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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.
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.
Attachment:
example.tar.gz
Description: Unix tar archive
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