read target register to decide breakpoint size

[Antoine Tremblay]

Yao Qi writes:

> On 16-12-13 13:30:02, Tim Newsome wrote:
>> Actually, this seems to work inside breakpoint_kind_from_pc():
>> ```
>>     struct frame_info *frame = get_current_frame ();
>>     uint32_t misa = get_frame_register_unsigned (frame, RISCV_CSR_MISA_REGNUM);
>> ```
>> 
>> Is that kosher? If so, is there any reason for me to implement
>> breakpoint_kind_from_current_state?
>
> I'd like not to do so.  Can't you decode the instruction to see whether
> it is compressed or uncompressed?  I also think it is a good idea to
> make a decision based on ELF info, as you mentioned in the first email.

Correct me if I'm wrong but reading the RISC-V arch manual, it seems
like the MISA register information is static.

So unlike arm where the ISA might change, on RISC-V it is static for the
life of the program ? 

If that's true, and that you can't decode the instruction, it doesn't
seem that bad to use the register in kind_from_pc ?

Yao, was your concern that this would be non-static, or there is another
reason ?

However thinking about the ELF info, could you elaborate on why it may
not reflect what is actually being executed ? And how this is a problem
?

A note about _from_current_state too this is used while single_stepping
since GDB knows it's about to set a breakpoint from a known state to
the next location, this differs from from_pc which is used to set a pc from
any state to any location.

So in your case it's really from_pc that you want.

Thanks,
Antoine