read target register to decide breakpoint size

Tim Newsome
Mon Nov 21 18:00:00 GMT 2016

Thanks, Antoine! That's exactly what I was looking for.


On Mon, Nov 21, 2016 at 8:36 AM, Antoine Tremblay
<> wrote:
> Tim Newsome writes:
> > I'm still working on RISC-V support for gdb. Any given RISC-V core may
> > support a compressed instruction set (2 bytes per instruction as
> > opposed to 4). There are corresponding 2-byte and 4-byte breakpoint
> > instructions. On cores that support the compressed instruction set it
> > is safe to just always use the 2-byte version, and there is a register
> > I can read to tell me whether the compressed instruction set is
> > supported. What I would like to do is read (and cache) that register
> > when breakpoint size is determined. That seems more robust than making
> > a decision based on ELF info, which may not reflect what is actually
> > being executed.
> >
> > Is that a good idea? Are there examples of operations that read target
> > registers to complete?
> Yes actually you can check how ARM does it, it has the same kind of
> problem with 3 breakpoints you can set for thumb, thumb2 and arm
> instruction sets.
> See arm-tdep.c:arm_sw_breakpoint_from_kind and
> arm_breakpoint_kind_from_current_state
> This is called in breakpoint.c:breakpoint_kind and it can use a register
> to make the decision from the current state of that register.
> So possibly just implementing the sw_breakpoint_from_kind and
> breakpoint_kind_from_current state would be ok your you.
> Regards,
> Antoine Tremblay

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