<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 --- - X86: Generate smaller code for atomic refcount decrement"
href="https://llvm.org/bugs/show_bug.cgi?id=31367">31367</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>X86: Generate smaller code for atomic refcount decrement
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hans@chromium.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mkuper@google.com, rnk@google.com
</td>
</tr>
<tr>
<th>Blocks</th>
<td>26299
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Example inspired by Chromium's RefcountedThreadSafe.
#include <windows.h>
void foo();
void f(long volatile *ptr) {
if (_InterlockedExchangeAdd(ptr, -1) == 1) {
foo(); // call delete on ptr
}
}
With MSVC:
?f@@YAXPECJ@Z (void __cdecl f(long volatile *)):
0000000000000000: 83 C8 FF or eax,0FFFFFFFFh
0000000000000003: F0 0F C1 01 lock xadd dword ptr [rcx],eax
0000000000000007: 83 F8 01 cmp eax,1
000000000000000A: 0F 84 00 00 00 00 je ?foo@@YAXXZ
0000000000000010: C3 ret
With clang-cl:
?f@@YAXPECJ@Z (void __cdecl f(long volatile *)):
0000000000000000: B8 FF FF FF FF mov eax,0FFFFFFFFh
0000000000000005: F0 0F C1 01 lock xadd dword ptr [rcx],eax
0000000000000009: 83 F8 01 cmp eax,1
000000000000000C: 75 05 jne 0000000000000013
000000000000000E: E9 00 00 00 00 jmp ?foo@@YAXXZ
0000000000000013: C3 ret
It annoys me that we spend 5 bytes putting -1 into ecx. We do the "or trick" at
/Os, but what we really want is subtraction, or even better: decrement.
Wouldn't just "lock dec" and then checking eflags work? That is, how about:
lock dec dword ptr [rcx]
jne <return label>
...
Those two instructions are 5 bytes total, as opposed to 14 bytes for mov--jne
in clang-cl's code above :-)
Am I missing anything that would make this not correct?</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>