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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Optimizing loop with sizeof("literal")"
href="https://llvm.org/bugs/show_bug.cgi?id=25429">25429</a>
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<tr>
<th>Summary</th>
<td>Optimizing loop with sizeof("literal")
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>3.6
</td>
</tr>
<tr>
<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>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hugo.guiroux@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</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=15233" name="attach_15233" title="minimal working example">attachment 15233</a> <a href="attachment.cgi?id=15233&action=edit" title="minimal working example">[details]</a></span>
minimal working example
Compiling with clang++ the minimal working example, I get the following result:
clang++ -O3 -Wall -o sizeof_vs_strlen sizeof_vs_strlen.cpp
./sizeof_vs_strlen
Strlen 36 ret=4
Sizeof 62092396 ret=5
With g++:
g++ -O3 -Wall -o sizeof_vs_strlen sizeof_vs_strlen.cpp
./sizeof_vs_strlen
Strlen 30 ret=4
Sizeof 30 ret=5
The assembly generated by clang++ for the loop with sizeof:
rdtsc
mov %edx,%r14d
shl $0x20,%r14
mov $0x3b9aca01,%ecx
xchg %ax,%ax
add $0xffffffed,%ecx // 0x400ad0
jne 0x400ad0 <main+192>
mov %eax,%eax
or %rax,%r14
rdtsc
And the one by g++:
rdtsc
mov %edx,%esi
mov %eax,%ecx
rdtsc
I don't understand why clang++ do the {add, jne} loop, it seems to be a bug
For information:
g++ (GCC) 5.1.0
clang version 3.6.2 (tags/RELEASE_362/final) (also tested with 3.7.0, same
bug).
The bug was originally describe on stackoverflow
(<a href="http://stackoverflow.com/questions/33562365/clang-sizeofliteral-optimization">http://stackoverflow.com/questions/33562365/clang-sizeofliteral-optimization</a>).</pre>
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