Re: [PATCH] gdb: Don't skip prologue for explicit line breakpoints in assembler

[Pedro Alves <palves at redhat dot com>]

On 6/21/19 12:23 AM, Andrew Burgess wrote:

> I spent some more time trying to find a path that would call both
> 'decode_digits_list_mode' and then 'skip_prologue_sal', but I still
> can't find one.

But won't that change affect any code path that ends up in
skip_prologue_sal with explicit_line set?

E.g.:

/* Helper function for break_command_1 and disassemble_command.  */

void
resolve_sal_pc (struct symtab_and_line *sal)
{
  CORE_ADDR pc;

  if (sal->pc == 0 && sal->symtab != NULL)
    {
      if (!find_line_pc (sal->symtab, sal->line, &pc))
	error (_("No line %d in file \"%s\"."),
	       sal->line, symtab_to_filename_for_display (sal->symtab));
      sal->pc = pc;

      /* If this SAL corresponds to a breakpoint inserted using a line
         number, then skip the function prologue if necessary.  */
      if (sal->explicit_line)
	skip_prologue_sal (sal);
    }

Is that path unreachable today?


> 
> Looking back at how the explicit_line flag was originally used when
> it was added in commit ed0616c6b78a0966, things have changed quite a
> bit in the 10+ years since.  There were some tests added along with
> the explicit_line flag (gdb.cp/mb-*.exp) and these all pass both in
> master and in my patched branch.
> 
> My current thinking is that the explicit_line flag was no longer doing
> anything useful in master, but if someone disagrees I'd love to
> understand more about this.

I seem to recall that GDB didn't use to update a breakpoint's line
number to advance to the next line number that includes some actual
compiled code.  Like if you set a breakpoint at line 10 below:

 10    // just a comment
 11    i++;

you end up with a breakpoint at line 11.  Maybe it's old code
related to that.

Maybe I'm misremembering.

In any case, if you change this, then you also should change
the function's entry comment:

 /* Adjust SAL to the first instruction past the function prologue.
    If the PC was explicitly specified, the SAL is not changed.
    If the line number was explicitly specified, at most the SAL's PC
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    is updated.  If SAL is already past the prologue, then do nothing.  */
    ^^^^^^^^^^
 
 void
 skip_prologue_sal (struct symtab_and_line *sal)
 {

Thanks,
Pedro Alves