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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - __builtin_constant_p resolves differently with loop unrolling at -O3"
href="https://bugs.llvm.org/show_bug.cgi?id=43420">43420</a>
</td>
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<th>Summary</th>
<td>__builtin_constant_p resolves differently with loop unrolling at -O3
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Interprocedural Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ndesaulniers@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>isanbard@gmail.com, jyknight@google.com, llvm-bugs@lists.llvm.org, srhines@google.com
</td>
</tr>
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<th>Blocks</th>
<td>4068
</td>
</tr></table>
<p>
<div>
<pre>void c(void);
void a(void) {
for (int b = 0x10; b; b <<= 1)
if (__builtin_constant_p(b))
__bad_udelay();
else
c();
}
Clang at -O2 and GCC at -O3 look similar. At -O3, LLVM unrolls the loop 27
times, but b is no longer an ICE or rather BCP evaluates to false. This
triggers a miscompile (__bad_udelay is purposely undefined) in the Linux kernel
at -O3.
<a href="https://godbolt.org/z/oDPVLO">https://godbolt.org/z/oDPVLO</a></pre>
</div>
</p>
<div id="referenced">
<hr style="border: 1px dashed #969696">
<b>Referenced Bugs:</b>
<ul>
<li>
[<a class="bz_bug_link
bz_status_CONFIRMED "
title="CONFIRMED - [Meta] Compiling the Linux kernel with clang"
href="https://bugs.llvm.org/show_bug.cgi?id=4068">Bug 4068</a>] [Meta] Compiling the Linux kernel with clang
</li>
</ul>
</div>
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