[gdb/symtab] Handle PU without import in "save gdb-index"

Consider the test-case added in this patch, with resulting dwarf:
...
  Compilation Unit @ offset 0xc7:
   Length:        0x2c (32-bit)
   Version:       4
   Abbrev Offset: 0x64
   Pointer Size:  8
 <0><d2>: Abbrev Number: 2 (DW_TAG_partial_unit)
    <d3>   DW_AT_language    : 2        (non-ANSI C)
    <d4>   DW_AT_name        : imported_unit.c
 <1><e4>: Abbrev Number: 3 (DW_TAG_base_type)
    <e5>   DW_AT_byte_size   : 4
    <e6>   DW_AT_encoding    : 5        (signed)
    <e7>   DW_AT_name        : int
 <1><eb>: Abbrev Number: 4 (DW_TAG_subprogram)
    <ec>   DW_AT_name        : main
    <f1>   DW_AT_type        : <0xe4>
    <f5>   DW_AT_external    : 1
 <1><f6>: Abbrev Number: 0
  Compilation Unit @ offset 0xf7:
   Length:        0x2c (32-bit)
   Version:       4
   Abbrev Offset: 0x85
   Pointer Size:  8
 <0><102>: Abbrev Number: 2 (DW_TAG_compile_unit)
    <103>   DW_AT_language    : 2       (non-ANSI C)
    <104>   DW_AT_name        : <artificial>
 <1><111>: Abbrev Number: 3 (DW_TAG_subprogram)
    <112>   DW_AT_abstract_origin: <0xeb>
    <116>   DW_AT_low_pc      : 0x4004a7
    <11e>   DW_AT_high_pc     : 0x4004b2
 <1><126>: Abbrev Number: 0
...

When run with target board cc-with-gdb-index, we run into:
...
(gdb) break main
warning: (Internal error: pc 0x4004a7 in read in CU, but not in symtab.)
<repeat>
warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
<repeat>
Breakpoint 1 at 0x4004ab
(gdb) PASS: gdb.dwarf2/imported-unit-runto-main.exp: setting breakpoint at main
run
Starting program: /data/gdb_versions/devel/build/gdb/testsuite/outputs/gdb.dwarf2/imported-unit-runto-main/imported-unit-runto-main
warning: (Internal error: pc 0x4004a7 in read in CU, but not in symtab.)
<repeat>
warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
<repeat>

Breakpoint 1, warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
<repeat>
0x00000000004004ab in main ()
warning: (Internal error: pc 0x4004ab in read in CU, but not in symtab.)
<repeat>
(gdb) FAIL: gdb.dwarf2/imported-unit-runto-main.exp: running to main in runto
...

Looking at the .gdb_index section contents using objdump --dwarf=gdb_index, we
have:
...
CU table:
[  0] 0x0 - 0x2d
[  1] 0x2e - 0xa4
[  2] 0xa5 - 0xc6
[  3] 0xf7 - 0x126
[  4] 0x127 - 0x2de
[  5] 0x2df - 0x300

Address table:
00000000004004a7 00000000004004b2 4

Symbol table:
[489] main: 4 [global, function]
...
We see that both the main symbol, and main address range map to CU 4, which has
offset range 0x127 - 0x2de, while main actually is contained in CU 3 at offset
range 0xf7 - 0x126.

This is caused by this continue in write_gdbindex, which triggers for the PU:
...
      /* CU of a shared file from 'dwz -m' may be unused by this main file.
        It may be referenced from a local scope but in such case it does not
        need to be present in .gdb_index.  */
      if (psymtab == NULL)
       continue;
...
The continue causes the PU to be skipped in the CU table (we can see that the
PU offset range 0xc7-0xf6 is missing) but the references are not taking that
into account.

I've tried fixing this in the optimal way, by updating the references, but ran
into trouble when follow_die_offset tries to find the CU for the inter-CU
ref.  Because the PU is missing from the CU table,
dwarf2_find_containing_comp_unit bisects to the wrong CU.

Fix this by not skipping the PU in the CU table.

