Re: [PATCHv2 1/4] gdb: more debug output for displaced stepping

[Pedro Alves <pedro@palves.net>]

On 2023-03-16 4:39 p.m., Andrew Burgess via Gdb-patches wrote:
> While investigating a displaced stepping issue I wanted an easy way to
> see what GDB thought the original instruction was, and what
> instruction GDB replaced that with when performing the displaced step.
> 
> We do print out the address that is being stepped, so I can track down
> the original instruction.
> 
> And we do print out the bytes of the new instruction, so I can figure
> out what the replacement instruction was, but it's not really easy.
> 
> In this commit, within displaced_step_buffers::prepare, I add new
> debug output which disassembles both the original instruction, and the
> replacement instruction, and prints both these instructions, along
> with the bytes that make these instructions, to the debug output.
> 
> This new debug improved on some debug that was already present in
> infrun.c, however, the old infrun.c debug (a) didn't actually
> disassemble the instruction, it just printed the bytes, and (b) there
> was an assumption that all instructions are 4-bytes long, which
> clearly isn't correct.
> 
> I think it makes more sense to have all this debug in the
> displaced-stepping.c file, so I've removed the infrun.c code, and
> added my new debug in displaced-stepping.c.

A downside of that move is that AMD GPUs (amd-dbgapi-target.c) actually support
displaced stepping (though that part of the port hasn't been upstreamed yet), but
we don't use displaced_step_buffers there.  So we'll be losing the debug output
in that target.  Adding Simon who may want to look into that.

Pedro Alves