[Bug c/100804] New: storage order swapped with specific opt

[george.thopas at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100804

            Bug ID: 100804
           Summary: storage order swapped with specific opt
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: george.thopas at gmail dot com
  Target Milestone: ---

/* 
    Target: x86_64-pc-linux-gnu gcc versie 10.2.0 (Gentoo 10.2.0-r4 p5) 

    failing:
    $ gcc -Wall -Wextra -O2 test.c && ./a.out
    Aborted

    passing:
    $ gcc -Wall -Wextra -O3 test.c && ./a.out
    $ gcc -Wall -Wextra -O1 test.c && ./a.out
    $ gcc -Wall -Wextra -O0 test.c && ./a.out

    no fallout with -fno-strict-aliasing -fwrapv    
    problem goes away with adding either 
    -fno-code-hoisting,
    -fno-guess-branch-probability,

    The example includes an unused string structure which seems to affect 
    the problem. The odd thing is that it is never used in this code

    Removing the untaken else path makes the problem go away too 

    Thanks

*/
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>

#define BE   __attribute__((scalar_storage_order("big-endian")))

struct _str {
    char str[10];
};
typedef struct _str t_str;

struct _le {
    char  is_val;
    union {
        int   val;
        t_str str;
    } data;
};

typedef struct _le t_le;

union _be_data {
    int   val;
    t_str str;
} BE;

typedef union _be_data t_be_data;

struct _be {
    char      is_val;
    t_be_data data;
} BE;

typedef struct _be t_be;

void validate(t_be * in)
{
    t_le chk = { is_val:1, data: { val:0x12345678 } };
    t_le out = { };

    out.is_val = in->is_val;
    if (out.is_val)
        out.data.val = in->data.val;
    else
        out.data.str = in->data.str;

    if (out.data.val != chk.data.val)
        abort();
}

int main(void)
{
    t_be val = { is_val:1, data: { val:0x12345678 } };
    validate(&val);
    return 0;
}