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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization std::atomic with 64bit struct"
href="https://bugs.llvm.org/show_bug.cgi?id=45055">45055</a>
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<th>Summary</th>
<td>Missed optimization std::atomic with 64bit struct
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>Other
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>-New Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>jvapen@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<div>
<pre>Logging for
<a href="https://stackoverflow.com/questions/60445848/clang-doesnt-inline-stdatomicload-for-loading-64-bit-structs">https://stackoverflow.com/questions/60445848/clang-doesnt-inline-stdatomicload-for-loading-64-bit-structs</a>
Code:
#include <atomic>
struct A {
int32_t x, y;
};
A f(std::atomic<A>& a) {
return a.load(std::memory_order_relaxed);
}
Expected assembly (by GCC):
f(std::atomic<A>&):
mov rax, QWORD PTR [rdi]
ret
Assembly by Clang:
f(std::atomic<A>&): # @f(std::atomic<A>&)
push rax
mov rsi, rdi
mov rdx, rsp
mov edi, 8
xor ecx, ecx
call __atomic_load
mov rax, qword ptr [rsp]
pop rcx
ret
mov rdi, rax
call __clang_call_terminate
__clang_call_terminate: # @__clang_call_terminate
push rax
call __cxa_begin_catch
call std::terminate()
This clang missed optimization only happens with libstdc++; clang on Godbolt
inlines as we expect for -stdlib=libc++. <a href="https://godbolt.org/z/Tt8XTX">https://godbolt.org/z/Tt8XTX</a>.
More details, see the stack overflow post</pre>
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