[Bug c/59259] New: [x32] Incorrect packing and/or alignment when using a 64 bit type as array of zero length in a structure

[Bug c/59259] New: [x32] Incorrect packing and/or alignment when using a 64 bit type as array of zero length in a structure

[cjanderson at yandex dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59259

            Bug ID: 59259
           Summary: [x32] Incorrect packing and/or alignment when using a
                    64 bit type as array of zero length in a structure
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cjanderson at yandex dot com

I believe that x32 has 32 bit pointers. I hope that I am not barking up the
wrong tree, but I have a problem with gcc versions 4.7.2 4.8.3 and the current
head on git with the following:

struct blah {
        long long a,b;
};
struct ipt_get_entries {
        unsigned int size;
        struct blah entrytable[0];
};

int x = sizeof(struct ipt_get_entries);

which produces the following when compiled with the -mx32 flag. ie. cc1 -mx32
produces:
    .file    "1.c"
    .globl    x
    .data
    .align 4
    .type    x, @object
    .size    x, 4
x:
    .long    8
    .ident    "GCC: (GNU) 4.9.0 20131120 (experimental)"
    .section    .note.GNU-stack,"",@progbits

When compiled with the -m32 flag the size is 4 and with the -m64 flag the size
is 8.

However, if you change the blah struct to a 32 bit value, ie 

struct blah {
        long long a,b;
};
...

The result for x is 4.

Also, when using -fpack-struct=4 the value of 4 is given, for all of -mx32 -m32
and -m64. 

The behaviour here seems to be that x32 is doing the same as for 64 bit. Is
this  correct.

[Bug c/59259] [x32] Incorrect packing and/or alignment when using a 64 bit type as array of zero length in a structure

[cjanderson at yandex dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59259

--- Comment #1 from cjanderson at yandex dot com ---
Created attachment 31276
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=31276&action=edit
Test file incorrectly compiled

[Bug c/59259] [x32] Incorrect packing and/or alignment when using a 64 bit type as array of zero length in a structure

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59259

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The alignment of ipt_get_entries for x32 is 64bit so the size is correct. This
is different from x86 where the alignment of long long in struct is 32bits.

[Bug c/59259] [x32] Incorrect packing and/or alignment when using a 64 bit type as array of zero length in a structure

[cjanderson at yandex dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59259

cjanderson at yandex dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |CLOSED

--- Comment #3 from cjanderson at yandex dot com ---
Thanks. 
> The alignment of ipt_get_entries for x32 is 64bit so the size is correct.
> This is different from x86 where the alignment of long long in struct is
> 32bits.

[Bug c/59259] [x32] Incorrect packing and/or alignment when using a 64 bit type as array of zero length in a structure

[cjanderson at yandex dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59259

cjanderson at yandex dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|CLOSED                      |UNCONFIRMED
         Resolution|INVALID                     |---

--- Comment #4 from cjanderson at yandex dot com ---
Sorry, this still troubles me. Isn't the last structure a pointer, ie:
struct blah entrytable[0];
is really just a pointer, and hence in most cases just an 32 bit value (I know
there are some tricks), but if it is a pointer then the alignment should surely
be the same as for ia32?

[Bug c/59259] [x32] Incorrect packing and/or alignment when using a 64 bit type as array of zero length in a structure

[schwab@linux-m68k.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59259

Andreas Schwab <schwab@linux-m68k.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #5 from Andreas Schwab <schwab@linux-m68k.org> ---
A flexible array member is not a pointer.

[Bug c/59259] [x32] Incorrect packing and/or alignment when using a 64 bit type as array of zero length in a structure

[cjanderson at yandex dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59259

cjanderson at yandex dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |CLOSED

--- Comment #6 from cjanderson at yandex dot com ---
ok