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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Wrong optimization when goto jumps backwards over declaration"
href="https://llvm.org/bugs/show_bug.cgi?id=27604">27604</a>
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<th>Summary</th>
<td>Wrong optimization when goto jumps backwards over declaration
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>unspecified
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<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>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>cherepan@mccme.ru
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The program:
#include <stdio.h>
int main()
{
int *p = 0;
start:
if (p) {
printf("%d\n", *p);
return 0;
}
int x;
x = 1;
p = &x;
goto start;
}
should print "1" but prints "0" when compiled with clang -O1.
Tested on clang version 3.9.0 (trunk 268228).
Tests with <a href="https://gcc.godbolt.org/">https://gcc.godbolt.org/</a> show that it started between 3.6 and 3.7.
The reason probably is insertion of llvm.lifetime.start where a declaration
occurs instead of where the corresponding block starts.
The relevant part of C11 is 6.2.4p5-6: "An object whose identifier is declared
with no linkage and without the storage-class specifier static has automatic
storage duration, as do some compound literals. [...] For such an object that
does not have a variable length array type, its lifetime extends from entry
into the block with which it is associated until execution of that block ends
in any way."</pre>
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</p>
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