<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 - [mca] modelling decoder unit throttling"
href="https://bugs.llvm.org/show_bug.cgi?id=42202">42202</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[mca] modelling decoder unit throttling
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</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>llvm-mca
</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>andrea.dibiagio@gmail.com, llvm-bugs@lists.llvm.org, matthew.davis@sony.com
</td>
</tr></table>
<p>
<div>
<pre>This is two-fold issue:
* Instructions that generate two macro-ops are called double instructions.
The decoders in the Piledriver, Steamroller, and Excavator can handle four
single instructions (1-1-1-1) or one double instruction and two single
(2-1-1) or two double instructions (2-2) in one clock cycle. The Bulldozer
can handle (1-1-1-1) and (2-1-1), but not (2-2).
I.e. e.g. this is incorrect: <a href="https://godbolt.org/z/pT2Tgz">https://godbolt.org/z/pT2Tgz</a>
* Instructions that generate more than two macro-ops are using microcode.
The decoders cannot do anything else while microcode is generated.
This means that a decoder can stop decoding for several clock cycles
after meeting an instruction that generates more than two macro-ops.
Interestingly, this seems to be already handled? <a href="https://godbolt.org/z/HUJH1R">https://godbolt.org/z/HUJH1R</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>