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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - lzcnt causes erroneous code generation (x86_64)"
href="https://bugs.llvm.org/show_bug.cgi?id=35399">35399</a>
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<th>Summary</th>
<td>lzcnt causes erroneous code generation (x86_64)
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>5.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
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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>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>michel.bernert@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=19461" name="attach_19461" title="Code compiled with : clang++ lzcnt.cpp -Wall -pedantic -O2 -mlzcnt -S">attachment 19461</a> <a href="attachment.cgi?id=19461&action=edit" title="Code compiled with : clang++ lzcnt.cpp -Wall -pedantic -O2 -mlzcnt -S">[details]</a></span>
Code compiled with : clang++ lzcnt.cpp -Wall -pedantic -O2 -mlzcnt -S
The use of lzcnt has an unexpected behavior, when using the -mlzcnt flag with
optimizations.
Compiling :
uint64_t test64_2(uint64_t x, bool& b1, bool& b2)
{
uint64_t a = (x == 0 ? 64 : __builtin_clzll(x));
b1 = (a != 64);
b2 = !b1;
return a;
}
Generates :
test64_2(unsigned long, bool&, bool&): # @test64_2(unsigned long, bool&, bool&)
lzcntq %rdi, %rax
setae (%rsi)
setae (%rdx)
retq
Which sets the two bools to the same value, even though 'b2 = !b1;' and I can't
see any undefined behavior.
This concerns all 16, 32 and 64 bits versions of lzcnt. Omitting -mlzcnt
produces the expected behavior for __builtin_clz versions, using bsr.</pre>
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