<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 --- - clang: x86: __force_align_arg_pointer__ assumes that stack arguments are aligned"
href="https://llvm.org/bugs/show_bug.cgi?id=26662">26662</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>clang: x86: __force_align_arg_pointer__ assumes that stack arguments are aligned
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>Macintosh
</td>
</tr>
<tr>
<th>OS</th>
<td>MacOS X
</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>huw@codeweavers.com
</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>When accessing stack arguments in a function marked with the
__force_align_arg_pointer__ attibute, the generated code assumes that the stack
args are 16-byte aligned. The point of re-aligning the stack with this
attribute is that the stack may not be so aligned.
This is of importance to Wine, where the caller is win32 code with 4-byte
alignment, yet the code is running on a OS/X / Linux host with 16-byte
alignment.
When the function below is compiled with:
clang -O1 -m32 -fno-stack-protector -c ~/test.c
(stack protection disabled just to simplify disassembly)
struct sixteen
{
char b[16];
};
void __attribute__((__force_align_arg_pointer__)) __attribute__((__stdcall__))
test_fn( struct sixteen s )
{
volatile struct sixteen d = s;
}
We get:
_test_fn:
00000000 pushl %ebp
00000001 movl %esp, %ebp
00000003 andl $-0x10, %esp
00000006 subl $0x20, %esp
00000009 movaps 0x8(%ebp), %xmm0
0000000d movaps %xmm0, (%esp)
00000011 movl %ebp, %esp
00000013 popl %ebp
00000014 ret $0x10
Note that while the stack is successfully re-aligned (andl $-0x10, %esp), the
movaps 0x8(%ebp), %xmm0 assumes that the stack was originally 16-byte aligned.
It should not make that assumption.</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>