Avoid bad breakpoints with --gc-sections

We found a case where --gc-sections can cause gdb to set an invalid
breakpoint.  In the included test case, gdb will set a breakpoint with
two locations, one of which is 0x0.

The code in lnp_state_machine::check_line_address is intended to
filter out this sort of problem, but in this case, the entire CU is
empty, causing unrelocated_lowpc==0x0 -- which circumvents the check.

It seems to me that if a CU is empty like this, then it is ok to
simply ignore the line table, as there won't be any locations anyway.

gdb/dwarf2/read.c

@@ -10630,8 +10630,13 @@ read_file_scope (struct die_info *die, struct dwarf2_cu *cu)
 
   /* Decode line number information if present.  We do this before
      processing child DIEs, so that the line header table is available
-     for DW_AT_decl_file.  */
-  handle_DW_AT_stmt_list (die, cu, fnd, lowpc);
+     for DW_AT_decl_file.  The PC check is here because, if LOWPC and
+     HIGHPC are both 0x0, then there won't be any interesting code in
+     the CU, but a check later on (in
+     lnp_state_machine::check_line_address) will fail to properly
+     exclude an entry that was removed via --gc-sections.  */
+  if (lowpc != highpc)
+    handle_DW_AT_stmt_list (die, cu, fnd, lowpc);
 
   /* Process all dies in compilation unit.  */
   if (die->child != NULL)

gdb/testsuite/gdb.ada/inline-section-gc.exp

@@ -0,0 +1,41 @@
+# Copyright 2022 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/>.
+
+load_lib "ada.exp"
+
+if { [skip_ada_tests] } { return -1 }
+
+standard_ada_testfile caller
+
+set options {
+    debug
+    optimize=-O2
+    additional_flags=-ffunction-sections
+    additional_flags=-largs
+    additional_flags=-Wl,--gc-sections
+    additional_flags=-margs
+    additional_flags=-gnatn
+}
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable $options] != ""} {
+    return -1
+}
+
+clean_restart ${testfile}
+
+set bp_location [gdb_get_line_number "BREAK" ${testdir}/callee.adb]
+# The bug here was that gdb would set a breakpoint with two locations,
+# one of them at 0x0.
+gdb_test "break callee.adb:$bp_location" \
+    "Breakpoint $decimal at $hex: file .*callee.adb, line $bp_location."

gdb/testsuite/gdb.ada/inline-section-gc/callee.adb

@@ -0,0 +1,23 @@
+--  Copyright 2022 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/>.
+
+with System;
+
+procedure Callee is
+   Data : Integer;
+   for Data'Address use System'To_Address (16#4000_0000#);
+begin
+   Data := 0; --  BREAK
+end Callee;

gdb/testsuite/gdb.ada/inline-section-gc/callee.ads

@@ -0,0 +1,17 @@
+--  Copyright 2022 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/>.
+
+procedure Callee;
+pragma Inline (Callee);

gdb/testsuite/gdb.ada/inline-section-gc/caller.adb

@@ -0,0 +1,21 @@
+--  Copyright 2022 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/>.
+
+with Callee;
+
+procedure Caller is
+begin
+   Callee;
+end Caller;