* [PATCH v2] [gdb] Support default locations in DWARF5 loclists
@ 2026-04-26 11:40 Gregori Mignerot
2026-04-27 3:12 ` Simon Marchi
0 siblings, 1 reply; 2+ messages in thread
From: Gregori Mignerot @ 2026-04-26 11:40 UTC (permalink / raw)
To: gdb-patches; +Cc: Gregori Mignerot
Add support and a test for default location entries in DWARF5 loclists
(DW_LLE_default_location, section 2.6.2 in the DWARF5 standard)
---
Applied the review comments and added the test for `info address`,
thanks for the quick feedback !
> 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?
I'm not sure either, there isn't a lot of producers or consumers for
this. LLVM and libdwarf don't seem to care about the position of the
default location entry.
I wasn't sure either about that test, it's probably superfluous, removed
it in v2.
gdb/dwarf2/loc.c | 65 ++++++---
.../gdb.dwarf2/loclists-default-location.c | 37 +++++
.../gdb.dwarf2/loclists-default-location.exp | 130 ++++++++++++++++++
gdb/testsuite/lib/dwarf.exp | 33 ++++-
4 files changed, 248 insertions(+), 17 deletions(-)
create mode 100644 gdb/testsuite/gdb.dwarf2/loclists-default-location.c
create mode 100644 gdb/testsuite/gdb.dwarf2/loclists-default-location.exp
diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
index a493a46dcfc..c728fa9c4b1 100644
--- a/gdb/dwarf2/loc.c
+++ b/gdb/dwarf2/loc.c
@@ -83,6 +83,10 @@ enum debug_loc_kind
the applicable base address. */
DEBUG_LOC_OFFSET_PAIR = 4,
+ /* This is followed by a normal location expression as in .debug_loc,
+ to use when no other entry matches the current PC */
+ DEBUG_LOC_DEFAULT_LOCATION = 5,
+
/* An internal value indicating there is insufficient data. */
DEBUG_LOC_BUFFER_OVERFLOW = -1,
@@ -275,9 +279,12 @@ decode_debug_loclists_addresses (dwarf2_per_cu *per_cu,
*new_ptr = loc_ptr;
return DEBUG_LOC_START_END;
+ case DW_LLE_default_location:
+ *new_ptr = loc_ptr;
+ return DEBUG_LOC_DEFAULT_LOCATION;
+
/* Following cases are not supported yet. */
case DW_LLE_startx_endx:
- case DW_LLE_default_location:
default:
return DEBUG_LOC_INVALID_ENTRY;
}
@@ -379,6 +386,11 @@ dwarf2_find_location_expression (const dwarf2_loclist_baton *baton,
unrelocated_addr base_address = baton->base_address;
const gdb_byte *loc_ptr, *buf_end;
+ /* When no DEBUG_LOC_DEFAULT_LOCATION is found
+ return NULL with length 0 (= no location found) */
+ const gdb_byte *default_loc_ptr = NULL;
+ size_t default_locexpr_length = 0;
+
loc_ptr = baton->data;
buf_end = baton->data + baton->size;
@@ -410,8 +422,8 @@ dwarf2_find_location_expression (const dwarf2_loclist_baton *baton,
switch (kind)
{
case DEBUG_LOC_END_OF_LIST:
- *locexpr_length = 0;
- return NULL;
+ *locexpr_length = default_locexpr_length;
+ return default_loc_ptr;
case DEBUG_LOC_BASE_ADDRESS:
base_address = high;
@@ -420,6 +432,7 @@ dwarf2_find_location_expression (const dwarf2_loclist_baton *baton,
case DEBUG_LOC_START_END:
case DEBUG_LOC_START_LENGTH:
case DEBUG_LOC_OFFSET_PAIR:
+ case DEBUG_LOC_DEFAULT_LOCATION:
break;
case DEBUG_LOC_BUFFER_OVERFLOW:
@@ -458,6 +471,14 @@ dwarf2_find_location_expression (const dwarf2_loclist_baton *baton,
loc_ptr += bytes_read;
}
