`std :: memory_order_acquire`的语义是否需要x86/x86_64上的处理器指令？

Question

众所周知,在x86上,操作load()和store()内存屏障memory_order_consume, memory_order_acquire, memory_order_release, memory_order_acq_rel不需要处理器指令用于缓存和管道,并且汇编程序的代码总是对应std::memory_order_relaxed,并且这些限制仅对编译器的优化是必需的:http:// www. stdthread.co.uk/forum/index.php?topic=72.0

此代码反汇编代码确认store()(MSVS2012 x86_64):

std::atomic<int> a;
    a.store(0, std::memory_order_relaxed);
000000013F931A0D  mov         dword ptr [a],0  
    a.store(1, std::memory_order_release);
000000013F931A15  mov         dword ptr [a],1  

但是这段代码并没有为load()(MSVS2012 x86_64)确认这一点,使用lock cmpxchg:

    int val = a.load(std::memory_order_acquire);
000000013F931A1D  prefetchw   [a]  
000000013F931A22  mov         eax,dword ptr [a]  
000000013F931A26  mov         edx,eax  
000000013F931A28  lock cmpxchg dword ptr [a],edx  
000000013F931A2E  jne         main+36h (013F931A26h)  

    std::cout << val << "\n";

但安东尼威廉姆斯说:

some_atomic.load(std :: memory_order_acquire)只是简单地加载到 一个简单的加载指令,some_atomic.store(std :: memory_order_release)直接转到一个简单的存储指令.

我错在哪里,并且语义是否std::memory_order_acquire需要x86/x86_64上的处理器指令lock cmpxchg或者只是一个简单的加载指令mov？

答案:它与此错误报告相同:http://connect.microsoft.com/VisualStudio/feedback/details/770885

Answer 1

不.语义std::memory_order_acquire不需要x86/x86_64上的处理器指令.

x86_64上的任何load()/ store()操作都不需要处理器指令(锁定/栅栏)atomic.store(val, std::memory_order_seq_cst);,这些指令需要(LOCK) XCHG或替代:MOV (into memory),MFENCE.

处理器内存屏障 - x86指令(CAS除外),以及ARM和PowerPC:http://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html

反汇编程序GCC 4.8.1 x86_64 - GDB - load():

    20      temp = a.load(std::memory_order_relaxed);
    21      temp = a.load(std::memory_order_acquire);
    22      temp = a.load(std::memory_order_seq_cst);
0x46140b  <+0x007b>         mov    0x38(%rsp),%ebx
0x46140f  <+0x007f>         mov    0x34(%rsp),%esi
0x461413  <+0x0083>         mov    0x30(%rsp),%edx

反汇编程序GCC 4.8.1 x86_64 - GDB - store():

a.store(temp, std::memory_order_relaxed);
a.store(temp, std::memory_order_release);
a.store(temp, std::memory_order_seq_cst);
0x4613dc  <+0x004c>         mov    %eax,0x20(%rsp)
0x4613e0  <+0x0050>         mov    0x38(%rsp),%eax
0x4613e4  <+0x0054>         mov    %eax,0x20(%rsp)
0x4613e8  <+0x0058>         mov    0x38(%rsp),%eax
0x4613ec  <+0x005c>         mov    %eax,0x20(%rsp)
0x4613f0  <+0x0060>         mfence
0x4613f3  <+0x0063>         mov    %ebx,0x20(%rsp)

反汇编程序MSVS 2012 x86_64 - load() - 它与此错误报告相同:http://connect.microsoft.com/VisualStudio/feedback/details/770885:

    temp = a.load(std::memory_order_relaxed);
000000013FE51A1F  prefetchw   [a]  
000000013FE51A24  mov         eax,dword ptr [a]  
000000013FE51A28  nop         dword ptr [rax+rax]  
000000013FE51A30  mov         ecx,eax  
000000013FE51A32  lock cmpxchg dword ptr [a],ecx  
000000013FE51A38  jne         main+40h (013FE51A30h)  
000000013FE51A3A  mov         dword ptr [temp],eax  
    temp = a.load(std::memory_order_acquire);
000000013FE51A3E  prefetchw   [a]  
000000013FE51A43  mov         eax,dword ptr [a]  
000000013FE51A47  nop         word ptr [rax+rax]  
000000013FE51A50  mov         ecx,eax  
000000013FE51A52  lock cmpxchg dword ptr [a],ecx  
000000013FE51A58  jne         main+60h (013FE51A50h)  
000000013FE51A5A  mov         dword ptr [temp],eax  
    temp = a.load(std::memory_order_seq_cst);
000000013FE51A5E  prefetchw   [a]  
    temp = a.load(std::memory_order_seq_cst);
000000013FE51A63  mov         eax,dword ptr [a]  
000000013FE51A67  nop         word ptr [rax+rax]  
000000013FE51A70  mov         ecx,eax  
000000013FE51A72  lock cmpxchg dword ptr [a],ecx  
000000013FE51A78  jne         main+80h (013FE51A70h)  
000000013FE51A7A  mov         dword ptr [temp],eax  

反汇编程序MSVS 2012 x86_64 - store():

    a.store(temp, std::memory_order_relaxed);
000000013F8C1A58  mov         eax,dword ptr [temp]  
000000013F8C1A5C  mov         dword ptr [a],eax  

    a.store(temp, std::memory_order_release);
000000013F8C1A60  mov         eax,dword ptr [temp]  
000000013F8C1A64  mov         dword ptr [a],eax  

    a.store(temp, std::memory_order_seq_cst);
000000013F8C1A68  mov         eax,dword ptr [temp]  
000000013F8C1A6C  xchg        eax,dword ptr [a]