[PATCHv2 2/2] gdb: Add support for tracking the DWARF line table is-stmt field

[Tom Tromey]

Hi.

This particular hunk causes a regression on an internal test case for a
RISC-V target:

Andrew> +  /* If we have a duplicate for the previous entry then ignore the new
Andrew> +     entry, except, if the new entry is setting the is_stmt flag, then
Andrew> +     ensure the previous entry respects the new setting.  */
Andrew> +  e = subfile->line_vector->item + subfile->line_vector->nitems - 1;
Andrew> +  if (e->line == line && e->pc == pc)
Andrew> +    {
Andrew> +      if (is_stmt && !e->is_stmt)
Andrew> +	e->is_stmt = 1;
Andrew> +      return;
Andrew> +    }

Now, I'm not 100% certain that this is a "true" regression.  The test
case in question already has a large number of XFAILs for various
targets, because in practice gdb reports different stop locations for
different compilers.

It's just 2 short files so I can share it here:

    /* r.c */
    #include "r.h"

    int
    main ()
    {
      callee ();
    }

    /* r.h */
    int counter = 42;

    inline void
    callee () {
      counter = 0;  /* break here */
    }

When I look at this with readelf I see:

    $ readelf --debug-dump=decodedline ./r 
    Contents of the .debug_line section:

    r.c:
    File name                            Line number    Starting address    View    Stmt
    r.c                                            6          0x80000042               x
    r.c                                            7          0x80000042       1       x

    r.h:
    r.h                                            6          0x80000042       2       x
    r.h                                            6          0x80000042       3

    r.c:
    r.c                                            8          0x8000004a       4
    r.c                                            8          0x8000004e       5

I can send an executable if you want.

If I run gdb on the resulting program and "break r.h:6", before the
is-statement patch it reports r.h:6, but afterward it says:

    (gdb) b r.h:6
    Breakpoint 1 at 0x8000004a: file r.c, line 8.

Backing out the hunk in question up above is enough to fix the problem.

What is happening is that gdb is no longer recognizing the special
prologue rule in skip_prologue_sal:

      /* For languages other than assembly, treat two consecutive line
	 entries at the same address as a zero-instruction prologue.
	 The GNU assembler emits separate line notes for each instruction
	 in a multi-instruction macro, but compilers generally will not
	 do this.  */

TBH this has always seemed like a big hack, at least in the DWARF era.
But, I suppose it's a hack we continue to need, at least until GCC emits
proper prologue end notes.

Just removing the "return" is enough to make it work for me.  I didn't
try a real test run, though.

I'd appreciate any thoughts you have on this.

thanks,
Tom