Build and reg-tested on x86_64-linux, with native and target boards
cc-with-gdb-index, cc-with-dwz and cc-with-dwz-m.

gdb/ChangeLog:

2020-04-16  Tom de Vries  <tdevries@suse.de>

	PR symtab/25791
	* dwarf2/index-write.c (write_gdbindex): Generate CU table entries for
	CUs without psymtab.

gdb/testsuite/ChangeLog:

2020-04-16  Tom de Vries  <tdevries@suse.de>

	PR symtab/25791
	* gdb.dwarf2/gdb-add-index.exp (add_gdb_index): Move ...
	(ensure_gdb_index): and factor out and move ...
	* lib/gdb.exp (add_gdb_index, ensure_gdb_index): ... here.
	* gdb.dwarf2/imported-unit-runto-main.exp: New file.

6 files changed, 160 insertions, 48 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 0130d06..b019ca9 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2020-04-16  Tom de Vries  <tdevries@suse.de>
+
+	PR symtab/25791
+	* dwarf2/index-write.c (write_gdbindex): Generate CU table entries for
+	CUs without psymtab.
+
 2020-04-16  Kevin Buettner  <kevinb@redhat.com>
 
 	* python/python.c (do_start_initialization): Don't call
diff --git a/gdb/dwarf2/index-write.c b/gdb/dwarf2/index-write.c
index 8c933dc..b6a13a0 100644
--- a/gdb/dwarf2/index-write.c
+++ b/gdb/dwarf2/index-write.c
@@ -1426,18 +1426,15 @@ write_gdbindex (struct dwarf2_per_objfile *dwarf2_per_objfile, FILE *out_file,
 	= dwarf2_per_objfile->all_comp_units[i];
       partial_symtab *psymtab = per_cu->v.psymtab;
 
-      /* CU of a shared file from 'dwz -m' may be unused by this main file.
-	 It may be referenced from a local scope but in such case it does not
-	 need to be present in .gdb_index.  */
-      if (psymtab == NULL)
-	continue;
-
-      if (psymtab->user == NULL)
-	recursively_write_psymbols (objfile, psymtab, &symtab,
-				    psyms_seen, i);
+      if (psymtab != NULL)
+	{
+	  if (psymtab->user == NULL)
+	    recursively_write_psymbols (objfile, psymtab, &symtab,
+					psyms_seen, i);
 
-      const auto insertpair = cu_index_htab.emplace (psymtab, i);
-      gdb_assert (insertpair.second);
+	  const auto insertpair = cu_index_htab.emplace (psymtab, i);
+	  gdb_assert (insertpair.second);
+	}
 
       /* The all_comp_units list contains CUs read from the objfile as well as
 	 from the eventual dwz file.  We need to place the entry in the
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index f12a5d7..eaa96b0 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,13 @@
 2020-04-16  Tom de Vries  <tdevries@suse.de>
 
+	PR symtab/25791
+	* gdb.dwarf2/gdb-add-index.exp (add_gdb_index): Move ...
+	(ensure_gdb_index): and factor out and move ...
+	* lib/gdb.exp (add_gdb_index, ensure_gdb_index): ... here.
+	* gdb.dwarf2/imported-unit-runto-main.exp: New file.
+
+2020-04-16  Tom de Vries  <tdevries@suse.de>
+
 	* gdb.base/maint-expand-symbols-header-file.exp: Set language before
 	loading exec.
 
diff --git a/gdb/testsuite/gdb.dwarf2/gdb-add-index.exp b/gdb/testsuite/gdb.dwarf2/gdb-add-index.exp
index 32d319f..708f4b1 100644
--- a/gdb/testsuite/gdb.dwarf2/gdb-add-index.exp
+++ b/gdb/testsuite/gdb.dwarf2/gdb-add-index.exp
@@ -27,48 +27,14 @@ if { [prepare_for_testing "failed to prepare" "${testfile}" \
     return -1
 }
 
-# Add a .gdb_index section to PROGRAM.
-# PROGRAM is assumed to be the output of standard_output_file.
-# Returns the 0 if there is a failure, otherwise 1.
-
-proc add_gdb_index { program } {
-    global srcdir GDB env BUILD_DATA_DIRECTORY
-    set contrib_dir "$srcdir/../contrib"
-    set env(GDB) "$GDB --data-directory=$BUILD_DATA_DIRECTORY"
-    set result [catch "exec $contrib_dir/gdb-add-index.sh $program" output]
-    if { $result != 0 } {
-	verbose -log "result is $result"
-	verbose -log "output is $output"
-	return 0
-    }
-
-    return 1
-}
-
-# Build a copy of the program with an index (.gdb_index/.debug_names).
-# But only if the toolchain didn't already create one: gdb doesn't support
-# building an index from a program already using one.
-
-set test "check if index present"
-gdb_test_multiple "mt print objfiles ${testfile}" $test {
-    -re "gdb_index.*${gdb_prompt} $" {
-	set binfile_with_index $binfile
-    }
-    -re "debug_names.*${gdb_prompt} $" {
-	set binfile_with_index $binfile
-    }
-    -re "Psymtabs.*${gdb_prompt} $" {
-	if { [add_gdb_index $binfile] != "1" } {
-	    return -1
-	}
-	set binfile_with_index $binfile
-    }
+if { [ensure_gdb_index $binfile] == -1 } {
+    return -1
 }
 