+ if (kind == DEBUG_LOC_DEFAULT_LOCATION)
+ {
+ default_loc_ptr = loc_ptr;
+ default_locexpr_length = length;
+ loc_ptr += length;
+ continue;
+ }
+
if (low == high && unrel_pc == low && at_entry)
{
/* This is entry PC record present only at entry point
@@ -4042,10 +4063,11 @@ loclist_describe_location (struct symbol *symbol, CORE_ADDR addr,
gdb_printf (stream, _(" Base address %s"),
paddress (gdbarch, (CORE_ADDR) base_address));
continue;
-
+
case DEBUG_LOC_START_END:
case DEBUG_LOC_START_LENGTH:
case DEBUG_LOC_OFFSET_PAIR:
+ case DEBUG_LOC_DEFAULT_LOCATION:
break;
case DEBUG_LOC_BUFFER_OVERFLOW:
@@ -4057,6 +4079,29 @@ loclist_describe_location (struct symbol *symbol, CORE_ADDR addr,
gdb_assert_not_reached ("bad debug_loc_kind");
}
+ 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;
+ }
+
+ if (kind == DEBUG_LOC_DEFAULT_LOCATION)
+ {
+ gdb_printf (stream, _(" Default location: "));
+ locexpr_describe_location_1 (symbol, addr, stream, loc_ptr, length,
+ addr_size, offset_size,
+ dlbaton->per_cu, per_objfile);
+ gdb_printf(stream, "\n");
+ loc_ptr += length;
+ continue;
+ }
+
/* Otherwise, a location expression entry. */
if (!dlbaton->from_dwo && kind == DEBUG_LOC_OFFSET_PAIR)
{
@@ -4069,18 +4114,6 @@ loclist_describe_location (struct symbol *symbol, CORE_ADDR addr,
CORE_ADDR low_reloc = per_objfile->relocate (low);
CORE_ADDR high_reloc = per_objfile->relocate (high);
- 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;
- }
-
/* (It would improve readability to print only the minimum
necessary digits of the second number of the range.) */
gdb_printf (stream, _(" Range %s-%s: "),
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..5b7889f3352
--- /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/>. */
+
+static int
+func1 (void)
+{
+ asm ("func1_label: .global func1_label\n");
+ return 1;
+}
+
+static int
+func2 (void)
+{
+ asm ("func2_label: .global func2_label\n");
+ return 2;
+}
+
+int
+main (void)
+{
+ func1 ();
+ func2 ();
+}
diff --git a/gdb/testsuite/gdb.dwarf2/loclists-default-location.exp b/gdb/testsuite/gdb.dwarf2/loclists-default-location.exp
new file mode 100644
index 00000000000..091cc0e01ed
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/loclists-default-location.exp
@@ -0,0 +1,130 @@
+# Copyright 2026 Free Software Foundation, Inc.
+
+# 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/>.
+
+# Test support for DW_LLE_default_location
+
+load_lib dwarf.exp
+
+require dwarf2_support
+
+# Test with 32-bit and 64-bit DWARF.
+foreach_with_prefix is_64 {false true} {
+ if { $is_64 } {
+ standard_testfile .c -dw64.S
+ set testfile ${testfile}-dw64
+ } else {
+ standard_testfile .c -dw32.S
+ set testfile ${testfile}-dw32
+ }
+
+ # Get the addresses / lengths of func1 and func2.
+ lassign [function_range func1 $srcdir/$subdir/$srcfile] func1_addr func1_len
+ lassign [function_range func2 $srcdir/$subdir/$srcfile] func2_addr func2_len
+
+ set asm_file [standard_output_file $srcfile2]
+ Dwarf::assemble $asm_file {
+ global func1_addr func1_len
+ global func2_addr func2_len
+ global is_64
+
+ # The CU uses the DW_FORM_loclistx form to refer to the .debug_loclists
+ # section.
