[Bug c++/71694] New: store-data race with bitfields and tail-padding in C++

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71694

            Bug ID: 71694
           Summary: store-data race with bitfields and tail-padding in C++
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

struct B {
    B() {}
    int x;
    int a : 6;
    int b : 6;
    int c : 6;
};

struct C : B {
    char d;
};

C c;

int main()
{
  c.c = 1;
  c.d = 2;
}

produces on x86_64

main:
.LFB6:
        .cfi_startproc
        movl    c+4(%rip), %eax
        andl    $-258049, %eax
        orb     $16, %ah
        movl    %eax, c+4(%rip)
        movb    $2, c+7(%rip)
        xorl    %eax, %eax
        ret

as you can see the store to c.c races with another thread accessing c.d.