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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_CONFIRMED "
title="CONFIRMED - [DebugInfo@O2] Bogus parameter value presented for struct argument"
href="https://bugs.llvm.org/show_bug.cgi?id=40188">40188</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[DebugInfo@O2] Bogus parameter value presented for struct argument
</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>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>CONFIRMED
</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>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</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, dblaikie@gmail.com, greg.bedwell@sony.com, international.phantom@gmail.com, llvm-bugs@lists.llvm.org, paul.robinson@am.sony.com
</td>
</tr>
<tr>
<th>Blocks</th>
<td>38768
</td>
</tr></table>
<p>
<div>
<pre>Using r350008 and the command line "clang-8 -O2 -g test.cpp -o a.out
-fno-inline", in the code below a bogus value is reported for the value of
'eyelids' when entering the 'foo' function. The code:
--------8<--------
struct bees {
int a;
int b;
};
int
foo(struct bees eyelids)
{
return eyelids.a + eyelids.b;
}
int
main()
{
struct bees xyzzy = { 1, 2 };
return foo(xyzzy);
}
-------->8--------
Gdbs interpretation on stepping into foo:
(gdb) frame
#0 foo (eyelids=...) at test.cpp:8
8 {
(gdb) print eyelids
$1 = {a = 1, b = 4195504}
Compiling with g++ 6.3 reports eyelids.b=2 at the same location. Examining
clangs output with llvm-dwarfdump, we generate the following expression for
"eyelids" for the body of "foo":
DW_OP_reg5 RDI, DW_OP_piece 0x4, DW_OP_reg0 RAX, DW_OP_piece 0x4
when the value of eyelids is actually only passed in rdi (upper and lower
portions). Compiling with g++ and -gstrict-dwarf correctly gives only
"DW_OP_reg5 RDI". The body of "foo" moves eyelids.b to eax initially, it
appears that the location of that fragment gets hoisted to cover the beginning
of the function, even when eax isn't def'd yet. I'm 95% confident the code that
does that is here [0], which appears to assume the first DBG_VALUE of a
parameter can have its location forwarded to the function start.
That assumption might have been valid given that there's code in SelectionDAG
[1] that emits DBG_VALUEs for parameters at the start of functions, although
for byvals only. Perhaps it used to produce entry DBG_VALUEs for every
parameter in the past?
There's a test in-tree that already exhibits this behaviour,
test/DebugInfo/ARM/partial-subreg.ll , which builds the 'self' parameter from
stack into registers over several instructions, but the location covers the
whole function, incorrectly. I ran into this problem when fiddling elsewhere,
the reproducer is to generalise the problem.
[0]
<a href="https://github.com/llvm-mirror/llvm/blob/6cd86b7cd2bb90a97d8a75196d6c0a4365c49a8b/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp#L219">https://github.com/llvm-mirror/llvm/blob/6cd86b7cd2bb90a97d8a75196d6c0a4365c49a8b/lib/CodeGen/AsmPrinter/DebugHandlerBase.cpp#L219</a>
[1]
<a href="https://github.com/llvm-mirror/llvm/blob/27f17bfee31bec92b918f4ca6a6f7a2a37e4d00c/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp#L818">https://github.com/llvm-mirror/llvm/blob/27f17bfee31bec92b918f4ca6a6f7a2a37e4d00c/lib/CodeGen/SelectionDAG/ScheduleDAGSDNodes.cpp#L818</a></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>
<br>
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