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c++cassemblyx86memory-fences

Do we need mfence when using xchg

I have a set and test xchg based assembly lock. my question is :

Do we need to use memory fencing (mfence, sfence or lfence ) when using xchg instruction ?

Edit :

64 Bit platform : with Intel nehalem

Solution

  • As said in the other answers the lock prefix is implicit, here, so there is no problem on the assembler level. The problem may lay on the C (or C++) level when you use that as inline assembler. Here you have to ensure that the compiler doesn't reorder instructions with respect to your xchg. If you are using gcc (or cousins) you would typically do something like:

      __asm__ __volatile__("xchgl %1, %0"
                       : "=r"(ret)
                       : "m"(*point), "0"(ret)
                       : "memory");

    that is declare the instruction as volatile and add the "memory" clobber.