[RFC][PATCH 0/6] Step program considering the source column information

[Tom de Vries]

On 16-05-2020 19:26, Hannes Domani via Gdb-patches wrote:
> Basically, this implements the new commands nextc (nc) and stepc (sc),
> which allow to step through the program until an instruction is reached,
> that belongs to another source column (according to the debug information).
> 
> The current column location is visualized in the frame info, and in the TUI.
> 
> Also, the column information is added to 'maint info line-table' and the
> python line table interface.
> 
> Since the frame info output is different, this certainly breaks the
> testsuite, so this column indicator needs a parameter to disable, but I
> wasn't concerned about this yet.
> 
> With the example code from PR25913, it looks like this:
> 
> (gdb) start
> Temporary breakpoint 2 at 0x40162d: file gdb-25911.c, line 4.
> Starting program: C:\src\tests\gdb-25911.exe
> 
> Temporary breakpoint 2, main () at gdb-25911.c:4
> 4         int a = 4;
>               ^
> (gdb) nc
> 6         a = 5; a = 6; a = 7;
>             ^
> (gdb) nc
> 6         a = 5; a = 6; a = 7;
>                    ^
> (gdb) nc
> 6         a = 5; a = 6; a = 7;
>                           ^
> (gdb) nc
> 8         return 0;
> 
> 
> What do you think of this so far?
> 

Very nice :)

As I've just learned by looking at this (
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95360 ) PR, interesting
code in relation to this patch series is dwarf_record_line_p.

The function implements filtering of line-table elements, and has as
related test-case gdb.dwarf2/dw2-single-line-discriminators.exp.

If we disable the filtering (by returning 1 at the end instead of 0), we
get for test-case gdb.dwarf2/dw2-single-line-discriminators.exp:
...
Temporary breakpoint 2, main () at
gdb.dwarf2/dw2-single-line-discriminators.c:26
26        x = 0;
(gdb) n
28        for (i = 0; i < 10; ++i) continue; /* stepi line */
(gdb) si
28        for (i = 0; i < 10; ++i) continue; /* stepi line */
...

That is: no progress is visible after the stepi.  If we re-enable the
filtering, we have:
...
Temporary breakpoint 1, main () at
gdb.dwarf2/dw2-single-line-discriminators.c:26
26        x = 0;
(gdb) n
28        for (i = 0; i < 10; ++i) continue; /* stepi line */
(gdb) si
0x00000000004004cd      28        for (i = 0; i < 10; ++i) continue; /*
stepi line */
...

So, progress is visible.

If we now use your patch series, and with the filtering enabled, we have:
...
Temporary breakpoint 1, main () at
gdb.dwarf2/dw2-single-line-discriminators.c:26
26        x = 0;
          ^
(gdb) n
28        for (i = 0; i < 10; ++i) continue; /* stepi line */
               ^
(gdb) si
0x00000000004004cd      28        for (i = 0; i < 10; ++i) continue; /*
stepi line */
                                       ^
(gdb)
0x00000000004004d1      28        for (i = 0; i < 10; ++i) continue; /*
stepi line */
                                       ^
(gdb)
0x00000000004004d3      28        for (i = 0; i < 10; ++i) continue; /*
stepi line */
                                       ^
(gdb)
0x00000000004004d5      28        for (i = 0; i < 10; ++i) continue; /*
stepi line */
                                       ^
...

So, we see progress in the insn addresses, but the column does not progress.

OTOH, if we disable filtering:
...
Temporary breakpoint 1, main () at
gdb.dwarf2/dw2-single-line-discriminators.c:26
26        x = 0;
          ^
(gdb) n
28        for (i = 0; i < 10; ++i) continue; /* stepi line */
               ^
(gdb) si
28        for (i = 0; i < 10; ++i) continue; /* stepi line */
               ^
(gdb) si
0x00000000004004d1      28        for (i = 0; i < 10; ++i) continue; /*
stepi line */
                                       ^
(gdb) si
28        for (i = 0; i < 10; ++i) continue; /* stepi line */
                                   ^
(gdb) si
28        for (i = 0; i < 10; ++i) continue; /* stepi line */
                              ^
(gdb) si
0x00000000004004d8      28        for (i = 0; i < 10; ++i) continue; /*
stepi line */
                                                      ^
...
we don't see progress after the first stepi, but eventually we do see
progress in the column.

Looking at the line-info:
...
  [0x00000147]  Set column to 8
  [0x00000149]  Special opcode 161: advance Address by 11 to 0x4004c6
and Line by 2 to 28
  [0x0000014a]  Extended opcode 4: set Discriminator to 4
  [0x0000014e]  Special opcode 103: advance Address by 7 to 0x4004cd and
Line by 0 to 28
...
perhaps we we should collapse entries that have the same column, so
something like this in dwarf_record_line_p:
...
  if (line != last_line)
    return 1;
  if (colum != last_column)
    return 1;
  return 0;
...

Thanks,
- Tom