<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 - [GlobalMerge] GlobalMerge pass hurts the performance when merging global arrays"
href="https://bugs.llvm.org/show_bug.cgi?id=37927">37927</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[GlobalMerge] GlobalMerge pass hurts the performance when merging global arrays
</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>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>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>haicheng@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Disabling GlobalMerge pass can improve 2/3 of spec20xx benchmarks and only 2
benchmarks have large regression. I looked into it and I believe the cause is
merging global arrays with other globals.
The motivation examples of this pass is merging global arrays using the same
index like below:
static int foo[N], bar[N], baz[N];
for (i = 0; i < N; ++i)
foo[i] = bar[i] * baz[i];
The same base registers can be reused among three memory accesses. However, if
the array accesses use different indices like this:
foo[i] = bar[j] * baz[k];
We need to use reg+reg+imm to access which cannot be folded into the addressing
mode of AArch64 or ARM.
I did a simple experiment by skipping all global arrays during the global
collecting phase and 3/4 benchmarks of spec20xx got improved and the
regressions are all noise.</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>