Question about __builtin_ia32_mfence and memory barriers

[dw]

>> 1) Am I right that __builtin_ia32_mfence() does not generate a memory
>> barrier?

> That is correct: it does not prevent the compiler from moving loads
> and stores across the call to __builtin_ia32_mfence.

Are you sure?  Based on your comment, I was fully expecting to be able to produce a failure case suitable for bugzilla.

In fact, I *can* generate failure cases if I comment the __builtin_ia32_mfence() call out of _mm_mfence and replace it with something else (like asm("mfence")).  But as soon as I put the __builtin_ia32_mfence call back in, my "failure scenario" clears right up.

In short, it looks like __builtin_ia32_mfence *does* generate a barrier.  But so do other builtins (like __builtin_ia32_pause).  Does that even seem possible?  It would be weird if every builtin (or even every ia32 builtin) implied a barrier.

dw