[Bug tree-optimization/124356] New: redundant initialization of inactive union member

[xry111 at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124356

            Bug ID: 124356
           Summary: redundant initialization of inactive union member
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xry111 at gcc dot gnu.org
  Target Milestone: ---

struct r
{
  int z;
  union
  {
    int p[1000];
    struct
    {
      int x, y;
    };
  };
} r;

int
main (void)
{
  int x, y;
  asm ("" : "=r"(x), "=r"(y));
  struct r test = { .x = x, .y = y, };
  asm ("" ::"m"(test));
  return 0;
}

GCC insists to redundantly zero the entire struct

        test = {};
        test.D.2966.D.2965.x = x;
        test.D.2966.D.2965.y = y;

Interestingly, explicitly initializing z to 0, i. e. struct r test = { .x = x,
.y = y, .z = 0 }; (IIUC it shouldn't cause a difference per the standard) makes
the zeroing gone:

        test.z = 0;
        test.D.2966.D.2965.x = x;
        test.D.2966.D.2965.y = y;