Re: [PATCH] [gdb] Support default locations in DWARF5 loclists

[Simon Marchi <simark@simark.ca>]

On 2026-04-25 03:39, Gregori Mignerot wrote:
> @@ -4031,6 +4052,18 @@ loclist_describe_location (struct symbol *symbol, CORE_ADDR addr,
>  						&low, &high, byte_order,
>  						addr_size, signed_addr_p);
>        loc_ptr = new_ptr;
> +      if (dlbaton->dwarf_version < 5)
> +      {
> +	length = extract_unsigned_integer (loc_ptr, 2, byte_order);
> +	loc_ptr += 2;
> +      }
> +      else
> +      {
> +	unsigned int bytes_read;
> +	length = read_unsigned_leb128 (NULL, loc_ptr, &bytes_read);
> +	loc_ptr += bytes_read;
> +      }
> +

This change doesn't seem right.  It will make it so that we'll read a
length even if the "kind" doesn't imply a length.  That is, when kind is
DEBUG_LOC_END_OF_LIST or DEBUG_LOC_BASE_ADDRESS (or
DEBUG_LOC_BUFFER_OVERFLOW / DEBUG_LOC_INVALID_ENTRY).

> diff --git a/gdb/testsuite/gdb.dwarf2/loclists-default-location.c b/gdb/testsuite/gdb.dwarf2/loclists-default-location.c
> new file mode 100644
> index 00000000000..fc388dcb106
> --- /dev/null
> +++ b/gdb/testsuite/gdb.dwarf2/loclists-default-location.c
> @@ -0,0 +1,37 @@
> +/* Copyright (C) 2026 Free Software Foundation, Inc.
> + *
> + *   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/>.  */

This copyright header is not formatted correctly (there shouldn't be the
leading asterisks.  Copy it from another existing source file in this
directory.

> +	loclists {is-64 $is_64} {
> +	    # The lists in this table are accessed by index (DW_FORM_loclistx).
> +	    table {post-header-label cu_table} {
> +		# With the default location first in the list (default_first)
> +		list_ {
> +		    # Applies by default in func2
> +		    default_location {
> +			DW_OP_constu 0x2222
> +			DW_OP_stack_value
> +		    }
> +
> +		    # When in func1.
> +		    start_end $func1_addr "$func1_addr + $func1_len" {
> +			DW_OP_constu 0x1111
> +			DW_OP_stack_value
> +		    }
> +		}

Just wondering, is this valid DWARF?  The spec implies that the default
location has the be last (if present):

    A location list consists of a sequence of zero or more bounded
    location description or base address entries, optionally followed by
    a default location entry, and terminated by an end-of-list entry.

I'm not sure why DWARF 5 requires that, it seems to me like it would
work just as well if the default location was allowed to be anywhere in
the list.  Do you know about producers that do this?

Also, could you add somme checks in the test to exercise
loclist_describe_location?

Simon