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>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>matthew.davis@sony.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>A discussion in <a href="https://reviews.llvm.org/D45637">https://reviews.llvm.org/D45637</a> mentions a possible
inconsistent debug representation when sinking debug-info instructions across
other debug-info instructions associated to the same user variable. I'll try
to distill the issue below, but the review provides a more detailed discussion.
While we have not yet seen such a case occur from user source code, it seems
that such a case might occur. To better illustrate this idea, the following
example was ripped from the link referenced above:
...
renamable $eax = COPY $edi
DBG_VALUE debug-use $eax, debug-use $noreg, !14, !DIExpression(),
debug-location !16
$esi = ADD $edi, 7
DBG_VALUE debug-use $esi, debug-use $noreg, !14, !DIExpression(),
debug-location !17
...
In this hypothetical example, both DBG_VALUEs are associated with some user
variable !14. If the MachineSink code were to sink the COPY and its associated
DBG_VALUE to some successor block, not displayed in the example above, then
there is the potential that a debugger will show incorrect data (old/original
debug data) when stepping pass the sunken instruction.
Another similar example, also ripped from the review is the following:
if (p == NULL)
b = 1;
p = q;
bar()
Represented as IR:
call dbg.value(!DIVariable("p", ...), null, !DIExpression(DW_OP_constu 0))
%b = predicated_move(%p, i32 1) // assuming such an instruction exists :-)
%p.1 = %q
call dbg.value(!DIVariable("p", ...), %p.1, !DIExpression()
In this example, moving the original dbg_value after the second would have the
effect of making variable 'p' appear NULL when stepping/inspecting instructions
after the second dbg.intrinsic.</pre>
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