<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 - Clang doesn't honor unroll #pragma to disable loop unrolling"
href="https://bugs.llvm.org/show_bug.cgi?id=50682">50682</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Clang doesn't honor unroll #pragma to disable loop unrolling
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>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>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kobalicek.petr@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Clang doesn't honor unroll #pragma to disable loop unrolling. I tried the
following 3 approaches to disable loop unrolling, but it's never honored. The
code is always autovectorized and unrolled regardless of #pragmas used.
Is there any way to make clang not unroll loops? Because in many cases I know
the loops will have very few iterations and I mark them with compiler specific
pragmas to not unroll, however, clang doesn't really honor them.
Compile flags:
-O2 -fno-math-errno -mavx2 -std=c++2a
The problem happens with -O2, -O3, and even -Os.
Sample code:
#include <stdint.h>
uint32_t countBits1(uint32_t* x, uint32_t n) {
uint32_t count = 0;
#pragma clang loop unroll(disable)
for (uint32_t i = 0; i < n; i++) {
count += __builtin_popcount(x[i]);
}
return count;
}
uint32_t countBits2(uint32_t* x, uint32_t n) {
uint32_t count = 0;
#pragma nounroll
for (uint32_t i = 0; i < n; i++) {
count += __builtin_popcount(x[i]);
}
return count;
}
uint32_t countBits3(uint32_t* x, uint32_t n) {
uint32_t count = 0;
#pragma unroll(1)
for (uint32_t i = 0; i < n; i++) {
count += __builtin_popcount(x[i]);
}
return count;
}
Compile explorer:
<a href="https://godbolt.org/z/EcoGqoaYG">https://godbolt.org/z/EcoGqoaYG</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>