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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - x86-64: std::atomic<T> is not atomic for a 16 byte object with -mno-cx16"
href="https://bugs.llvm.org/show_bug.cgi?id=34498">34498</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>x86-64: std::atomic<T> is not atomic for a 16 byte object with -mno-cx16
</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>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>peter@cordes.ca
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>struct A{
int a;
int b;
double c;
};
#include <atomic>
std::atomic<A> shared_struct;
void update_shared2(A tmp) {
shared_struct.store(tmp, std::memory_order_relaxed);
// shared_struct = tmp; // same buggy asm with libc++
}
Godbolt compiler explorer: <a href="https://godbolt.org/g/drmtAe">https://godbolt.org/g/drmtAe</a>
clang 6.0.0 (trunk 312520) -std=c++11 -stdlib=libc++ -Wall -Wextra -O3
-march=skylake -mno-cx16
update_shared2(A): # @update_shared2(A)
mov qword ptr [rip + shared_struct], rdi
vmovsd qword ptr [rip + shared_struct+8], xmm0
ret
Two separate stores are obviously not atomic!!
This might actually be a libc++ bug. The asm output is different with
-stdlib=libstdc++:
sub rsp, 24
mov qword ptr [rsp + 8], rdi
vmovsd qword ptr [rsp + 16], xmm0
lea rdx, [rsp + 8]
mov edi, 16
mov esi, shared_struct
xor ecx, ecx
call __atomic_store
add rsp, 24
ret
mov rdi, rax
call __clang_call_terminate
-----
correct clang4.0 output with libc++ (same options)
update_shared2(A): # @update_shared2(A)
push rax
mov rax, rdi
vmovq rdx, xmm0
mov edi, shared_struct
mov rsi, rax
call __sync_lock_test_and_set_16
pop rax
ret</pre>
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