[Bug 1883268] Re: random errors on aarch64 when executing __aarch64_cas8_acq_rel

[Alex Bennée]

** Tags added: arm testcase

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https://bugs.launchpad.net/bugs/1883268

Title:
  random errors on aarch64 when executing __aarch64_cas8_acq_rel

Status in QEMU:
  New

Bug description:
  Hello,

  Since I upgraded to qemu-5.0 when executing the GCC testsuite,
  I've noticed random failures of g++.dg/ext/sync-4.C.

  I'm attaching the source of the testcase, the binary executable and
  the qemu traces (huge, 111MB!) starting at main (with qemu-aarch64
  -cpu cortex-a57 -R 0 -d
  in_asm,int,exec,cpu,unimp,guest_errors,nochain)

  The traces where generated by a CI build, I built the executable
  manually but I expect it to be the same as the one executed by CI.

  In seems the problem occurs in f13, which leads to a call to abort()

  The preprocessed version of f13/t13 are as follows:
  static bool f13 (void *p) __attribute__ ((noinline));
  static bool f13 (void *p)
  {
    return (__sync_bool_compare_and_swap((ditype*)p, 1, 2));
  }
  static void t13 ()
  {
    try {
      f13(0);
    }
    catch (...) {
      return;
    }
    abort();
  }

  
  When looking at the execution traces at address 0x00400c9c, main calls f13, 
which in turn calls __aarch64_cas8_acq_rel (at 0x00401084)
  __aarch64_cas8_acq_rel returns to f13 (address 0x0040113c), then f13 returns 
to main (0x0040108c) which then calls abort (0x00400ca0)

  I'm not quite sure what's wrong :-(

  I've not noticed such random problems with native aarch64 hardware.

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