<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 - missing loop unrolling and vectorization for loop with peeled branch"
href="https://bugs.llvm.org/show_bug.cgi?id=47671">47671</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>missing loop unrolling and vectorization for loop with peeled branch
</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>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>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Testcase (x86_64):
int broken_sum(int *p, int *q) {
bool first = true;
int sum = 0;
while (p != q) {
if (!first) {
sum += 2;
}
sum += *p;
first = false;
++p;
}
return sum;
}
We peel off the first ('first == true') iteration, but don't unroll or
vectorize the resulting ('first == false') loop [<a href="https://godbolt.org/z/9chsMr">https://godbolt.org/z/9chsMr</a>].
However, if we create that same resulting loop by initializing 'first' to
'false', then we do unroll and vectorize the loop
[<a href="https://godbolt.org/z/588ovf">https://godbolt.org/z/588ovf</a>].
There's also a potential regression from LLVM 10 to trunk here: under -mno-sse,
with 'first' initialized to 'false', we used to unroll the loop even though we
couldn't vectorize [<a href="https://godbolt.org/z/bovvE9">https://godbolt.org/z/bovvE9</a>], but LLVM trunk does not
unroll the loop under -mno-sse [<a href="https://godbolt.org/z/4h3x5b">https://godbolt.org/z/4h3x5b</a>].</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>