<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - miscompile with Loop Vectorizer on a simple double-nested loop"
href="https://bugs.llvm.org/show_bug.cgi?id=42384">42384</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>miscompile with Loop Vectorizer on a simple double-nested loop
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>fedor.v.sergeev@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=22137" name="attach_22137" title="bad-vector.ll">attachment 22137</a> <a href="attachment.cgi?id=22137&action=edit" title="bad-vector.ll">[details]</a></span>
bad-vector.ll
Consider this C testcase:
] cat vector.c
extern int printf(const char*, ...);
long mainTest() {
int byFld = 0;
long instanceCount=0;
int i24=0;
for (int i25 = 0; ++i25 < 260;) {
i24 += i25;
for (int i30 = 1; i30 < 31; i30++) {
instanceCount += (i30 ^ byFld);
byFld -= (char)i24;
}
}
return instanceCount;
}
int main() {
printf("%ld\n", mainTest());
}
] gcc -m64 vector.c; ./a.out
-139279619
]
Attached is IR produced and reduced from an equivalent Java program,
snapshotted right before Loop Vectorizer. It looks basically as what you would
expect to see by translating the above C code into IR, yet I did not bother to
find exact sequence of clang optimizations that could show me the same result.
] grep -n define bad-vector.ll
6:define i64 @mainTest() {
49:define i32 @main() {
] bin/opt bad-vector.ll | bin/lli
-139279619
As you see, the result matches the expected output shown above.
Now adding loop-vectorizer:
] bin/opt -passes=loop-vectorize bad-vector.ll | bin/lli
-139446659
and here is a wrong result.</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>