Is std::atomic_compare_exchange_weak thread-unsafe by design?

Question

It was brought up on cppreference atomic_compare_exchange Talk page that the existing implementations of std::atomic_compare_exchange_weak compute the boolea

Answer 1

I was the one who originally found this bug. For the last few days I have been e-mailing Anthony Williams regarding this issue and vendor implementations. I didn't realize Cubbi had raise a StackOverFlow question. It's not just Clang or GCC it's every vendor that is broken (all that matters anyway). Anthony Williams also author of Just::Thread (a C++11 thread and atomic library) confirmed his library is implemented correctly (only known correct implementation).

Anthony has raised a GCC bug report http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60272

Simple example:

   #include 
   struct Node { Node* next; };
   void Push(std::atomic head, Node* node)
   {
       node->next = head.load();
       while(!head.compare_exchange_weak(node->next, node))
           ;
   }

g++ 4.8 [assembler]

       mov    rdx, rdi
       mov    rax, QWORD PTR [rdi]
       mov    QWORD PTR [rsi], rax
   .L3:
       mov    rax, QWORD PTR [rsi]
       lock cmpxchg    QWORD PTR [rdx], rsi
       mov    QWORD PTR [rsi], rax !!!!!!!!!!!!!!!!!!!!!!!
       jne    .L3
       rep; ret

clang 3.3 [assembler]

       movq    (%rdi), %rcx
       movq    %rcx, (%rsi)
   .LBB0_1:
       movq    %rcx, %rax
       lock
       cmpxchgq    %rsi, (%rdi)
       movq    %rax, (%rsi) !!!!!!!!!!!!!!!!!!!!!!!
       cmpq    %rcx, %rax !!!!!!!!!!!!!!!!!!!!!!!
       movq    %rax, %rcx
       jne    .LBB0_1
       ret

icc 13.0.1 [assembler]

       movl      %edx, %ecx
       movl      (%rsi), %r8d
       movl      %r8d, %eax
       lock
       cmpxchg   %ecx, (%rdi)
       movl      %eax, (%rsi) !!!!!!!!!!!!!!!!!!!!!!!
       cmpl      %eax, %r8d !!!!!!!!!!!!!!!!!!!!!!!
       je        ..B1.7
   ..B1.4:
       movl      %edx, %ecx
       movl      %eax, %r8d
       lock
       cmpxchg   %ecx, (%rdi)
       movl      %eax, (%rsi) !!!!!!!!!!!!!!!!!!!!!!!
       cmpl      %eax, %r8d !!!!!!!!!!!!!!!!!!!!!!!
       jne       ..B1.4
   ..B1.7:
       ret

Visual Studio 2012 [No need to check assembler, MS uses _InterlockedCompareExchange !!!]

   inline int _Compare_exchange_seq_cst_4(volatile _Uint4_t *_Tgt, _Uint4_t *_Exp, _Uint4_t _Value)
   {    /* compare and exchange values atomically with
       sequentially consistent memory order */
       int _Res;
       _Uint4_t _Prev = _InterlockedCompareExchange((volatile long
*)_Tgt, _Value, *_Exp);
       if (_Prev == *_Exp) !!!!!!!!!!!!!!!!!!!!!!!
           _Res = 1;
       else
       { /* copy old value */
           _Res = 0;
           *_Exp = _Prev;
       }
       return (_Res);
   }