[Bug sanitizer/105141] New: #pragma pack(1) causes incorrect UBSAN warning

[shaohua.li at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105141

            Bug ID: 105141
           Summary: #pragma pack(1) causes incorrect UBSAN warning
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: shaohua.li at inf dot ethz.ch
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at gcc dot gnu.org
  Target Milestone: ---

For the following code, UBSAN would warn a "load of misaligned address".
Removing #pragma pack(1) will make the warning away.

$cat a.c
#pragma pack(1)
struct {
  char a;
  int b;
} c;
int *d = &c.b;
int main() {
    __builtin_printf("%d\n", *d);
}
$
$gcc -fsanitize=undefined -O0 -w a.c;./a.out
~/a.c:8:5: runtime error: load of misaligned address 0x0000004040ca for type
'int', which requires 4 byte alignment
0x0000004040ca: note: pointer points here
 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00
00  00 00 00 00 00 00
              ^ 
0
$
$