<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 - 'hoisting conditional out of loop' missed optimization opportunity"
href="https://bugs.llvm.org/show_bug.cgi?id=34364">34364</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>'hoisting conditional out of loop' missed optimization opportunity
</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>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>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>lebedev.ri@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Old code:
int get();
void func(int y, int width, int* img) {
__builtin_assume(width > 4);
int vpred[2][2] = {{0, 0}, {0, 0}};
int hpred[2];
for (int x = 0; x < width; x++) {
int diff = get();
if (x < 2)
hpred[x] = vpred[y & 1][x] += diff;
else
hpred[x & 1] += diff;
img[x] = hpred[x & 1];
}
}
Equivalent code without inner if:
int get();
void func(int y, int width, int* img) {
__builtin_assume(width > 4);
int vpred[2][2] = {{0, 0}, {0, 0}};
int hpred[2];
int x;
int diff;
x = 0;
diff = get();
hpred[x] = vpred[y & 1][x] += diff;
img[x] = hpred[x & 1];
x = 1;
diff = get();
hpred[x] = vpred[y & 1][x] += diff;
img[x] = hpred[x & 1];
for (; x < width; x++) {
diff = get();
hpred[x & 1] += diff;
img[x] = hpred[x & 1];
}
}
According to godbolt, clang trunk can not do that: <a href="https://godbolt.org/g/cS6RD5">https://godbolt.org/g/cS6RD5</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>