Re: [PATCH] Program Breakpoints

[Ross Morley <ross@tensilica.com>]

Pedro,

Did that make sense to you?
Are you waiting for me to redo the patch with -Nurp before
you look at the code? I had assumed you could look at the
code and I'll redo the patch once I incorporate any feedback.

Or if someone else has time to look at it... ?
Refer to http://sourceware.org/ml/gdb-patches/2009-04/msg00579.html
for discussion and patch.

Thanks,
Ross


Ross Morley wrote:

>
> Pedro Alves wrote:
>
>> Before looking at the code, I'd like to see the interaction of
>> program breakpoints with decr_pc_after_break adjustment formalized.
>> I did a quick skim and couldn't find it handled.
>>  
>>
>> E.g, if you were to implement support for this on x86 gdbserver,
>> assuming int3 traps, it appears to me that the only option is
>> for the target to always rewind the pc before reporting to GDB, and
>> for GDB to never adjust it itself, even for regular breakpoint hits,
>> otherwise, e.g., consecutive breakpoints will be mishandled.  This
>> also suggests that there has to be prior negotiation (qSupported) to
>> enable the support.  Has this been considered?
>>  
>>
> I have thought about this. I understand decr_pc_after_break is for archs
> that have already incremented the PC after hitting a trap, so GDB needs
> to decrement it back to the trap in order to replace it with the original
> code to step over it.
>
> No special handling is needed for program breakpoints because:
>
> - decr_pc_after_break only applies to breakpoints inserted by GDB
>  and therefore known to GDB (adjust_pc_after_break in infrun.c
>  appears to only adjust the PC if it hit a software break known
>  to GDB). Program breakpoints are by definition not those, and
>  their special handling is not applied to those. If a target were
>  to adjust the PC for a program breakpoint, it would then have to
>  report a non-zero size in STOPPED_BY_TRAP_INSTRUCTION (see next
>  bullet), but that would not be the normal handling for such archs.
>
> - decr_pc_after_break archs report size==0 in their implementation
>  of STOPPED_BY_TRAP_INSTRUCTION(&size). That means the PC does not
>  need adjustment to step over a program breakpoint. GDB will report
>  it stopped for a program breakpoint (if it wasn't in the breakpoint
>  table) but will not do anything special on resume. Perhaps I should
>  make that clearer in the comment in target.h on that macro, by
>  explicit reference to decr_pc_after_break.
>
>> (A small request: please include the -p switch in your `cvs diff'
>> commands, or add it to .cvsrc.  I use -Nurp myself.)
>>
>>  
>>
> OK.
>
> Thanks,
> Ross
>
>