Cydox wrote:
I'm gonna write a better explanation with some drawings of my arguments in a
bit, but to start of with, clang's answer is inconsistent.
If you rewrite the reproducer in the issue to this version without the
`__attribute__((counted_by(a_count)))`, and compile with `-O2`, clang says that
the size is now 40 instead of 36:
```C
#include <stdio.h>
#include <stdlib.h>
typedef struct {
int counter;
} atomic_t;
typedef struct refcount_struct {
atomic_t refs;
} refcount_t;
struct callback_head {
struct callback_head *next;
void (*func)(struct callback_head *head);
} __attribute__((aligned(sizeof(void *))));
#define rcu_head callback_head
typedef struct {
uid_t val;
} kuid_t;
typedef struct {
gid_t val;
} kgid_t;
struct posix_acl_entry {
short e_tag;
unsigned short e_perm;
union {
kuid_t e_uid;
kgid_t e_gid;
};
};
struct posix_acl {
refcount_t a_refcount;
struct rcu_head a_rcu;
unsigned int a_count;
struct posix_acl_entry a_entries[];
};
int main() {
unsigned int count = 1;
struct posix_acl *acl;
acl = malloc(sizeof(struct posix_acl) +
sizeof(struct posix_acl_entry) * count);
acl->a_count = count;
acl->a_entries[0].e_tag = 0x1;
printf("%zu\n", __builtin_dynamic_object_size(acl, 0));
printf("%zu\n", __builtin_dynamic_object_size(acl->a_entries, 0));
return 0;
}
```
```
$ clang -O2 test.c
$ ./a.out
40
```
Compiling the original reproducer in #111009 (only change is the counted_by
attribute) you get this:
```
$ clang -O2 test.c
$ ./a.out
36
```
https://github.com/llvm/llvm-project/pull/111015
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