Re: DWARF5 DW_AT_data_bit_offset

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, Oct 02, 2020 at 06:11:06PM +0200, Mark Wielaard escreveu:
> 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.

Thanks for the detailed report, I'm releasing v1.18 right now, will look
into that for v1.19.

- Arnaldo