https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94337
Bug ID: 94337
Summary: Incorrect "dereferencing type-punned pointer will
break strict-aliasing rules" warning
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: vincent-gcc at vinc17 dot net
Target Milestone: ---
Consider the following example.
#include <stdio.h>
struct s1
{
int a;
};
struct s2
{
int a, b;
};
int main (void)
{
union {
struct s1 m1[1];
struct s2 m2[1];
} u;
(u.m2)->b = 17;
printf ("%d\n", ((struct s2 *) (struct s1 *) u.m2)->b);
printf ("%d\n", ((struct s2 *) u.m1)->b);
return 0;
}
zira:~> gcc-10 tst.c -o tst -O2 -Wstrict-aliasing
tst.c: In function ‘main’:
tst.c:22:20: warning: dereferencing type-punned pointer will break
strict-aliasing rules [-Wstrict-aliasing]
22 | printf ("%d\n", ((struct s2 *) u.m1)->b);
| ~^~~~~~~~~~~~~~~~~~~
But there is no type-punning here. All accesses are done via struct s2.
Everything else is pointer conversions, which are not related to the aliasing
rules.
