[reverse] A "toy" implementation of reverse execution

[Michael Snyder]

And I forgot to attach my example.  This contains a trace-frame
file for the testsuite/gdb.base/break.c program, with one frame
per instruction.  Using the rda server and any x86 gdb you can
debug this forward and backward, by instruction or by source line.


Michael Snyder wrote:
> Hey folks,
> 
> Here's a "toy" implementation of a target that can,
> to a very limited extent, act as if it's offering
> reverse execution to gdb.  I hope it will be useful
> for trying out ideas.  It should be useful for both
> the "bookmark" approach and the "reverse-stepi" method.
> 
> What I've done is, starting from the "sample" program in RDA,
> I added the ability to understand "tfind" requests, and then
> added on a module that could read simplified tracepoint frames
> from a file, and then serve them back to gdb.
> 
> Then I generated a trace frame file from the testsuite program
> "break.c", starting at main and saving a tracepoint for every
> instruction.  It only took about 157 frames.   ;-)
> 
> Now, if I feed that file to my rda-derived trace frame server,
> "tfind next" becomes equivalent to "stepi", and "tfind prev"
> becomes equivalent to "reverse-stepi".
> 
> With those as primatives, reverse-step turns out to be,
> to first approximation at least, fairly easy:
> 
>     define reverse-si
>       tfind -
>     end
> 
>     define si
>       tfind
>     end
> 
>     define unstep
>       reverse-si
>       set $foo=$trace_line
>       while $foo==$trace_line
>         reverse-si
>       end
>       si
>     end
> 
> As several of us speculated, the algorhythm for stepping backward
> involves first taking a backstep, then establishing the line range
> and stepping out of it, and finally taking one forward step.
> 
> I've created a branch and checked in my modified rda, along with
> a hacked-up gdb that I'm using to help create trace frame files.
> The branch is "msnyder-tracepoint-checkpoint-branch", and y'all
> are welcome to play with it.  There's a README-CHECKPOINTS file.
> 
> 

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