 # Ok, we have a copy of $binfile with an index.
 # Restart gdb and verify the index was used.
 
-clean_restart ${binfile_with_index}
+clean_restart ${binfile}
 gdb_test "mt print objfiles ${testfile}" \
     "(gdb_index|debug_names).*" \
     "index used"
diff --git a/gdb/testsuite/gdb.dwarf2/imported-unit-runto-main.exp b/gdb/testsuite/gdb.dwarf2/imported-unit-runto-main.exp
new file mode 100644
index 0000000..2794684
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/imported-unit-runto-main.exp
@@ -0,0 +1,92 @@
+# Copyright 2020 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 dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+};
+
+standard_testfile main.c .S
+
+set executable ${testfile}
+set asm_file [standard_output_file ${srcfile2}]
+
+# We need to know the size of integer types in order to write some of the
+# debugging info we'd like to generate.
+if [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] {
+    return -1
+}
+
+# Create the DWARF.
+Dwarf::assemble $asm_file {
+    declare_labels cu_label main_label int_label
+    declare_labels aaa_label
+    set int_size [get_sizeof "int" 4]
+
+    global srcdir subdir srcfile
+
+    extern main
+
+    set main_range [function_range main ${srcdir}/${subdir}/${srcfile}]
+    set main_start [lindex $main_range 0]
+    set main_length [lindex $main_range 1]
+
+    cu {} {
+	cu_label: partial_unit {
+	    {language @DW_LANG_C}
+	    {name "imported_unit.c"}
+	} {
+	    int_label: base_type {
+		{byte_size $int_size sdata}
+		{encoding @DW_ATE_signed}
+		{name int}
+	    }
+
+	    main_label: subprogram {
+		{name main}
+		{type :$int_label}
+		{external 1 flag}
+	    }
+	}
+    }
+
+    cu {} {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {name "<artificial>"}
+	} {
+	    subprogram {
+		{abstract_origin %$main_label}
+		{low_pc $main_start addr}
+		{high_pc "$main_start + $main_length" addr}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+if { [ensure_gdb_index $binfile] == -1 } {
+    return -1
+}
+
+clean_restart ${binfile}
+
+runto main message
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 52687ad..8418c3d 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -7038,5 +7038,48 @@ proc verify_psymtab_expanded { filename readin } {
     }
 }
 
+# Add a .gdb_index section to PROGRAM.
+# PROGRAM is assumed to be the output of standard_output_file.
+# Returns the 0 if there is a failure, otherwise 1.
+
+proc add_gdb_index { program } {
+    global srcdir GDB env BUILD_DATA_DIRECTORY
+    set contrib_dir "$srcdir/../contrib"
+    set env(GDB) "$GDB --data-directory=$BUILD_DATA_DIRECTORY"
+    set result [catch "exec $contrib_dir/gdb-add-index.sh $program" output]
+    if { $result != 0 } {
+	verbose -log "result is $result"
+	verbose -log "output is $output"
+	return 0
+    }
+
+    return 1
+}
+
+# Add a .gdb_index section to PROGRAM, unless it alread has an index
+# (.gdb_index/.debug_names).  Gdb doesn't support building an index from a
+# program already using one.  Return 1 if a .gdb_index was added, return 0
+# if it already contained an index, and -1 if an error occurred.
+
+proc ensure_gdb_index { binfile } {
+    set testfile [file tail $binfile]
+    set test "check if index present"
+    gdb_test_multiple "mt print objfiles ${testfile}" $test {
+	-re -wrap "gdb_index.*" {
+	    return 0
+	}
+	-re -wrap "debug_names.*" {
+	    return 0
+	}
+	-re -wrap "Psymtabs.*" {
+	    if { [add_gdb_index $binfile] != "1" } {
+		return -1
+	    }
+	    return 1
+	}
+    }
+    return -1
+}
+
 # Always load compatibility stuff.
 load_lib future.exp