<html>
<head>
<base href="http://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 --- - Many allocations in clang aren't correctly aligned."
href="http://llvm.org/bugs/show_bug.cgi?id=22728">22728</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Many allocations in clang aren't correctly aligned.
</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>All
</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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ahmed.bougacha@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider "DeclRefExpr::Create(.., NameInfo, ..)". It allocates itself
and a few other structs in a single allocation, using the well-known
pattern:
std::size_t Size = sizeof(DeclRefExpr);
if (...)
Size += sizeof(...);
...
void *Mem = Context.Allocate(Size, llvm::alignOf<DeclRefExpr>());
The alignment should also take into account the trailing classes,
really. In every Allocate call that has a non-trivial size, the
alignment should be correspondingly non-trivial. Some calls just use
ASTContext's default, 8 (enough for most purposes, at least; better
than the explicit alignOf, which might be 4).
[skip forward if you don't care about the investigation]
Currently, I think we get away with this because 1) X86 is lenient, 2)
most people don't self-host on another architecture, and 3) at least
on ARM, alignment checking has to be explicitly enabled, statically or
dynamically.
On ARM, if you try running clang with either:
- SCTRL.A==1 (alignment checking on non-strict-aligned instructions)
(I should say I couldn't get this to work); or
- different LLVM CodeGen picking stricter instructions that actually
enforce alignment (what made me investigate),
it should crash, at some point. On the LIT-tests, the first (of many)
to fail was
test/Analysis/dead-stores.cpp
Indeed, on trunk clang, the "this" pointer in
ASTTemplateArgumentListInfo::initializeFrom(Info, b, b, b)
is sometimes dynamically 4-aligned, whereas it is supposed to be
8-aligned (according to alignof and the IR we generate.)
This pattern is pervasive throughout clang, and even though a few
classes try to get it right (with a trailing AlignedCharArray), most
don't.
UBSan's alignment sanitizer would be *very* useful here. On ARM,
self-hosting clang using clang, with the alignment sanitizer:
-fsanitize=alignment -fsanitize-undefined-trap-on-error
Should exhibit the problems.</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>