read target register to decide breakpoint size

Antoine Tremblay
Mon Nov 21 16:37:00 GMT 2016

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

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.

Antoine Tremblay

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