<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 - Compiler no longer optimizing a vector add+shuffle to identity to a horizontal add"
href="https://bugs.llvm.org/show_bug.cgi?id=34111">34111</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Compiler no longer optimizing a vector add+shuffle to identity to a horizontal add
</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>normal
</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>douglas_yung@playstation.sony.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>It seems commit r309812 has broken the compiler's ability to optimize a vector
add+shuffle to identity to a simple horizontal add instruction. Consider the
following code:
__m128 add_ps_001(__m128 a, __m128 b) {
__m128 r = (__m128){ a[0] + a[1], a[2] + a[3], b[0] + b[1], b[2] + b[3] };
return __builtin_shufflevector(r, a, -1, -1, 2, 3);
}
Prior to commit r309812, when targeting x86+avx with optimizations (clang -S
-O2 -mavx), the compiler would generate the following code:
.cfi_startproc
# BB#0: # %entry
vhaddps %xmm1, %xmm0, %xmm0
retq
But starting with commit r309812, the compiler is now generating less efficient
code:
.cfi_startproc
# BB#0: # %entry
vpermilps $232, %xmm1, %xmm0 # xmm0 = xmm1[0,2,2,3]
vpermilps $237, %xmm1, %xmm1 # xmm1 = xmm1[1,3,2,3]
vaddps %xmm1, %xmm0, %xmm0
vmovddup %xmm0, %xmm0 # xmm0 = xmm0[0,0]
retq</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>