<html>
<head>
<base href="https://llvm.org/bugs/" />
</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 --- - memset optimization for initialization is sometimes a pessimization"
href="https://llvm.org/bugs/show_bug.cgi?id=24700">24700</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>memset optimization for initialization is sometimes a pessimization
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Extracted from PR24698, consider:
union U {
int a[2];
int b[256];
};
void foo(int a)
{
union U u = {.a = {1}};
union U v = {.a = {a}};
}
Clang emits both of these as a memset of 1024 bytes followed by a store due to
our memset optimizations for initialization (see
shouldUseMemSetPlusStoresToInitialize and CheckAggExprForMemSetUse for the
constant and non-constant cases respectively). That's a pessimization: only the
first 8 bytes of u and v have specified values, so we should emit each of these
initializations as just one or two stores.
Another, more difficult to fix, version of the same problem:
struct S {
union U a, b;
int n;
};
void bar() {
struct S s = { {{1}}, {{2}}, 3 };
}
This can be emitted as three or five stores, but we'll instead memset all 2K of
stack space.</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>