+ cu {
+ version 5
+ is_64 $is_64
+ } {
+ declare_labels int_type
+
+ DW_TAG_compile_unit {
+ DW_AT_loclists_base cu_table DW_FORM_sec_offset
+ } {
+ int_type: DW_TAG_base_type {
+ DW_AT_byte_size 4 DW_FORM_data1
+ DW_AT_encoding @DW_ATE_signed
+ DW_AT_name "int"
+ }
+
+ DW_TAG_variable {
+ DW_AT_name "x"
+ DW_AT_location 0 DW_FORM_loclistx
+ DW_AT_type :$int_type
+ }
+
+ DW_TAG_subprogram {
+ DW_AT_name "func1"
+ DW_AT_low_pc $func1_addr
+ DW_AT_high_pc $func1_len DW_FORM_udata
+ }
+
+ DW_TAG_subprogram {
+ DW_AT_name "func2"
+ DW_AT_low_pc $func2_addr
+ DW_AT_high_pc $func2_len DW_FORM_udata
+ }
+ }
+ }
+
+ 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 last in the list (default_last)
+ list_ {
+ # When in func1.
+ start_end $func1_addr "$func1_addr + $func1_len" {
+ DW_OP_constu 1111
+ DW_OP_stack_value
+ }
+
+ # Applies by default in func2
+ default_location {
+ DW_OP_constu 2222
+ DW_OP_stack_value
+ }
+ }
+ }
+ }
+ }
+
+ if { [prepare_for_testing "failed to prepare" ${testfile} \
+ [list $srcfile $asm_file] {nodebug}] } {
+ return -1
+ }
+
+ if { ![runto_main] } {
+ return
+ }
+
+ gdb_breakpoint "func1"
+ gdb_breakpoint "func2"
+
+ with_test_prefix "non-default location" {
+ gdb_continue_to_breakpoint "func1"
+ gdb_test "print x" " = 1111"
+ }
+
+ with_test_prefix "default location" {
+ gdb_continue_to_breakpoint "func2"
+ gdb_test "print x" " = 2222"
+ }
+
+ gdb_test_sequence "info address x" "info address" [list \
+ "Symbol \"x\" is multi-location:" \
+ "\\s*Range $hex-$hex: a complex DWARF expression:" \
+ "\\s*0: DW_OP_constu 1111" \
+ "\\s*3: DW_OP_stack_value" \
+ "\\s*Default location: a complex DWARF expression:" \
+ "\\s*0: DW_OP_constu 2222" \
+ "\\s*3: DW_OP_stack_value"]
+}
diff --git a/gdb/testsuite/lib/dwarf.exp b/gdb/testsuite/lib/dwarf.exp
index 5358dc317a1..5be57a74d42 100644
--- a/gdb/testsuite/lib/dwarf.exp
+++ b/gdb/testsuite/lib/dwarf.exp
@@ -2432,8 +2432,9 @@ namespace eval Dwarf {
with_override Dwarf::start_length Dwarf::_loclists_start_length {
with_override Dwarf::base_address Dwarf::_loclists_base_address {
with_override Dwarf::start_end Dwarf::_loclists_start_end {
+ with_override Dwarf::default_location Dwarf::_loclists_default_location {
uplevel $_level $body
- }}}
+ }}}}
# Emit end of list.
_op .byte 0x00 "DW_LLE_end_of_list"
@@ -2520,6 +2521,36 @@ namespace eval Dwarf {
incr _debug_loclists_locdesc_count
}
+ # Emit a DW_LLE_default_location entry.
+ #
+ # This proc is meant to be used within proc _loclists_list's body. It is
+ # made available as `default_location` while inside proc _loclists_list's body.
