<table border="1" cellspacing="0" cellpadding="8">
    <tr>
        <th>Issue</th>
        <td>
            <a href=https://github.com/llvm/llvm-project/issues/83710>83710</a>
        </td>
    </tr>

    <tr>
        <th>Summary</th>
        <td>
            -fsanitize=alignment false positive with intended unaligned struct member access
        </td>
    </tr>

    <tr>
      <th>Labels</th>
      <td>
            new issue
      </td>
    </tr>

    <tr>
      <th>Assignees</th>
      <td>
      </td>
    </tr>

    <tr>
      <th>Reporter</th>
      <td>
          akihikodaki
      </td>
    </tr>
</table>

<pre>
    `-fsanitize=alignment` generates a false positive error for an intended unaligned struct member access. The intention of unaligned struct member access is expressed with `__builtin_memcpy()` as done by QEMU or packed struct access as done by Linux. GCC translates such a construct to code to access memory unaligned for architectures like rv64gc as intended but also emits code to enforce the alignment.

The relevant code of QEMU is at: https://gitlab.com/qemu-project/qemu/-/blob/v8.2.1/include/qemu/bswap.h?ref_type=tags
The relevant code of Linux is at: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/asm-generic/unaligned.h?h=v6.7

FYI, this issue is reproducible also with 13.2.0, and the relevant ticket is available at: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114217

To reproduce the issue, compile the code shown below with `-O2 -fsanitize=alignment` for rv64gc:

```C
#include <stdint.h>

typedef uint64_t u64;

/* https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/compiler_attributes.h?h=v6.7 */

/*
 *   gcc: https://gcc.gnu.org/onlinedocs/gcc/Common-Type-Attributes.html#index-packed-type-attribute
 * clang: https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-packed-variable-attribute
 */
#define __packed __attribute__((__packed__))

/* https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/asm-generic/unaligned.h?h=v6.7 */

#define __get_unaligned_t(type, ptr) ({                                         \
        const struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr);      \
        __pptr->x;                                                           \
})

#define __put_unaligned_t(type, val, ptr) do {                                  \
        struct { type x; } __packed *__pptr = (typeof(__pptr))(ptr);            \
        __pptr->x = (val);                                                   \
} while (0)

#define get_unaligned(ptr)      __get_unaligned_t(typeof(*(ptr)), (ptr))
#define put_unaligned(val, ptr) __put_unaligned_t(typeof(*(ptr)), (val), (ptr))

/* https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/btrfs/inode.c?h=v6.7 */

struct dir_entry {
        u64 ino;
        u64 offset;
        unsigned type;
        int name_len;
};

/*
 * This function is intended to perform an unaligned access.
 * GCC emits code for an unaligned operation as intended,
 * but also emits code to assert alignment.
 */
u64 f(struct dir_entry *entry)
{
    return get_unaligned(&entry->offset);
}

/*
 * This function is intended to perform an aligned access.
 * GCC emits code for an aligned operation,
 * and emits code to assert alignment.
 */
u64 g(struct dir_entry *entry)
{
    return entry->offset;
}
```
</pre>
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