Ticket #496 (Open)Tue Nov 03 10:22:28 UTC 2020

cc 5.83: Assignment of struct return value incorrectly optimised away

Details by Julie Stamp (8365):

If I run the below code I get

*cc c.test 
Norcroft RISC OS ARM C vsn 5.83 [01 Jun 2020]
*test
00008ce0 7
00008ce0 14

whereas I would expect to get different addresses, which I do with debug:

*cc c.test  -g
Norcroft RISC OS ARM C vsn 5.83 [01 Jun 2020]
*test
00008d00 7
00008d0c 14

#include <stdio.h>
#include <stdbool.h>

typedef struct
{
  int x; /* If you remove this, the problem goes away */
  char y[1];
  int z; /* Just to show it's not y being at the end causing it */
} t_t;

typedef struct
{
  int a; /* This must come first */
  t_t *t;
} s_t;

static t_t t1 = {0,{7},0};
static t_t t2 = {0,{14},0};

s_t f(void)
{
  s_t s = { 0, &t2 };
  return s;
}

int main(int argc, char **argv)
{
  s_t s = { 0, &t1 };
  printf("%p %d\n", s.t->y, s.t->y[0]);
  s = f();
  printf("%p %d\n", s.t->y, s.t->y[0]);
  return 0;
}