C++ undefined reference to `__atomic_load_16'

Question

I have linking errors, when trying to do an atomic load of a 16 byte block. I have the following code:

#include <atomic>

struct MyStruct{
  long x; long y;
};

struct X{
  std::atomic<MyStruct> myStruct;
};

int main(){
  X x;
  MyStruct s = atomic_load(&x.myStruct);
}

When I compile this with (g++ version 5.3.1):

g++ --std=c++11 test.cpp

I get the error

/tmp/ccrvzLMq.o: In function `std::atomic<MyStruct>::load(std::memory_order) const':
test.cpp:(.text._ZNKSt6atomicI8MyStructE4loadESt12memory_order[_ZNKSt6atomicI8MyStructE4loadESt12memory_order]+0x1c): undefined reference to `__atomic_load_16'
collect2: error: ld returned 1 exit status

If (following a hint in another post) I add the "-latomic" flag, I get the error "/bin/ld: cannot find /usr/lib64/libatomic.so.1.1.0". And indeed that file does not exist.

Any suggestions?

Gavin

Answer 1

Same problem for Clang compiler, in short word: install libatomic and linking with it. (In RHEL the library named libatomic.so.1 so you may need to -l:libatomic.so.1 to set the name)

https://releases.llvm.org/14.0.0/tools/clang/docs/Toolchain.html#libatomic-gnu

If compiler don't know how to translate your 'c++_atomic_operation_code' into CPU instruction, it would ask for help from libatomic. The default compile arguments make the program runnable on x86/64 CPU more generally, so some CPU instructions are disabled.

On the other side, using libatomic would have chance to execute more mordern instruction with faster speed, see comments below. Thanks for @Peter Cordes

Within -march=native, the compiler could utilize more instruction set to translate code. (your CPU are better than Generial-x86/64-CPU)

Or linking with libatomic to avoid assigning -march.

For example, with using tbb, it would often asked for libatomic to make algorithm generally.