std ::原子trivially可复制的结构 [英] std::atomic trivially copyable structs

问题描述

C ++参考说： http://en.cppreference.com/w/cpp / atomic / atomic

std :: atomic可以用任何TriviallyCopyable类型T实例化

然而下面的例子在g ++ 6.2.0下无效

  #include< atomic> 
 #include< functional> 
 
 struct Test11 {
 int x; 
}; 
 struct Test12 {
 char x; 
}; 
 struct Test13 {
 long x; 
}; 
 struct Test2 {
 char x; 
 int y; 
}; 
 struct Test3 {
 int y; 
 long x; 
}; 
 
模板< typename T，typename ... ARGS> 
 void test（ARGS&& ... args）{
 static_assert（std :: is_trivially_copyable< T> :: value）; 
 
 std :: atomic< T>一个; 
 a.store（T {std :: forward< ARGS>（args）...}）; 
 
 
 int main（）{
 test< Test11>（1）; 
 test< Test12>（'\1'）; 
 test< Test13>（1L）; 
 test< Test2>（'\1'，2）; 
 test< Test3>（1,2L）; 
返回0; 
} 
  

编译： g ++ - 6 -std = c + +14 -latomic test.cpp

/tmp/cchademz.o：在函数 std :: atomic< Test3> :: store（Test3，std :: memory_order）：
test.cpp :(。text._ZNSt6atomicI5Test3E5storeES0_St12memory_order [_ZNSt6atomicI5Test3E5storeES0_St12memory_order] + 0x3e）：undefined reference to __ atomic_store_16
collect2：错误：ld返回1退出状态

g ++ - 6 --version

g ++（Ubuntu 6.2.0-7ubuntu11） 6.2.0 20161018

特别是我不明白为什么 Test2 code> Test3 不会。

有什么想法？

编辑：添加-latomic标志和g ++版本

解决方案

正如@TartanLlama在其现已删除 answer ，你需要链接 libatomic ：

  g ++  -  6 -std = c ++ 14 test.cpp -latomic 
  

您需要在编辑行末尾添加 -latomic 。如果在 test.cpp 之前放置 -latomic （例如），则某些编译器（链接器） > g ++ 在Coliru上），但有些不会（请参阅为什么库链接的顺序有时会导致GCC错误？）。

< 免责声明：我不是联系方面的专家，所以我无法详细解释为什么它在 -latomic 平台，而不是其他人（我猜连接器是不同的，但...）。

---

至于为什么你的代码编译，如果你删除 Test3 ，这是依赖于编译器和体系结构。如果您使用 -O2 和 g ++ 6.2 在 godbolt 上查看生成的ASM， ：

  sub rsp，24 
 movabs rax，8589934593 
 mov ecx，5 
 mov DWORD PTR [rsp]，1 
 mov rdi，rsp 
 mov esi，1 
 mov edx，2 
 mfence 
 mov BYTE PTR [rsp]，1 
 mfence 
 mov QWORD PTR [rsp]，1 
 mfence 
 mov QWORD PTR [rsp]，rax 
 mfence 
 call __atomic_store_16 
  

您会发现，对于少于8个字节的结构（ Test1X ， Test2 ），编译器可以使用 mov QWORD 指令（在当今体系结构中qword通常为8个字节），但它不能生成单个指令来处理尺寸严格大于8的情况（ sizeof（Test3）通常为16）。

基本上，可能有专门的 std :: atomic< T> （或一些 g ++ ¹ 当 T>时，c $ c> std :: atomic< T> 是small，而small的定义可能是与架构相关的。

免责声明：我不是关于< atomic> 的专家，所以这主要来自对godbolt生成的ASM和 g ++ 和 clang 在Coliru上。





_{¹ clang 有一个 __ atomic_store_8 过程和一个 __ atomic_store 过程，并且没有 -latomic 它不会编译为 Test2 和 Test3 。然而，即使 sizeof（Test13）是8，它仍然可以编译 Test13 ，所以它不会使用 __attribute_store_8 用于某些结构。 icc 具有完全不同的行为，并且不会生成任何调用（无法在Coliru上测试，但您可以查看它在godbolt上）。}