+
+ proc _loclists_default_location { locdesc } {
+ variable _debug_loclists_addr_size
+ variable _debug_loclists_offset_size
+ variable _debug_loclists_table_count
+ variable _debug_loclists_list_count
+ variable _debug_loclists_locdesc_count
+ variable _level
+
+ set locdesc [uplevel $_level [list subst $locdesc]]
+
+ _op .byte 0x05 "DW_LLE_default_location"
+
+ # Length of location description.
+ set locdesc_start_label ".Lloclists_table_${_debug_loclists_table_count}_list_${_debug_loclists_list_count}_locdesc_${_debug_loclists_locdesc_count}_start"
+ set locdesc_end_label ".Lloclists_table_${_debug_loclists_table_count}_list_${_debug_loclists_list_count}_locdesc_${_debug_loclists_locdesc_count}_end"
+ _op .uleb128 "$locdesc_end_label - $locdesc_start_label" "locdesc length"
+
+ define_label $locdesc_start_label
+ set dwarf_version 5
+ _location $locdesc $dwarf_version $_debug_loclists_addr_size $_debug_loclists_offset_size
+ define_label $locdesc_end_label
+
+ incr _debug_loclists_locdesc_count
+ }
+
# Emit a DWARF .debug_macro section.
#
# BODY must be Tcl code that emits the content of the section. It is
--
2.53.0
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH v2] [gdb] Support default locations in DWARF5 loclists
2026-04-26 11:40 [PATCH v2] [gdb] Support default locations in DWARF5 loclists Gregori Mignerot
@ 2026-04-27 3:12 ` Simon Marchi
0 siblings, 0 replies; 2+ messages in thread
From: Simon Marchi @ 2026-04-27 3:12 UTC (permalink / raw)
To: Gregori Mignerot, gdb-patches
On 2026-04-26 07:40, Gregori Mignerot wrote:
> Add support and a test for default location entries in DWARF5 loclists
> (DW_LLE_default_location, section 2.6.2 in the DWARF5 standard)
> ---
> Applied the review comments and added the test for `info address`,
> thanks for the quick feedback !
>
>> 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?
>
> I'm not sure either, there isn't a lot of producers or consumers for
> this. LLVM and libdwarf don't seem to care about the position of the
> default location entry.
> I wasn't sure either about that test, it's probably superfluous, removed
> it in v2.
Are you able to mention some producers that produce default locations,
so we can look?
If we don't need to support non-conforming implementations, I think we
could stick to what the spec says, it would make the code a little bit
simpler: when encountering a DW_LLE_default_location, you can return
this that expression right away, instead of saving it to a local
variable to be returned later.
> diff --git a/gdb/dwarf2/loc.c b/gdb/dwarf2/loc.c
> index a493a46dcfc..c728fa9c4b1 100644
> --- a/gdb/dwarf2/loc.c
> +++ b/gdb/dwarf2/loc.c
> @@ -83,6 +83,10 @@ enum debug_loc_kind
> the applicable base address. */
> DEBUG_LOC_OFFSET_PAIR = 4,
>
> + /* This is followed by a normal location expression as in .debug_loc,
> + to use when no other entry matches the current PC */
Finish the sentence with <dot><space><space>:
/* This is followed by a normal location expression as in .debug_loc,
to use when no other entry matches the current PC. */
The concept of "default location" appeared in DWARF 5, right? If so, I
think this comment should say ".debug_loclists" instead of ".debug_loc".
> @@ -379,6 +386,11 @@ dwarf2_find_location_expression (const dwarf2_loclist_baton *baton,
> unrelocated_addr base_address = baton->base_address;
> const gdb_byte *loc_ptr, *buf_end;
>
> + /* When no DEBUG_LOC_DEFAULT_LOCATION is found
> + return NULL with length 0 (= no location found) */
Finish sentence with <dot><space><space>.
> + const gdb_byte *default_loc_ptr = NULL;
nullptr (instead of NULL) for new (or modified) code.
