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

[Hannes Domani]

 Am Mittwoch, 27. Mai 2020, 17:33:37 MESZ hat Tom de Vries <tdevries@suse.de> Folgendes geschrieben:

> 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;
>
> ...

Interesting.
I'm just wondering, if dwarf_record_line_p merges all entries of the same line,
why does my column-based stepping even work at all?

Is it because of line_has_non_zero_discriminator?
If yes, what is the discriminator?


Hannes