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[powerpc64le] seq_cst memory order possibly not honored


I'm having a problem with one of the Boost.Atomic tests on a PowerPC64 LE test platform. The test is running two threads which are looping code like this:

  Thread 1               Thread 2
     [initially a == 0 && b == 0], seq_cst);, seq_cst);
  a.load(relaxed);       b.load(relaxed);
  x = b.load(relaxed);   y = a.load(relaxed);

On each iteration the test verifies that !(x == 0 && y == 0) and this check fails. As far as I can tell the test is valid and it indeed passes on x86. Boost.Atomic uses __atomic* intrinsics in both cases, so from C++ perspective the implementation is the same.

I don't have the access to the tester that fails, so I can't tell the exact GCC version that is used there, only that it is labeled as 6.0. I did some experimenting on the version 4.9.2 that I have locally and found out that for code like this:

  __atomic_store_n(&n, 1, __ATOMIC_SEQ_CST);

gcc generates this assembly:

  1c:   01 00 40 39     li      r10,1
  20:   ac 04 00 7c     sync
  24:   00 00 49 91     stw     r10,0(r9)

I would expect that for seq_cst there should be a second 'sync' right after the store, but it is absent. If 6.0 generates the similar code then this might explain the test failures I'm seeing.

So my questions are:

1. Is my test valid or is there a flaw that I'm missing?
2. Am I correct about the missing 'sync' instruction?


PS: If you're interested, here's the full code snippet I compiled:

  int n = 0;

  int main()
    __atomic_store_n(&n, 1, __ATOMIC_SEQ_CST);
    return n;

The command line was:

  powerpc64le-linux-gnu-g++ -g -O0 -o seq_cst_ppc64el.o -c ./seq_cst.cpp

And here's the link to the original Boost.Atomic test:

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