Hi,
Seems pahole with a recent version of elfutils libdw already handles
most DWARF5 encodings. One thing it doesn't handle yet is
DW_AT_data_bit_offset (this is actually a DWARF4 thing, but gcc only
emits it for -gdwarf-5).
Note that the actual bit offset for the different attributes is defined
differently:
DW_AT_bit_offset: The bit offset attribute describes the offset in bits
of the high order bit of a value of the given type from the high order
bit of the storage unit used to contain that value.
DW_AT_data_bit_offset: the value is an integer constant that specifies
the number of bits from the beginning of the containing entity to the
beginning of the data member.
If there is a DW_AT_data_bit_offset instead of a
DW_AT_data_member_location then there will be no DW_AT_byte_size and no
DW_AT_bit_offset.
DWARF5 has some example for big and little endian in D.2.8 C/C++ Bit-
Field Examples
dwarf_loader.c already seems to do the right thing for little-endian
machines with DW_AT_data_member_location and DW_AT_bit_offset. For
DW_AT_data_bit_offset it doesn't have to do this fixup because it is
already defined as you would expect.
Example that shows the issue:
$ cat bf.c
struct pea
{
int type;
long a:1, b:1, c:1;
};
struct pea p;
$ gcc -gdwarf-4 -c bf.c
$ ./pahole ./bf.o
struct pea {
int type; /* 0 4 */
/* Bitfield combined with previous fields */
long int a:1; /* 0:32 8 */
long int b:1; /* 0:33 8 */
long int c:1; /* 0:34 8 */
/* size: 8, cachelines: 1, members: 4 */
/* bit_padding: 29 bits */
/* last cacheline: 8 bytes */
};
$ gcc -gdwarf-5 -c bf.c
$ ./pahole ./bf.o
DW_AT_<0xd>=0x21
DW_AT_<0xd>=0x21
DW_AT_<0xd>=0x21
struct pea {
int type; /* 0 4 */
static long int a; /* 0 0 */
static long int b; /* 0 0 */
static long int c; /* 0 0 */
/* size: 8, cachelines: 1, members: 1, static members: 3 */
/* padding: 4 */
/* last cacheline: 8 bytes */
};
Note that GCC11 might default to DWARF5.
Cheers,
Mark
