<html>
<head>
<base href="https://bugs.freedesktop.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 - blend failures on llvmpipe with llvm 3.7 due to vector selects"
href="https://bugs.freedesktop.org/show_bug.cgi?id=94972">94972</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>blend failures on llvmpipe with llvm 3.7 due to vector selects
</td>
</tr>
<tr>
<th>Product</th>
<td>Mesa
</td>
</tr>
<tr>
<th>Version</th>
<td>git
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>medium
</td>
</tr>
<tr>
<th>Component</th>
<td>Mesa core
</td>
</tr>
<tr>
<th>Assignee</th>
<td>mesa-dev@lists.freedesktop.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sroland@vmware.com
</td>
</tr>
<tr>
<th>QA Contact</th>
<td>mesa-dev@lists.freedesktop.org
</td>
</tr></table>
<p>
<div>
<pre>Using vector selects (cdf7c6b83dad7eb6a7600af61403315b02dcf13f) caused some
regressions when using llvm 3.7 (large number of blend tests in deqp, also
mentioned in <a class="bz_bug_link
bz_status_NEW "
title="NEW - dEQP failures on llvmpipe"
href="show_bug.cgi?id=94957">bug 94957</a>, piglit gl-1.0-blend-func, and dolphin was mentioned on
IRC).
The problem is actually entirely llvm's fault after some digging (initially
wasn't sure if we weren't relying on some undefined behavior, hence tracked
this down), which very obviously miscompiles rather simple vector selects (I
suspect this only affects per-byte selects, possibly only when constant values
are involved).
It only seems to affect 3.7, I've tried 3.3 through 3.8 and everything else
worked.
Mostly filing a bug so I've got something to refer to when working around it in
mesa.
define <16 x i8> @novsel(<16 x i8> %val1, <16 x i8> %val2) {
entry:
%val1a = and <16 x i8> %val1, <i8 0, i8 0, i8 0, i8 -1, i8 0, i8 0, i8 0, i8
-1, i8 0, i8 0, i8 0, i8 -1, i8 0, i8 0, i8 0, i8 -1>
%val2rgb = and <16 x i8> %val2, <i8 -1, i8 -1, i8 -1, i8 0, i8 -1, i8 -1, i8
-1, i8 0, i8 -1, i8 -1, i8 -1, i8 0, i8 -1, i8 -1, i8 -1, i8 0>
%res = or <16 x i8> %val1a, %val2rgb
ret <16 x i8> %res
}
define <16 x i8> @vsel(<16 x i8> %val1, <16 x i8> %val2) {
entry:
%res = select <16 x i1> <i1 false, i1 false, i1 false, i1 true, i1 false, i1
false, i1 false, i1 true, i1 false, i1 false, i1 false, i1 true, i1 false, i1
false, i1 false, i1 true>, <16 x i8> %val1, <16 x i8> %val2
ret <16 x i8> %res
}
The former gets compiled to:
vmovdqa .LCPI0_0(%rip), %xmm2 # xmm2 =
[255,255,255,0,255,255,255,0,255,255,255,0,255,255,255,0]
vpblendvb %xmm2, %xmm1, %xmm0, %xmm0
retq
But the latter to:
vmovdqa .LCPI1_0(%rip), %xmm2 # xmm2 =
[0,0,0,255,0,0,0,255,255,255,255,255,255,255,255,255]
vpblendvb %xmm2, %xmm0, %xmm1, %xmm0
retq
So only the first 8 of the 16 values are correct (and yes the .LCPI1_0 byte
values look the same as indicated in the comment).</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are the QA Contact for the bug.</li>
<li>You are the assignee for the bug.</li>
</ul>
</body>
</html>