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, 44 insertions, 3 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index 4724738..e6566f9 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -20040,6 +20040,15 @@ private:
   /* The last file a line number was recorded for.  */
   struct subfile *m_last_subfile = NULL;
 
+  /* The address of the last line entry.  */
+  CORE_ADDR m_last_address;
+
+  /* Set to true when a previous line at the same address (using
+     m_last_address) had m_is_stmt true.  This is reset to false when a
+     line entry at a new address (m_address different to m_last_address) is
+     processed.  */
+  bool m_stmt_at_address = false;
+
   /* When true, record the lines we decode.  */
   bool m_currently_recording_lines = false;
 
@@ -20233,14 +20242,34 @@ lnp_state_machine::record_line (bool end_sequence)
       fe->included_p = 1;
       if (m_record_lines_p)
 	{
-	  if (m_last_subfile != m_cu->get_builder ()->get_current_subfile ()
-	      || end_sequence)
+	  /* When we switch files we insert an end maker in the first file,
+	     switch to the second file and add a new line entry.  The
+	     problem is that the end marker inserted in the first file will
+	     discard any previous line entries at the same address.  If the
+	     line entries in the first file are marked as is-stmt, while
+	     the new line in the second file is non-stmt, then this means
+	     the end marker will discard is-stmt lines so we can have a
+	     non-stmt line.  This means that there are less addresses at
+	     which the user can insert a breakpoint.
+
+	     To improve this we track the last address in m_last_address,
+	     and whether we have seen an is-stmt at this address.  Then
+	     when switching files, if we have seen a stmt at the current
+	     address, and we are switching to create a non-stmt line, then
+	     discard the new line.  */
+	  bool file_changed
+	    = m_last_subfile != m_cu->get_builder ()->get_current_subfile ();
+	  bool ignore_this_line
+	    = (file_changed && !end_sequence && m_last_address == m_address
+	       && !m_is_stmt && m_stmt_at_address);
+
+	  if ((file_changed && !ignore_this_line) || end_sequence)
 	    {
 	      dwarf_finish_line (m_gdbarch, m_last_subfile, m_address,
 				 m_currently_recording_lines ? m_cu : nullptr);
 	    }
 
-	  if (!end_sequence)
+	  if (!end_sequence && !ignore_this_line)
 	    {
 	      bool is_stmt = producer_is_codewarrior (m_cu) || m_is_stmt;
 
@@ -20259,6 +20288,15 @@ lnp_state_machine::record_line (bool end_sequence)
 	    }
 	}
     }
+
+  /* Track whether we have seen any m_is_stmt true at m_address in case we
+     have multiple line table entries all at m_address.  */
+  if (m_last_address != m_address)
+    {
+      m_stmt_at_address = false;
+      m_last_address = m_address;
+    }
+  m_stmt_at_address |= m_is_stmt;
 }
 
 lnp_state_machine::lnp_state_machine (struct dwarf2_cu *cu, gdbarch *arch,
@@ -20278,6 +20316,9 @@ lnp_state_machine::lnp_state_machine (struct dwarf2_cu *cu, gdbarch *arch,
   m_address = gdbarch_adjust_dwarf2_line (arch, 0, 0);
   m_is_stmt = lh->default_is_stmt;
   m_discriminator = 0;
+
+  m_last_address = m_address;
+  m_stmt_at_address = false;
 }
 
 void