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

[Gregori Mignerot <gregori.mignerot@laposte.net>]

Add support and a test for default location entries in DWARF5 loclists
(DW_LLE_default_location, section 2.6.2 in the DWARF5 standard)
---
 gdb/dwarf2/loc.c                              |  60 ++++++--
 .../gdb.dwarf2/loclists-default-location.c    |  37 +++++
 .../gdb.dwarf2/loclists-default-location.exp  | 144 ++++++++++++++++++
 gdb/testsuite/lib/dwarf.exp                   |  33 +++-
 4 files changed, 258 insertions(+), 16 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..214c81ec4ea 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
@@ -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;
+      }
+
       switch (kind)
 	{
 	case DEBUG_LOC_END_OF_LIST:
@@ -4043,6 +4076,15 @@ loclist_describe_location (struct symbol *symbol, CORE_ADDR addr,
 		      paddress (gdbarch, (CORE_ADDR) base_address));
 	  continue;
 
+	case 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;
+	
 	case DEBUG_LOC_START_END:
 	case DEBUG_LOC_START_LENGTH:
 	case DEBUG_LOC_OFFSET_PAIR:
@@ -4069,18 +4111,6 @@ 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..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/>.  */
+
+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..76d46a4bc18
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/loclists-default-location.exp
@@ -0,0 +1,144 @@
+# 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 $is_64
+	} {
+	    declare_labels int_type
+
+	    DW_TAG_compile_unit {
+		DW_AT_loclists_base cu_table DW_FORM_sec_offset
+	    } {
+		int_type: DW_TAG_base_type {
+		    DW_AT_byte_size 4 DW_FORM_data1
+		    DW_AT_encoding @DW_ATE_signed
+		    DW_AT_name "int"
+		}
+
+		DW_TAG_variable {
+		    DW_AT_name "default_first"
+		    DW_AT_location 0 DW_FORM_loclistx
+		    DW_AT_type :$int_type
+		}
+
+		DW_TAG_variable {
+		    DW_AT_name "default_last"
+		    DW_AT_location 1 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 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
+		    }
+		}
+
+		# 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 0x3333
+			DW_OP_stack_value
+		    }
+
+		    # Applies by default in func2
+		    default_location {
+			DW_OP_constu 0x4444
+			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 default_first" " = 0x1111"
+	gdb_test "print /x default_last" " = 0x3333"
+    }
+
+    with_test_prefix "default location" {
+	gdb_continue_to_breakpoint "func2"
+	gdb_test "print /x default_first" " = 0x2222"
+	gdb_test "print /x default_last" " = 0x4444"
+    }
+}
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