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

[Kevin Buettner <kevinb@redhat.com>]

On Wed, 12 Jun 2019 13:34:03 +0100
Andrew Burgess <andrew.burgess@embecosm.com> wrote:

> It was observed that in some cases, placing a breakpoint in an
> assembler file using filename:line-number syntax would result in the
> breakpoint being placed at a different line within the file.
> 
> For example, consider this x86-64 assembler:
> 
>     test:
>             push   %rbp		/* Break here.  */
>             mov    %rsp, %rbp
>             nop			/* Stops here.  */
> 
> The user places the breakpoint using file:line notation targeting the
> line marked 'Break here', GDB actually stops at the line marked 'Stops
> here'.
> 
> The reason is that the label 'test' is identified as the likely start
> of a function, and the call to symtab.c:skip_prologue_sal causes GDB
> to skip forward over the instructions that GDB believes to be part of
> the prologue.
> 
> I believe however, that when debugging assembler code, where the user
> has instruction-by-instruction visibility, if they ask for a specific
> line, GDB should (as far as possible) stop on that line, and not
> perform any prologue skipping.  I don't believe that the behaviour of
> higher level languages should change, in these cases skipping the
> prologue seems like the correct thing to do.

I agree with all of this.

> In order to implement this change I needed to extend our current
> tracking of when the user has requested an explicit line number.  We
> already tracked this in some cases, but not in others (see the changes
> in linespec.c).  However, once I did this I started to see some
> additional failures (in tests gdb.base/break-include.exp
> gdb.base/ending-run.exp gdb.mi/mi-break.exp gdb.mi/mi-reverse.exp
> gdb.mi/mi-simplerun.exp) where we currently expected a breakpoint
> placed at one file and line number to be updated to reference a
> different line number, this was fixed by removing some code in
> symtab.c:skip_prologue_sal.  My concern here is that removing this
> check didn't cause anything else to fail.

Did you investigate the reason for the failures with this hunk
left in place?...

> @@ -3812,12 +3821,6 @@ skip_prologue_sal (struct symtab_and_line *sal)
>  
>    sal->pc = pc;
>    sal->section = section;
> -
> -  /* Unless the explicit_line flag was set, update the SAL line
> -     and symtab to correspond to the modified PC location.  */
> -  if (sal->explicit_line)
> -    return;
> -
>    sal->symt> FAIL: gdb.base/break-include.exp: break break-include.c:53
ab = start_sal.symtab;
>    sal->line = start_sal.line;
>    sal->end = start_sal.end;

The rest of the patch looks fine to me.  Deleting those lines might
be okay also, but I'd like to understand why adding additional
explicit line number tracking caused these failures:

FAIL: gdb.base/break-include.exp: break break-include.c:53
FAIL: gdb.base/ending-run.exp: Cleared 2 by line
FAIL: gdb.base/ending-run.exp: clear 2 by default

Kevin