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binutils-gdb/gdb/dwarf2/attribute.c
Tom Tromey 21b25b168d Fix regression with DW_AT_bit_offset handling 
Internal AdaCore testing using -gdwarf-4 found a spot where GCC will
emit a negative DW_AT_bit_offset.  However, my recent signed/unsigned
changes assumed that this value had to be positive.

I feel this bug somewhat invalidates my previous thinking about how
DWARF attributes should be handled.

In particular, both GCC and LLVM at understand that a negative bit
offset can be generated -- but for positive offsets they might use a
smaller "data" form, which is expected not to be sign-extended.  LLVM
has similar code but GCC does:

  if (bit_offset < 0)
    add_AT_int (die, DW_AT_bit_offset, bit_offset);
  else
    add_AT_unsigned (die, DW_AT_bit_offset, (unsigned HOST_WIDE_INT) bit_offset);

What this means is that this attribute is "signed but default
unsigned".

To fix this, I've added a new attribute::confused_constant method.
This should be used when a constant value might be signed, but where
narrow forms (e.g., DW_FORM_data1) should *not* cause sign extension.

I examined the GCC and LLVM DWARF writers to come up with the list of
attributes where this applies, namely DW_AT_bit_offset,
DW_AT_const_value and DW_AT_data_member_location (GCC only, but LLVM
always emits it as unsigned, so we're safe here).

This patch corrects the bug and imports the relevant test case.

Regression tested on x86-64 Fedora 41.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32680
Bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118837
Approved-By: Simon Marchi <simon.marchi@efficios.com>
2025-06-06 10:13:23 -06:00

