[Clang][CodeGen] __bdos off by 4

[Cydox]

Under certain circumstances __bdos can be off by 4 bytes compared to gcc.

Seen in the linux kernel:
https://lore.kernel.org/linux-kernel/[email protected]/

Reproducer from Thorsten Blum (also here: https://godbolt.org/z/vvK9PE1Yq):

#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[] __attribute__((counted_by(a_count)));
};

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;

    printf("%zu", __builtin_dynamic_object_size(acl, 0));
    return 0;
}

[Cydox]

I do have a fix for this I'll submit shortly.

[llvmbot]

@llvm/issue-subscribers-clang-codegen

Author: Jan Hendrik Farr (Cydox)

Under certain circumstances __bdos can be off by 4 bytes compared to gcc.

Seen in the linux kernel:
https://lore.kernel.org/linux-kernel/3D0816D1-0807-4D37-8D5F-3C55CA910FAA@linux.dev/

Reproducer from Thorsten Blum (also here: https://godbolt.org/z/vvK9PE1Yq):

#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[] __attribute__((counted_by(a_count)));
};

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;

    printf("%zu", __builtin_dynamic_object_size(acl, 0));
    return 0;
}

[nickdesaulniers]

cc @nathanchance @kees @bwendling

[Cydox]

Fix is at: https://github.com/Cydox/llvm-project/tree/fix-bdos-offby4
Just adding an additional test based on the reproducer now (and waiting for a full recompile as I just rebased on top of main)