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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - InstCombine does invalid transformation"
href="https://bugs.llvm.org/show_bug.cgi?id=33695">33695</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>InstCombine does invalid transformation
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>babokin@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>clang trunk (also reproducible with 4.0, not reproducible with 3.9), x86_64.
<span class="quote">> cat f.cpp</span >
#include <stdio.h>
unsigned char uc = 170;
unsigned char var_1 = 252;
unsigned int var_33 = 1346838541U;
unsigned char var_36;
void foo() {
var_36 = (((var_33 | (0xaf2597a4 | uc)) << 26) - var_1) * 70983U;
}
int main() {
foo();
printf("result: %d\n", var_36);
return 0;
}
<span class="quote">> clang++ -O0 f.cpp -o out0
> ./out0</span >
result: 28
<span class="quote">> clang++ -O2 f.cpp -o out2
> ./out2</span >
result: 0
Opt-bisect points to "Combine redundant instructions on function", which
removes the calculations and foo() and stores 0 to var_36, which is incorrect
value.</pre>
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</p>
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