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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM optimizes away local variables making them alias"
href="http://llvm.org/bugs/show_bug.cgi?id=18304">18304</a>
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<th>Summary</th>
<td>LLVM optimizes away local variables making them alias
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>3.3
</td>
</tr>
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<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>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>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>uzytkownik2@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=11771" name="attach_11771" title="Test case in C++">attachment 11771</a> <a href="attachment.cgi?id=11771&action=edit" title="Test case in C++">[details]</a></span>
Test case in C++
The LLVM opt -O2 optimizes the:
define i32 @main() #2 {
%1 = alloca i32, align 4
%ptr1 = alloca i32, align 4
%ptr2 = alloca i32, align 4
store i32 0, i32* %1
call void @_Z4testPi(i32* %ptr2)
%2 = load i32* %ptr2, align 4
store i32 %2, i32* %ptr1, align 4
call void @_Z4testPi(i32* %ptr1)
ret i32 0
}
into:
define i32 @main() #2 {
%ptr1 = alloca i32, align 4
call void @_Z4testPi(i32* %ptr1)
call void @_Z4testPi(i32* %ptr1)
ret i32 0
}
The problem is that ptr1 and 'new' ptr2 alias in second case while they did not
in first case. As ptr2 is still formally alive (at least in case of C++) the
test might access it. So the attached testcase produce:
clang++ 3.3/gcc 4.8 -O3:
0
2
clang++ 3.3 -O2:
0
AAA
1
(I've observed it while playing with how clang handles aliases)</pre>
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