Re: [discuss] Support for reverse-execution

["Eli Zaretskii" <eliz at gnu dot org>]

> Date: Thu, 19 May 2005 15:00:52 +0200
> From: Johan Rydberg <jrydberg@virtutech.com>
> Cc: gdb@sources.redhat.com
> 
> Here's a patch against CVS that implements the following commands:
> 
>    - rstep, rstepi
>    - rnext, rnexti
> 
> The obvious commands "rcontinue" and "rfinished" are not yet
> implemented, and therefor not included in this patch.  The command
> names are of course up for discussion.

I don't mind these names; reverse-next etc. seem awkward, but rnext
etc. are okay, I think.  However, see my other comments in response to
Daniel.

> What I have done is add a new variable, step_direction, that can either
> be STEP_DIR_FORWARD or STEP_DIR_REVERSE.

I'd prefer STEP_FORWARD and STEP_BACKWARD.

> To implement "rnext" I had to add a new target-specific function that
> tries to figure out the address of the call instruction based on the
> last executed instruction and the instruction to be executed, IF the
> current instruction can be identified as a return insn.

Sorry, I'm confused.  Isn't it right that if I'm stepping backwards
through code such as this:

    i += 1;
    foo (i);
  =>i = 0;

where "=>" shows the current source line, then typing `rnext' once
will get me to this:

    i += 1;
  =>foo (i);
    i = 0;

which means I'm now before the CALL insn that calls `foo'?  If so,
what ``return insn'' were you talking about?  If it's the return
instruction that returns from `foo', then that insn is inside the code
generatedfor `foo', which GDB won't see.  What am I missing?

> +  add_com ("rnext", class_run, rnext_command,
> +           "Step program until it reaches the previous source line.\n\

"Go backwards until the program reaches the source line before the
current one."

> +   add_com_alias ("rn", "rnext", class_run, 1);

Do we want another alias called "previous"?

> +extern enum step_direction_kind step_direction;

Yuk! a global variable!

> +      if (step_direction == STEP_DIR_FORWARD)
> +	target_resume (resume_ptid, step, sig);
> +      else
> +	{
> +	  target_reverse (resume_ptid, step);
> +	}

I'd prefer that target_resume accepted the direction argument.  Is
there something that I'm missing that makes this hard or impossible?