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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - atomics are 20x slower compared to gcc if atomic<T>'s T is a packed struct"
href="https://llvm.org/bugs/show_bug.cgi?id=25335">25335</a>
</td>
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<tr>
<th>Summary</th>
<td>atomics are 20x slower compared to gcc if atomic<T>'s T is a packed struct
</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>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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sami.liedes@iki.fi
</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>Consider this code:
------------------------------------------------------------
#include <array>
#include <atomic>
#include <cstdint>
#include <iostream>
struct Entry {
uint32_t pos;
};
constexpr size_t N = 1000000000;
std::array<std::atomic<Entry>, N> *arr;
int main() {
arr = new std::array<std::atomic<Entry>, N>();
std::cout << "lock_free(): " << (*arr)[100].is_lock_free() << std::endl;
for (size_t i=0; i<N; i++) {
Entry e;
e.pos = 1;
(*arr)[i].store(e, std::memory_order_relaxed);
}
}
------------------------------------------------------------
When compiled with gcc 4.9 -std=c++14 -O3 for x86_64:
------------------------------------------------------------
$ time ./atomic_bench.gcc
lock_free(): 1
real 0m0.689s
user 0m0.490s
sys 0m0.198s
------------------------------------------------------------
When compiled with recent clang HEAD, -std=c++14 -O3:
------------------------------------------------------------
$ time ./atomic_bench.clang
lock_free(): 1
real 0m11.120s
user 0m10.931s
sys 0m0.187s
------------------------------------------------------------
Profiling shows that the bunch of the time is spent in __atomic_store_4.
Curiously, removing the packed attribute from the struct makes clang as fast as
gcc. However, it is a 4-byte struct; there hardly is a reason why writing it
entirely could not be just as fast as a 4-byte integer.</pre>
</div>
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