[Bug c/14786] New: Struct misaligned containing unsigned char bit-field

[Bug c/14786] New: Struct misaligned containing unsigned char bit-field

[ryanma at cpsc dot ucalgary dot ca]

Description:

According to RS/6000 Alignment Rules, structs containing   
bit fields are 4-byte (word) aligned. The following testcase 
illustrates that this rule is violated, when Substruct B is not  
word aligned. 

Testcase:

extern int      printf(const char *, ...);
typedef unsigned char UCHAR;

typedef struct {
    UCHAR               a   : 1 ;
    UCHAR               b   : 1 ;
    UCHAR               c   : 1 ;
    UCHAR               d   : 1 ;
    UCHAR               e   : 1 ;
    UCHAR               f   : 1 ;
    UCHAR               g   : 1 ;
    UCHAR               h   : 1 ;
    UCHAR               i   : 1 ;
    UCHAR               J   : 1 ;
    UCHAR               K   : 6 ;
} SubStruct ;

typedef struct {
    UCHAR               A[5];
    SubStruct           B   ;
    UCHAR               C   ;
    UCHAR               D[4];
} Struct ;

int main() {
  Struct data;
  printf("sizeof(SubStruct) =%d\n",sizeof(SubStruct));
  printf("sizeof(Struct)    =%d\n",sizeof(Struct));
  printf(" offset of B      =%d\n",
        ((unsigned int)&(data.B)-(unsigned int)&data));
  printf(" offset of C      =%d\n",
        ((unsigned int)&(data.C)-(unsigned int)&data));
  printf(" offset of D      =%d\n",
        ((unsigned int)&(data.D)-(unsigned int)&data));
  return 0;
}

Actual Output:

$ gcc t.c
$ ./a.out
Address of Struct =2ff22a20
sizeof(SubStruct) =2
sizeof(Struct)    =12
 offset of B      =5
 offset of C      =7
 offset of D      =8
$

Expected Output:

$ gcc t.c
$ ./a.out
Address of Struct =2ff22a20
sizeof(SubStruct) =2
sizeof(Struct)    =20
 offset of B      =8
 offset of C      =12
 offset of D      =13


GCC Version:

$ /home/ryan/gcc-3.3.2/aix51/bin/gcc -v
Reading specs from /home/ryan/gcc-3.3.2/aix51/bin/../lib/gcc-lib/powerpc-ibm-
aix5.1.0.0/3.3.
2/specs
Configured with: ../gcc-3.3.2/configure --disable-nls
Thread model: aix
gcc version 3.3.2

Machine Type:

Model: 7044 -170
Processor: 1 x pwr3 
Processor Speed: 400 MHz
Memory: 1 GB
AIX Level: AIX 5.1 ML 5
Hardware Arch: 64 bit
Kernel Arch: 32 bit

-- 
           Summary: Struct misaligned containing unsigned char bit-field
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ryanma at cpsc dot ucalgary dot ca
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: powerpc-ibm-aix5.1.0.0
  GCC host triplet: powerpc-ibm-aix5.1.0.0
GCC target triplet: powerpc-ibm-aix5.1.0.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14786

[Bug c/14786] Struct misaligned containing unsigned char bit-field

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-03-30 19:36 -------
>From Mac OS PEF ABI which is based on the PowerOpen ABI which is the ABI for AIX:
The preferred alignment of a bit field is the same as its declared type.

So if the same wording is in the Power Open ABI then this is invalid as it is correctly aligned and bit-
fields are aligned as the same as their declared type.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14786

[Bug c/14786] Struct misaligned containing unsigned char bit-field

[ryanma at cpsc dot ucalgary dot ca]

------- Additional Comments From ryanma at cpsc dot ucalgary dot ca  2004-03-31 15:35 -------
http://publib.boulder.ibm.com/infocenter/comphelp/topic/com.ibm.vacpp6a.doc/prog
uide/ref/rvclralg.htm

The xlc compiler exhibits the expected behavior.

$ xlc t.c -qlanglvl=extended
"t.c", line 5.25: 1506-159 (E) Bit-field type specified for a is not valid. 
Type unsigned assumed.
"t.c", line 6.25: 1506-159 (E) Bit-field type specified for b is not valid. 
Type unsigned assumed.
"t.c", line 7.25: 1506-159 (E) Bit-field type specified for c is not valid. 
Type unsigned assumed.
"t.c", line 8.25: 1506-159 (E) Bit-field type specified for d is not valid. 
Type unsigned assumed.
"t.c", line 9.25: 1506-159 (E) Bit-field type specified for e is not valid. 
Type unsigned assumed.
"t.c", line 10.25: 1506-159 (E) Bit-field type specified for f is not valid. 
Type unsigned assumed.
"t.c", line 11.25: 1506-159 (E) Bit-field type specified for g is not valid. 
Type unsigned assumed.
"t.c", line 12.25: 1506-159 (E) Bit-field type specified for h is not valid. 
Type unsigned assumed.
"t.c", line 13.25: 1506-159 (E) Bit-field type specified for i is not valid. 
Type unsigned assumed.
"t.c", line 14.25: 1506-159 (E) Bit-field type specified for J is not valid. 
Type unsigned assumed.
"t.c", line 15.25: 1506-159 (E) Bit-field type specified for K is not valid. 
Type unsigned assumed.
./a.out
$ ./a.out
Address of Struct =2ff22a20
sizeof(SubStruct) =4
sizeof(Struct)    =20
 offset of B      =8
 offset of C      =12
 offset of D      =13


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14786

[Bug target/14786] Struct misaligned containing unsigned char bit-field

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-04-04 06:37 -------
Note in OpenPower ABI, bitfields are only defined for short and int.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c                           |target
           Keywords|                            |ABI, wrong-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14786

[Bug target/14786] Struct misaligned containing unsigned char bit-field

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2004-06-03 22:50 -------
Invalid as you see from the XLC output that bitfields alignment is not defined on char.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14786