Re: [PATCH/RFC] Re: DWARF5 DW_AT_data_bit_offset

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

Em Thu, Jan 28, 2021 at 12:54:43PM +0000, Daniel P. Berrangé escreveu:
> On Thu, Jan 28, 2021 at 09:11:22AM -0300, Arnaldo Carvalho de Melo wrote:
> > Em Thu, Jan 21, 2021 at 12:35:16PM +0100, Mark Wielaard escreveu:
> > > 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).
> > 
> > <SNIP>
> > 
> > > > > $ 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.
> > 
> > So, 1.20 will have the fix below, please check if what is in:
> > 
> > https://git.kernel.org/pub/scm/devel/pahole/pahole.git/log/?h=DW_AT_data_bit_offset
> > 
> > Fixes it for you, the cset with all the tests performed is this one:
> > 
> > https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?h=DW_AT_data_bit_offset&id=a77f039bb49bc97badf938049245f013fe3de4aa
> > 
> > Now looking at https://bugzilla.redhat.com/show_bug.cgi?id=1919965 ...
> 
> The XDR generated code example I give in that bug *does* appear
> to be fixed when using your  DW_AT_data_bit_offset branch.

Thanks for checking, I'll double check on my side now, got sidetracked
in the last hour with perf patch processing :-)

- Arnaldo