[Bug c/87367] New: GCC gives false warning on -Wnull-dereference when using -O2

[mytbk920423 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87367

            Bug ID: 87367
           Summary: GCC gives false warning on -Wnull-dereference when
                    using -O2
           Product: gcc
           Version: 8.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mytbk920423 at gmail dot com
  Target Milestone: ---

When compiling the following program with -Wnull-dereference, GCC doesn't warn
when using -O1, but warns when using -O2.


#include <stdlib.h>

typedef struct list_element_s {
  void *data;
  struct list_element_s *next;
} list_element_s, list_element_p[1];

list_element_s *list_new() {
  list_element_s *n = malloc(sizeof(list_element_s));
  if (!n) {
    return NULL;
  }

  n->data = NULL;
  n->next = NULL;

  return n;
}

size_t otrng_list_len(list_element_s *head) {
  list_element_s *cursor = head;
  size_t size = 0;

  while (cursor) {
    if (cursor->data) {
      size++;
    }
    cursor = cursor->next;
  }

  return size;
}

list_element_s *otrng_list_copy(list_element_s *head) {
  if (otrng_list_len(head) == 0) {
    return NULL;
  }

  list_element_s *cursor = head;
  list_element_s *copy = list_new();
  if (!copy) {
    return NULL;
  }
  copy->data = cursor->data;
  copy->next = NULL;

  list_element_s *ret = copy;

  cursor = cursor->next;
  while (cursor) {
    copy->next = list_new();
    copy = copy->next;
    copy->data = cursor->data;
    copy->next = NULL;

    cursor = cursor->next;
  }

  return ret;
}


$ gcc -c -O2 -Wnull-dereference list.c
list.c: In function 'otrng_list_copy':
list.c:54:16: warning: potential null pointer dereference [-Wnull-dereference]
     copy->next = NULL;
                ^
list.c:53:16: warning: potential null pointer dereference [-Wnull-dereference]
     copy->data = cursor->data;
     ~~~~~~~~~~~^~~~~~~~~~~~~~