<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 - Atomic operation emits an __atomic_* call rather than an instruction for trivial structure"
href="https://bugs.llvm.org/show_bug.cgi?id=47577">47577</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Atomic operation emits an __atomic_* call rather than an instruction for trivial structure
</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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>antoshkka@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Consider the example:
#include <atomic>
struct SleepState {
unsigned short flags;
unsigned short epoch;
};
void test(std::atomic<SleepState>& i) {
SleepState ss{2, 4};
i.compare_exchange_weak(ss, SleepState{4,2});
}
Function test() could be compiled to the following:
test(std::atomic<SleepState>&):
mov eax, 262146
mov edx, 131076
mov DWORD PTR [rsp-4], 262146
lock cmpxchg DWORD PTR [rdi], edx
ret
However clang calls __atomic_compare_exchange when libstdc++ is used:
test(std::atomic<SleepState>&)
push rax
mov rsi, rdi
mov dword ptr [rsp], 262146
mov dword ptr [rsp + 4], 131076
mov rdx, rsp
lea rcx, [rsp + 4]
mov edi, 4
mov r8d, 5
mov r9d, 5
call __atomic_compare_exchange
pop rax
ret
Godbolt playground: <a href="https://godbolt.org/z/Eajn57">https://godbolt.org/z/Eajn57</a></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>