<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 - Stack realignment produces inefficient code"
href="https://bugs.llvm.org/show_bug.cgi?id=41318">41318</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Stack realignment produces inefficient code
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>henrik@gramner.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>-mstackrealign or __attribute__((force_align_arg_pointer)) in combination with
-mstack-alignment=32 (or larger) emits an unnecessary instruction.
Minimal test case:
void foo(void);
__attribute__((force_align_arg_pointer))
void bar(void) {
foo();
}
-O3 -mstack-alignment=16
<bar>:
55 push rbp
48 89 e5 mov rbp,rsp
48 83 e4 f0 and rsp,0xfffffffffffffff0
e8 00 00 00 00 call <foo>
48 89 ec mov rsp,rbp
5d pop rbp
c3 ret
-O3 -mstack-alignment=32
<bar>:
55 push rbp
48 89 e5 mov rbp,rsp
48 83 e4 e0 and rsp,0xffffffffffffffe0
48 83 ec 20 sub rsp,0x20 <- This is completely redundant
e8 00 00 00 00 call <foo>
48 89 ec mov rsp,rbp
5d pop rbp
c3 ret
Any stack alignment value larger than 16 results in the unnecessary subtraction
seen above. GCC does not have the same issue.</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>