https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89133
Bug ID: 89133
Summary: bogus -Wcast-align=strict for a member of an aligned
struct or union
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: msebor at gcc dot gnu.org
Target Milestone: ---
Assigning the address of an overaligned member of a struct or union that is of
an otherwise less aligned type to a pointer to a type with the same alignment
triggers a -Wcast-align=strict false positive. The alignment of the member
must be the same as that of the pointed-to type regardless of the target in
order to ensure the alignment of the next member which of the pointed-to type.
Note that because the member whose address is cast is the first member, it's
necessarily at the same address and thus has the same alignment as the struct
itself for which the warning is not issued.
Clang issues the same false positive here.
$ cat u.c && gcc -S -Wall -Wextra -Wcast-align=strict u.c
union {
char c;
long long x;
} u;
long long *p0 = (long long*)&u;
long long *q0 = (long long*)&u.x;
long long *r0 = (long long*)&u.c;
struct {
char c;
long long x;
} s;
long long *p1 = (long long*)&s;
long long *q1 = (long long*)&s.x;
long long *r1 = (long long*)&s.c;
u.c:9:17: warning: cast increases required alignment of target type
[-Wcast-align]
9 | long long *r0 = (long long*)&u.c;
| ^
u.c:19:17: warning: cast increases required alignment of target type
[-Wcast-align]
19 | long long *r1 = (long long*)&s.c;
| ^
