gdb: Preserve is-stmt lines when switch between files

After the is-stmt support commit:

  commit 8c95582da858ac981f689a6f599acacb8c5c490f
  Date:   Mon Dec 30 21:04:51 2019 +0000

      gdb: Add support for tracking the DWARF line table is-stmt field

A regression was observed where a breakpoint could no longer be placed
in some cases.

Consider a line table like this:

  File 1: test.c
  File 2: test.h

  | Addr | File | Line | Stmt |
  |------|------|------|------|
  | 1    | 1    | 16   | Y    |
  | 2    | 1    | 17   | Y    |
  | 3    | 2    | 21   | Y    |
  | 4    | 2    | 22   | Y    |
  | 4    | 1    | 18   | N    |
  | 5    | 2    | 23   | N    |
  | 6    | 1    | 24   | Y    |
  | 7    | 1    | END  | Y    |
  |------|------|------|------|

Before the is-stmt patch GDB would ignore any non-stmt lines, so GDB
built two line table structures:

  File 1                 File 2
  ------                 ------

  | Addr | Line |        | Addr | Line |
  |------|------|        |------|------|
  | 1    | 16   |        | 3    | 21   |
  | 2    | 17   |        | 4    | 22   |
  | 3    | END  |        | 6    | END  |
  | 6    | 24   |        |------|------|
  | 7    | END  |
  |------|------|

After the is-stmt patch GDB now records non-stmt lines, so the
generated line table structures look like this:

  File 1                   File 2
  ------                   ------

  | Addr | Line | Stmt |  | Addr | Line | Stmt |
  |------|------|------|  |------|------|------|
  | 1    | 16   | Y    |  | 3    | 21   | Y    |
  | 2    | 17   | Y    |  | 4    | 22   | Y    |
  | 3    | END  | Y    |  | 4    | END  | Y    |
  | 4    | 18   | N    |  | 5    | 23   | N    |
  | 5    | END  | Y    |  | 6    | END  | Y    |
  | 6    | 24   | Y    |  |------|------|------|
  | 7    | END  | Y    |
  |------|------|------|

The problem is that in 'File 2', end END marker at address 4 causes
the previous line table entry to be discarded, so we actually end up
with this:

  File 2
  ------

  | Addr | Line | Stmt |
  |------|------|------|
  | 3    | 21   | Y    |
  | 4    | END  | Y    |
  | 5    | 23   | N    |
  | 6    | END  | Y    |
  |------|------|------|

When a user tries to place a breakpoint in file 2 at line 22, this is
no longer possible.

The solution I propose here is that we ignore line table entries that
would trigger a change of file if:

  1. The new line being added is at the same address as the previous
  line, and

  2. We have previously seen an is-stmt line at the current address.

The result of this is that GDB switches file, and knows that some line
entry (or entries) are going to be discarded, prefer to keep is-stmt
lines and discard non-stmt lines.

After this commit the lines tables are now:

  File 1                   File 2
  ------                   ------

  | Addr | Line | Stmt |  | Addr | Line | Stmt |
  |------|------|------|  |------|------|------|
  | 1    | 16   | Y    |  | 3    | 21   | Y    |
  | 2    | 17   | Y    |  | 4    | 22   | Y    |
  | 3    | END  | Y    |  | 5    | 23   | N    |
  | 5    | END  | Y    |  | 6    | END  | Y    |
  | 6    | 24   | Y    |  |------|------|------|
  | 7    | END  | Y    |
  |------|------|------|

We've lost the non-stmt entry for file 1, line 18, but retained the
is-stmt entry for file 2, line 22.  The user can now place a
breakpoint at that location.

One problem that came from this commit was the test
gdb.cp/step-and-next-inline.exp, which broke in several places.  After
looking at this test again I think that in some cases this test was
only ever passing by pure luck.  The debug GCC is producing for this
test is pretty broken.  I raised this GCC bug:

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94474

for this and disabled one entire half of the test.  There are still
some cases in here that do pass, and if/when GCC is fixed it would be
great to enable this test again.

gdb/ChangeLog:

	* dwarf2/read.c (class lnp_state_machine) <m_last_address>: New
	member variable.
	<m_stmt_at_address>: New member variable.
	(lnp_state_machine::record_line): Don't record some lines, update
	tracking of is_stmt at the same address.
	(lnp_state_machine::lnp_state_machine): Initialise new member
	variables.

gdb/testsuite/ChangeLog:

	* gdb.cp/step-and-next-inline.exp (do_test): Skip all tests in the
	use_header case.
	* gdb.dwarf2/dw2-inline-header-1.exp: New file.
	* gdb.dwarf2/dw2-inline-header-2.exp: New file.
	* gdb.dwarf2/dw2-inline-header-3.exp: New file.
	* gdb.dwarf2/dw2-inline-header-lbls.c: New file.
	* gdb.dwarf2/dw2-inline-header.c: New file.
	* gdb.dwarf2/dw2-inline-header.h: New file.