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/* DWARF attributes
Copyright (C) 1994-2025 Free Software Foundation, Inc.
Adapted by Gary Funck (gary@intrepid.com), Intrepid Technology,
Inc. with support from Florida State University (under contract
with the Ada Joint Program Office), and Silicon Graphics, Inc.
Initial contribution by Brent Benson, Harris Computer Systems, Inc.,
based on Fred Fish's (Cygnus Support) implementation of DWARF 1
support.
This file is part of GDB.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */
#include "dwarf2/attribute.h"
#include "dwarf2/stringify.h"
#include "complaints.h"
/* See attribute.h. */
unrelocated_addr
attribute::as_address () const
{
unrelocated_addr addr;
gdb_assert (!requires_reprocessing);
if (form != DW_FORM_addr && form != DW_FORM_addrx
&& form != DW_FORM_GNU_addr_index)
{
/* Aside from a few clearly defined exceptions, attributes that
contain an address must always be in DW_FORM_addr form.
Unfortunately, some compilers happen to be violating this
requirement by encoding addresses using other forms, such
as DW_FORM_data4 for example. For those broken compilers,
we try to do our best, without any guarantee of success,
to interpret the address correctly. It would also be nice
to generate a complaint, but that would require us to maintain
a list of legitimate cases where a non-address form is allowed,
as well as update callers to pass in at least the CU's DWARF
version. This is more overhead than what we're willing to
expand for a pretty rare case. */
addr = (unrelocated_addr) u.unsnd;
}
else
addr = u.addr;
return addr;
}
/* See attribute.h. */
bool
attribute::form_is_string () const
{
return (form == DW_FORM_strp || form == DW_FORM_line_strp
|| form == DW_FORM_string
|| form == DW_FORM_strx
|| form == DW_FORM_strx1
|| form == DW_FORM_strx2
|| form == DW_FORM_strx3
|| form == DW_FORM_strx4
|| form == DW_FORM_GNU_str_index
|| form == DW_FORM_GNU_strp_alt
|| form == DW_FORM_strp_sup);
}
/* See attribute.h. */
const char *
attribute::as_string () const
{
gdb_assert (!requires_reprocessing);
if (form_is_string ())
return u.str;
return nullptr;
}
/* See attribute.h. */
bool
attribute::form_is_block () const
{
return (form == DW_FORM_block1
|| form == DW_FORM_block2
|| form == DW_FORM_block4
|| form == DW_FORM_block
|| form == DW_FORM_exprloc
|| form == DW_FORM_data16);
}
/* See attribute.h. */
bool
attribute::form_is_section_offset () const
{
return (form == DW_FORM_data4
|| form == DW_FORM_data8
|| form == DW_FORM_sec_offset
|| form == DW_FORM_loclistx);
}
/* See attribute.h. */
bool
attribute::form_is_constant () const
{
switch (form)
{
case DW_FORM_sdata:
case DW_FORM_udata:
case DW_FORM_data1:
case DW_FORM_data2:
case DW_FORM_data4:
case DW_FORM_data8:
case DW_FORM_implicit_const:
return true;
default:
return false;
}
}
/* See attribute.h. */
void
attribute::get_ref_die_offset_complaint () const
{
complaint (_("unsupported die ref attribute form: '%s'"),
dwarf_form_name (form));
}
/* See attribute.h. */
LONGEST
attribute::constant_value (int default_value) const
{
if (form == DW_FORM_sdata || form == DW_FORM_implicit_const)
return u.snd;
else if (form == DW_FORM_udata
|| form == DW_FORM_data1
|| form == DW_FORM_data2
|| form == DW_FORM_data4
|| form == DW_FORM_data8)
return u.unsnd;
else
{
/* For DW_FORM_data16 see attribute::form_is_constant. */
complaint (_("Attribute value is not a constant (%s)"),
dwarf_form_name (form));
return default_value;
}
}
/* See attribute.h. */
std::optional<ULONGEST>
attribute::unsigned_constant () const
{
if (form_is_strictly_signed ())
{
if (u.snd >= 0)
return u.snd;
complaint (_("Attribute value is not unsigned (%s)"),
dwarf_form_name (form));
}
else if (form_is_constant ())
return u.unsnd;
/* For DW_FORM_data16 see attribute::form_is_constant. */
complaint (_("Attribute value is not a constant (%s)"),
dwarf_form_name (form));
return {};
}
/* See attribute.h. */
std::optional<LONGEST>
attribute::signed_constant () const
{
if (form_is_strictly_signed ())
return u.snd;
switch (form)
{
case DW_FORM_data8:
case DW_FORM_udata:
/* Not sure if DW_FORM_udata should be handled or not. Anyway
for DW_FORM_data8, there's no need to sign-extend. */
return u.snd;
case DW_FORM_data1:
return sign_extend (u.unsnd, 8);
case DW_FORM_data2:
return sign_extend (u.unsnd, 16);
case DW_FORM_data4:
return sign_extend (u.unsnd, 32);
}
/* For DW_FORM_data16 see attribute::form_is_constant. */
complaint (_("Attribute value is not a constant (%s)"),
dwarf_form_name (form));
return {};
}
/* See attribute.h. */
std::optional<LONGEST>
attribute::confused_constant () const
{
if (form_is_strictly_signed ())
return u.snd;
else if (form_is_constant ())
return u.unsnd;
/* For DW_FORM_data16 see attribute::form_is_constant. */
complaint (_("Attribute value is not a constant (%s)"),
dwarf_form_name (form));
return {};
}
/* See attribute.h. */
bool
attribute::form_is_unsigned () const
{
return (form == DW_FORM_ref_addr
|| form == DW_FORM_GNU_ref_alt
|| form == DW_FORM_ref_sup4
|| form == DW_FORM_ref_sup8
|| form == DW_FORM_data2
|| form == DW_FORM_data4
|| form == DW_FORM_data8
|| form == DW_FORM_sec_offset
|| form == DW_FORM_data1
|| form == DW_FORM_flag
|| form == DW_FORM_flag_present
|| form == DW_FORM_udata
|| form == DW_FORM_rnglistx
|| form == DW_FORM_loclistx
|| form == DW_FORM_ref1
|| form == DW_FORM_ref2
|| form == DW_FORM_ref4
|| form == DW_FORM_ref8
|| form == DW_FORM_ref_udata);
}
/* See attribute.h. */
bool
attribute::form_is_strictly_signed () const
{
return form == DW_FORM_sdata || form == DW_FORM_implicit_const;
}
/* See attribute.h. */
bool
attribute::form_requires_reprocessing () const
{
return (form == DW_FORM_strx
|| form == DW_FORM_strx1
|| form == DW_FORM_strx2
|| form == DW_FORM_strx3
|| form == DW_FORM_strx4
|| form == DW_FORM_GNU_str_index
|| form == DW_FORM_addrx
|| form == DW_FORM_GNU_addr_index
|| form == DW_FORM_rnglistx
|| form == DW_FORM_loclistx);
}
/* See attribute.h. */
dwarf_defaulted_attribute
attribute::defaulted () const
{
std::optional<ULONGEST> value = unsigned_constant ();
if (value.has_value ())
{
switch (*value)
{
case DW_DEFAULTED_no:
case DW_DEFAULTED_in_class:
case DW_DEFAULTED_out_of_class:
return (dwarf_defaulted_attribute) *value;
default:
complaint (_("unrecognized DW_AT_defaulted value (%s)"),
plongest (*value));
break;
}
}
return DW_DEFAULTED_no;
}
/* See attribute.h. */
dwarf_virtuality_attribute
attribute::as_virtuality () const
{
std::optional<ULONGEST> value = unsigned_constant ();
if (value.has_value ())
{
switch (*value)
{
case DW_VIRTUALITY_none:
case DW_VIRTUALITY_virtual:
case DW_VIRTUALITY_pure_virtual:
return (dwarf_virtuality_attribute) *value;
default:
complaint (_("unrecognized DW_AT_virtuality value (%s)"),
plongest (*value));
break;
}
}
return DW_VIRTUALITY_none;
}
/* See attribute.h. */
bool
attribute::as_boolean () const
{
if (form == DW_FORM_flag_present)
return true;
else if (form == DW_FORM_flag)
return u.unsnd != 0;
/* Using signed_constant here will work even for the weird case
where a negative value is provided. Probably doesn't matter but
also seems harmless. */
return signed_constant ().value_or (0) != 0;
}
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