[LLVMbugs] [Bug 23262] New: undefined reference to `__atomic_load_8', when it comes to 64-bit atomic load.

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=23262

            Bug ID: 23262
           Summary: undefined reference to `__atomic_load_8', when it
                    comes to 64-bit atomic load.
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: rwindz0 at gmail.com
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

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
http://preshing.com/20150402/you-can-do-any-kind-of-atomic-read-modify-write-operation/
:

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)

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