1 files changed, 186 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/dw2-inline-header-1.exp b/gdb/testsuite/gdb.dwarf2/dw2-inline-header-1.exp
new file mode 100644
index 0000000..dc7ec92
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/dw2-inline-header-1.exp
@@ -0,0 +1,186 @@
+# 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/>.
+
+# Setup a line table where:
+#
+# |      |      |      |      | Inline | Inline |
+# | Addr | File | Line | Stmt | Rng A  | Rng B  |
+# |------|------|------|------|--------|--------|
+# | 1    | 1    | 16   | Y    |        |        |
+# | 2    | 1    | 17   | Y    |        |        |
+# | 3    | 2    | 21   | Y    | X      |        |
+# | 4    | 2    | 22   | Y    | X      |        |
+# | 4    | 1    | 18   | N    | X      |        |
+# | 5    | 2    | 23   | N    | X      | X      |
+# | 6    | 1    | 24   | Y    |        |        |
+# | 7    | 1    | END  | Y    |        |        |
+# |------|------|------|------|--------|--------|
+#
+# Places a brekpoint at file 2, line 22.  Previously GDB would discard
+# the line table entry for this line due to switching files for the
+# file 1, line 18 non-statement line.  After patching however, GDB now
+# discards the file 1, line 18 entry instead, and the breakpoint at
+# line 22 should succeed.
+#
+# The two inlined subroutine ranges 'A' and 'B' represent two possible
+# ways that a compiler might represent this siuatio in the DWARF.
+#
+# Range 'B' is something that has been seen in the wild using GCC 8.2.
+# In this case the compilers range information is clearly wrong, but
+# this shouldn't impact the main point of the test.
+#
+# Range 'A' is a hypothetical case of how the compiler might choose to
+# represent this range, this has never been seen in the wild, but is
+# an improved debug experiece over range 'B'.  However, if we ever run
+# in to the situation where GDB can support the range 'A' test, or
+# support some real DWARF seen in the wild, then the range 'A' case
+# should be dropped in favour of supporting real world cases.  This is
+# included here as it "just worked" once the range 'B' case was
+# working.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+# The .c files use __attribute__.
+if [get_compiler_info] {
+    return -1
+}
+if !$gcc_compiled {
+    return 0
+}
+
+# Prepare and run the test.
+proc do_test { start_label func_name tag } {
+    global srcfile srcfile2 srcfile3 srcfile4 testfile
+
+    standard_testfile dw2-inline-header-lbls.c dw2-inline-header-${tag}.S \
+	dw2-inline-header.c dw2-inline-header.h
+
+    set build_options {nodebug optimize=-O1}
+
+    set asm_file [standard_output_file $srcfile2]
+    Dwarf::assemble $asm_file {
+	global srcdir subdir srcfile srcfile3 srcfile4 testfile
+	upvar build_options build_options
+	upvar start_label start_label
+	declare_labels lines_label callee_subprog_label
+
+	get_func_info main $build_options
+
+	cu {} {
+	    compile_unit {
+		{producer "gcc" }
+		{language @DW_LANG_C}
+		{name ${srcfile3}}
+		{low_pc 0 addr}
+		{stmt_list ${lines_label} DW_FORM_sec_offset}
+	    } {
+		callee_subprog_label: subprogram {
+		    {external 1 flag}
+		    {name callee}
+		    {inline 3 data1}
+		}
+		subprogram {
+		    {external 1 flag}
+		    {name main}
+		    {low_pc $main_start addr}
+		    {high_pc "$main_start + $main_len" addr}
+		} {
+		    inlined_subroutine {
+			{abstract_origin %$callee_subprog_label}
+			{low_pc $start_label addr}
+			{high_pc line_label_6 addr}
+			{call_file 1 data1}
+			{call_line 18 data1}
+		    }
+		}
+	    }
+	}
+
+	lines {version 2 default_is_stmt 1} lines_label {
+	    include_dir "${srcdir}/${subdir}"
+	    file_name "$srcfile3" 1
+	    file_name "$srcfile4" 1
+
+	    program {
+		{DW_LNE_set_address line_label_1}
+		{DW_LNS_advance_line 15}
+		{DW_LNS_copy}
+
+		{DW_LNE_set_address line_label_2}
+		{DW_LNS_advance_line 1}
+		{DW_LNS_copy}
+
+		{DW_LNS_set_file 2}
+		{DW_LNE_set_address line_label_3}
+		{DW_LNS_advance_line 4}
+		{DW_LNS_copy}
+
+		{DW_LNE_set_address line_label_4}
+		{DW_LNS_advance_line 1}
+		{DW_LNS_copy}
+
+		{DW_LNS_advance_line -4}
+		{DW_LNS_set_file 1}
+		{DW_LNS_negate_stmt}
+		{DW_LNS_copy}
+
+		{DW_LNS_set_file 2}
+		{DW_LNE_set_address line_label_5}
+		{DW_LNS_advance_line 5}
+		{DW_LNS_copy}
+
+		{DW_LNS_negate_stmt}
+		{DW_LNS_set_file 1}
+		{DW_LNE_set_address line_label_6}
+		{DW_LNS_advance_line 1}
+		{DW_LNS_copy}
+
+		{DW_LNE_set_address line_label_7}
+		{DW_LNE_end_sequence}
+	    }
+	}
+    }
+
+    if { [prepare_for_testing "failed to prepare" ${testfile}-${tag} \
+	      [list $srcfile $asm_file] $build_options] } {
+	return -1
+    }
+
+    if ![runto_main] {
+	return -1
+    }
+
+    # Delete all breakpoints so that the output of "info breakpoints"
+    # below will only contain a single breakpoint.
+    delete_breakpoints
+
+    # Place a breakpoint within the function in the header file.
+    gdb_breakpoint "${srcfile4}:22"
+
+    # Check that the breakpoint was placed where we expected.  It should
+    # appear at the requested line.  When the bug in GDB was present the
+    # breakpoint would be placed on one of the following lines instead.
+    gdb_test "info breakpoints" \
+	".* in $func_name at \[^\r\n\]+${srcfile4}:22\\y.*" \
+	"info breakpoints, $tag"
+}
+
+do_test line_label_3 "callee" "range-a"
+do_test line_label_5 "main" "range-b"