> @@ -4042,10 +4063,11 @@ loclist_describe_location (struct symbol *symbol, CORE_ADDR addr,
> gdb_printf (stream, _(" Base address %s"),
> paddress (gdbarch, (CORE_ADDR) base_address));
> continue;
> -
> +
Spurious tab added.
> @@ -4057,6 +4079,29 @@ loclist_describe_location (struct symbol *symbol, CORE_ADDR addr,
> gdb_assert_not_reached ("bad debug_loc_kind");
> }
>
> + 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;
> + }
> +
> + if (kind == DEBUG_LOC_DEFAULT_LOCATION)
> + {
> + gdb_printf (stream, _(" Default location: "));
> + locexpr_describe_location_1 (symbol, addr, stream, loc_ptr, length,
> + addr_size, offset_size,
> + dlbaton->per_cu, per_objfile);
Alight the last line with the one before:
locexpr_describe_location_1 (symbol, addr, stream, loc_ptr, length,
addr_size, offset_size,
dlbaton->per_cu, per_objfile);
> + gdb_printf(stream, "\n");
Space before parenthesis.
> diff --git a/gdb/testsuite/gdb.dwarf2/loclists-default-location.exp b/gdb/testsuite/gdb.dwarf2/loclists-default-location.exp
> new file mode 100644
> index 00000000000..091cc0e01ed
> --- /dev/null
> +++ b/gdb/testsuite/gdb.dwarf2/loclists-default-location.exp
> @@ -0,0 +1,130 @@
> +# Copyright 2026 Free Software Foundation, Inc.
> +
> +# 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/>.
> +
> +# Test support for DW_LLE_default_location
> +
> +load_lib dwarf.exp
> +
> +require dwarf2_support
> +
> +# Test with 32-bit and 64-bit DWARF.
> +foreach_with_prefix is_64 {false true} {
> + if { $is_64 } {
> + standard_testfile .c -dw64.S
> + set testfile ${testfile}-dw64
> + } else {
> + standard_testfile .c -dw32.S
> + set testfile ${testfile}-dw32
> + }
> +
> + # Get the addresses / lengths of func1 and func2.
> + lassign [function_range func1 $srcdir/$subdir/$srcfile] func1_addr func1_len
> + lassign [function_range func2 $srcdir/$subdir/$srcfile] func2_addr func2_len
> +
> + set asm_file [standard_output_file $srcfile2]
> + Dwarf::assemble $asm_file {
> + global func1_addr func1_len
> + global func2_addr func2_len
> + global is_64
> +
> + # The CU uses the DW_FORM_loclistx form to refer to the .debug_loclists
> + # section.
> + cu {
> + version 5
> + is_64 $is_64
> + } {
> + declare_labels int_type
> +
> + DW_TAG_compile_unit {
> + DW_AT_loclists_base cu_table DW_FORM_sec_offset
> + } {
> + int_type: DW_TAG_base_type {
> + DW_AT_byte_size 4 DW_FORM_data1
> + DW_AT_encoding @DW_ATE_signed
> + DW_AT_name "int"
> + }
> +
> + DW_TAG_variable {
> + DW_AT_name "x"
> + DW_AT_location 0 DW_FORM_loclistx
> + DW_AT_type :$int_type
> + }
> +
> + DW_TAG_subprogram {
> + DW_AT_name "func1"
> + DW_AT_low_pc $func1_addr
> + DW_AT_high_pc $func1_len DW_FORM_udata
> + }
> +
> + DW_TAG_subprogram {
> + DW_AT_name "func2"
> + DW_AT_low_pc $func2_addr
> + DW_AT_high_pc $func2_len DW_FORM_udata
> + }
> + }
> + }
> +
> + 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 last in the list (default_last)
This comment is stale.
Simon
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2026-04-26 11:40 [PATCH v2] [gdb] Support default locations in DWARF5 loclists Gregori Mignerot
2026-04-27 3:12 ` Simon Marchi
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