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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 --- - undefined reference to `__atomic_load_8', when it comes to 64-bit atomic load."
href="https://llvm.org/bugs/show_bug.cgi?id=23262">23262</a>
</td>
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<tr>
<th>Summary</th>
<td>undefined reference to `__atomic_load_8', when it comes to 64-bit atomic load.
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</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>C++11
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>rwindz0@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>clang fails to deal with 64-bit atomic load and compare_exchange_weak when it
comes with -stdlib=libstdc++, while gcc can deal with it correctly.
The test program is from
<a href="http://preshing.com/20150402/you-can-do-any-kind-of-atomic-read-modify-write-operation/">http://preshing.com/20150402/you-can-do-any-kind-of-atomic-read-modify-write-operation/</a>
:
cat >test-atomic.cc<<EOF
#include <atomic>
struct Terms
{
uint32_t x;
uint32_t y;
};
std::atomic<Terms> terms;
void atomicFibonacciStep()
{
Terms oldTerms = terms.load();
Terms newTerms;
do
{
newTerms.x = oldTerms.y;
newTerms.y = oldTerms.x + oldTerms.y;
}
while (!terms.compare_exchange_weak(oldTerms, newTerms));
}
int main() {
atomicFibonacciStep();
}
EOF
test with:
$clang++-3.7 -std=c++11 -stdlib=libstdc++ -O3 test-atomic.cc
/tmp/test-atomic-190571.o: In function
`std::atomic<Terms>::load(std::memory_order) const':
test-atomic.cc:(.text[_ZNKSt6atomicI5TermsE4loadESt12memory_order]+0x14):
undefined reference to `__atomic_load_8'
/tmp/test-atomic-190571.o: In function
`std::atomic<Terms>::compare_exchange_weak(Terms&, Terms, std::memory_order,
std::memory_order)':
test-atomic.cc:(.text[_ZNSt6atomicI5TermsE21compare_exchange_weakERS0_S0_St12memory_orderS3_]+0x2b):
undefined reference to `__atomic_compare_exchange_8'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
$g++-4.9 -std=c++11 -O3 test-atomic.cc
(exited status 0)
$clang++-3.7 -std=c++11 -stdlib=libstdc++ -O3 -S test-atomic.cc -o -
...
callq __atomic_load_8
movq %rax, 8(%rsp)
movq %rax, %rcx
shrq $32, %rcx
movl %ecx, %edx
addl %eax, %edx
shlq $32, %rdx
orq %rcx, %rdx
leaq 8(%rsp), %rsi
movl $terms, %edi
movl $5, %ecx
movl $5, %r8d
callq __atomic_compare_exchange_8
testb %al, %al
jne .LBB0_3
...
$g++-4.9 -std=c++11 -O3 -S test-atomic.cc -o -
...
.LFB329:
.cfi_startproc
movq terms(%rip), %rax
movq %rax, %rdx
movl %eax, -24(%rsp)
shrq $32, %rdx
movl %edx, -20(%rsp)
.p2align 4,,10
.p2align 3
.L4:
addl %edx, %eax
movl %edx, %edx
salq $32, %rax
orq %rax, %rdx
movq -24(%rsp), %rax
lock cmpxchgq %rdx, terms(%rip)
jne .L6
rep ret
.p2align 4,,10
.p2align 3
...
BTW: clang++ works just fine with libc++. (the generated assembly code looks
almost the same to gcc's)</pre>
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