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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - merging of code can lead to misleading line number information; add flag to disable it in some cases?"
href="http://llvm.org/bugs/show_bug.cgi?id=22579">22579</a>
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<th>Summary</th>
<td>merging of code can lead to misleading line number information; add flag to disable it in some cases?
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>3.3
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>FreeBSD
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>-New Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>llvm@insonuit.org
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>In cases where several lines of code are merged together to share instructions,
llvm
currently picks a single line number to 'represent' those in the DWARF. This
can lead
to misleading backtraces.
An example from our source base is a large function with some 20 assertion
failure
points. If any of those assertions fails, the backtrace will point to a single
one
of them, because all of them share the code to our assertion failure function.
This
is, in part, because that function has __attribute((noreturn)).
We could remove that attribute, but that opens us up to warnings based on
static
analysis of the code.
If there were a flag to disable the code merging which is happening here, we
could
get useful backtraces.
For example, code like this
extern int do_assert(const char *, int, const char *, ...)
__attribute((noreturn));
void try(int a)
{
if (a & 1)
do_assert("one", 0, "");
if (a & 2)
do_assert("two", 0, "");
if (a & 4)
do_assert("three", 0, "");
}
generates a single call to do_assert. This is normally a Good Thing, but it
has been
confusing some of our developers (and we don't care that much).
Changing __attribute((noreturn)) to __attribute((analyzer_noreturn)) seems like
it
might be a path forward, but it triggers missing-return-value warnings for code
like
int val(int a)
{
if (a == 1) return 1;
if (a == 2) return 2;
do_assert("bad a", 0, "");
}
Interestingly, it does *not* trigger a number of other warnings, such as
uninitialized
variables in the default case of a switch, so perhaps this is a case where this
warning
should have been disabled [somehow] by the analyzer_noreturn attribute?
An alternative would be to avoid generating line numbers in the DWARF at all
for this
type of merged code. That might be reasonable too.</pre>
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