[Bug middle-end/104067] New: wrong code compiling QEMU

[bonzini at gnu dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104067

            Bug ID: 104067
           Summary: wrong code compiling QEMU
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: blocker
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bonzini at gnu dot org
  Target Milestone: ---
            Target: x86_64-pc-linux-gnu

Compiling QEMU's tests/unit/rcutorture.c file with GCC 12 causes an incorrect
optimization

Here is a reduced testcase:

extern void abort();
struct rcu_stress {
    int x;
};

struct rcu_stress rcu_stress_array[10] = { { 0 } };
struct rcu_stress *rcu_stress_current;

int main(int argc, char *argv[])
{
    int i, rcu_stress_idx = 0;
    struct rcu_stress *cp = rcu_stress_array;

    for (i = 100; --i > 0; ) {
        struct rcu_stress *p;
        rcu_stress_idx++;
        if (rcu_stress_idx >= 10) {
            rcu_stress_idx = 0;
        }
        p = &rcu_stress_array[rcu_stress_idx];

       if (p == cp) abort();
        __atomic_thread_fence(5);
        cp = p;
    }
}

$ /opt/gcc-latest/bin/gcc -m64 -mcx16 -std=gnu11 -O2 -fno-strict-aliasing
-fno-common -fwrapv gccbug.c -o gccbug  -pthread  && ./gccbug
Aborted (core dumped)
$ gcc -m64 -mcx16 -std=gnu11 -O2 -fno-strict-aliasing -fno-common -fwrapv
gccbug.c -o gccbug  -pthread  && ./gccbug

The issue seems to be that when "p" is assigned &rcu_stress_array[10], gcc
decides it's undefined behavior and triggers the abort.  The first problematic
dump is .194t.threadfull2:

  <bb 5> [count: 0]:
  abort ();

...
  <bb 7> [local count: 1052374367]:
  # rcu_stress_idx_21 = PHI <rcu_stress_idx_2(6)>
  # cp_7 = PHI <prephitmp_16(6)>
  # ivtmp_12 = PHI <ivtmp_3(6)>
  rcu_stress_idx_20 = rcu_stress_idx_21 + 1;
  if (rcu_stress_idx_20 == 10)
    goto <bb 8>; [34.00%]
  else
    goto <bb 3>; [66.00%]

  <bb 8> [local count: 357807289]:
  # rcu_stress_idx_5 = PHI <0(7)>
  # prephitmp_9 = PHI <&rcu_stress_array(7)>
  goto <bb 5>; [100.00%]