C++ reference says: http://en.cppreference.com/w/cpp/atomic/atomic

std::atomic may be instantiated with any TriviallyCopyable type T

However following example does not work under g++ 6.2.0

#include <atomic>
#include <functional>

struct Test11 {
    int x;
};
struct Test12 {
    char x;
};
struct Test13 {
    long x;
};
struct Test2 {
    char x;
    int y;
};
struct Test3 {
    int y;
    long x;
};

template<typename T, typename... ARGS>
void test(ARGS&& ... args) {
    static_assert(std::is_trivially_copyable<T>::value);

    std::atomic<T> a;
    a.store(T{std::forward<ARGS>(args)...});
}

int main() {
    test<Test11>(1);
    test<Test12>('\1');
    test<Test13>(1L);
    test<Test2>('\1',2);
    test<Test3>(1,2L);
    return 0;
}

Compile: g++-6 -std=c++14 -latomic test.cpp

/tmp/cchademz.o: In function std::atomic<Test3>::store(Test3, std::memory_order): test.cpp:(.text._ZNSt6atomicI5Test3E5storeES0_St12memory_order[_ZNSt6atomicI5Test3E5storeES0_St12memory_order]+0x3e): undefined reference to __atomic_store_16 collect2: error: ld returned 1 exit status

g++-6 --version

g++ (Ubuntu 6.2.0-7ubuntu11) 6.2.0 20161018

Especially I do not understand why Test2 works but Test3 does not.

Any ideas?

EDIT: added -latomic flag and g++ version

解决方案

As mentioned by @TartanLlama in its now-deleted answer, you need to link against libatomic:

g++-6 -std=c++14 test.cpp -latomic

You need to add -latomic at the end of the compilation line. Some compilers (linkers) may work correctly if you put -latomic before test.cpp (e.g. g++ on Coliru), but some won't (see Why does the order in which libraries are linked sometimes cause errors in GCC?).

Disclaimer: I am not an expert in linkage, so I cannot provide a detailed explanation as to why it works with -latomic before on some platform and not on other (I am guessing the linkers are different, but... ).

---

As to why your code compiles if you remove Test3, this is compiler and architecture dependent. If you look at the generated ASM with -O2 and g++6.2 on godbolt:

sub     rsp, 24
movabs  rax, 8589934593
mov     ecx, 5
mov     DWORD PTR [rsp], 1
mov     rdi, rsp
mov     esi, 1
mov     edx, 2
mfence
mov     BYTE PTR [rsp], 1
mfence
mov     QWORD PTR [rsp], 1
mfence
mov     QWORD PTR [rsp], rax
mfence
call    __atomic_store_16

You see that for structure that takes less than 8 bytes (Test1X, Test2), the compiler can use the mov QWORD instruction (a qword is usually 8 bytes long on nowadays architectures), but it cannot generate a single instruction to handle the cases where the size is strictly greater than 8 (sizeof(Test3) will usually be 16).

Basically, there is probably a specialization of std::atomic<T> (or of some operations of std::atomic<T>) in g++¹ when T is "small", and the definition of "small" is probably architecture dependent.

Disclaimer: Again, I am not an expert regarding <atomic> so this is mainly from experiments regarding generated ASM on godbolt and behaviour of g++ and clang on Coliru.

_{¹ clang has a __atomic_store_8 procedure and a __atomic_store procedure, and without -latomic it will not compile for Test2 and Test3. However it manages to compile Test13 even if sizeof(Test13) is 8 so it does not use __atomic_store_8 for some structures. icc has a completely different behavior and does not generate any call (cannot test on Coliru, but you can look it up on godbolt).}

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