Re: DWARF5 DW_AT_data_bit_offset

[Mark Wielaard <mark@klomp.org>]

Hi,

On Fri, Oct 02, 2020 at 06:18:19PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Fri, Oct 02, 2020 at 06:11:06PM +0200, Mark Wielaard escreveu:
> > 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.

Note that GCC11 indeed just switched to producing DWARF5 by default.
It is not released yet, but already in stage4 and Fedora will start
doing a mass-rebuild with it soon to shake out the last remaining bugs.

Cheers,

Mark