From: Mark Wielaard <mark@klomp.org>
To: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: dwarves@vger.kernel.org
Subject: Re: DWARF5 DW_AT_data_bit_offset
Date: Thu, 21 Jan 2021 12:35:16 +0100 [thread overview]
Message-ID: <20210121113516.GA8492@wildebeest.org> (raw)
In-Reply-To: <20201002211819.GA127275@kernel.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
next prev parent reply other threads:[~2021-01-21 11:47 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-02 16:11 DWARF5 DW_AT_data_bit_offset Mark Wielaard
2020-10-02 21:18 ` Arnaldo Carvalho de Melo
2021-01-21 11:35 ` Mark Wielaard [this message]
2021-01-28 12:11 ` [PATCH/RFC] " Arnaldo Carvalho de Melo
2021-01-28 12:54 ` Daniel P. Berrangé
2021-01-28 13:48 ` Arnaldo Carvalho de Melo
2021-01-28 13:54 ` Arnaldo Carvalho de Melo
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