<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 - [X86] X86ISelLowering combineShiftLeft contains a transform that isn't undef safe."
href="https://bugs.llvm.org/show_bug.cgi?id=50468">50468</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[X86] X86ISelLowering combineShiftLeft contains a transform that isn't undef safe.
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>craig.topper@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, pengfei.wang@intel.com, spatel+llvm@rotateright.com
</td>
</tr></table>
<p>
<div>
<pre>This function contains this transform which increases the uses of V. This isn't
correct if V is undef or could become undef later.
// Hardware support for vector shifts is sparse which makes us scalarize the
// vector operations in many cases. Also, on sandybridge ADD is faster than
// shl.
// (shl V, 1) -> add V,V
if (auto *N1BV = dyn_cast<BuildVectorSDNode>(N1))
if (auto *N1SplatC = N1BV->getConstantSplatNode()) {
assert(N0.getValueType().isVector() && "Invalid vector shift type");
// We shift all of the values by one. In many cases we do not have
// hardware support for this operation. This is better expressed as an
ADD
// of two values.
if (N1SplatC->isOne())
return DAG.getNode(ISD::ADD, SDLoc(N), VT, N0, N0);
}
For i16/i32/i64 we can probably just use an isel pattern instead like we do for
scalar shift left. Not sure how to preserve i8.</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>